Wow. In 9 minutes, you completely explained the function and basis of the Fender Presence control. I have been twisting that knob for a long time, and could tell that it brightened the sound, but now I understand why and therefore, how to use it better. I know that these are now very old videos, but thank you for taking the time to make them and for your incredibly clear explanations.
I forgot to mention, your videos remind me of the training I use to get when I spent a lot up at McIntosh Laboratory in Binghamton New York. I would go up there quite a bit over a 25 year period and learn from the engineers up there. One of my favorite teachers, was my uncle Gordon J Gow, the president and co founder of McIntosh back in 1949. Thanks again, Bob
Another great video and a great mod! Once in a while we see people who say to completely remove the NFB loop, and that is not a good idea. At low gain levels, it's not that apparent, but when you start pushing an amp, you can quickly turn the amp into mud. Low frequency overload. Removing the NFB loop makes an amp sound terrible and it's counterproductive. Leo used NFB in order to control the gain so that we could hear all that wonderful chime. I've been using an adjustable NFB for quite some time after I came to appreciate its purpose. I typically use a 3k resistor with a b50k linear pot. With this setup you are able to carefully dial in some great breakup. I was pleased to see that you use it, too! I also add the Presence Control. Try auditioning a few different cap values or use a switch to select from several different values so that you can dial in the low frequency control. This really comes into play if you're a Blues player. In a Triode, the electrons flow from the Grid to the Plate and from the Cathode to the Plate. When you use NFB, the triode acts like a mixer. The grid contains your guitar signal and the cathode "mixes" a low amplitude guitar signal from the output transformer; just like you said, the cathode signal subtracts, (not adds) the opposite of amplifies, the incoming signal from the grid. The Plate in the triode sends this mixed tamed signal on to the power tube(s). NFB subdues the signal in that triode. The neat part about the Presence Control is that you can subdue the lower frequencies, that causes farting and blocking distortion, which sounds terrible, and allow the (lower amplitude) higher treble frequencies to run wild. Again, selecting from various Presence capacitors to dial in the low frequency cutoff is recommended. Furthermore, guitar pickups will reach all the way down into an good tube amp. Use a higher value presence capacitor for a humbucker equipped guitar - 0.1uf, 0.047, because they produce more low end harmonics. Use a lower value capacitor for a single coil equipped guitar - 0.047, 0.022, 0.01 because their low frequency output is lower. Adding a 3 or 4 pole rotary switch that allows you to select the Presence capacitor is a great mod. Try it and while you're strumming a chord, select different cap values. I think you'll be pleasantly surprised. Thank you Doug for providing us a forum to share our experiences! We are all blessed to have you around...
You're welcome, Mark, and thanks for your very detailed and informative review of the applications and values of NFB. We are fortunate to have a contributor of your knowledge and experience.
Uncle Doug I added the variable NFB loop to my 69 Princeton. It has really opened up my amp and made it so much more versatile. Before the modification, to get the sound I wanted, I was either too loud for the audience in the front row or (if I was in a small room) playing so quite I couldn't hear my amp on stage. Now, after the mod, I can adjust the amps volume to the level I need, then adjust the NFB. I also play with a pedalboard and they sound even better now. Thank you Uncle Doug for another great video
Greetings, Donny. I'm glad to hear that this modification worked so well for you. It really makes these videos worth producing when viewers (like you) apply the information in useful ways. Thanks so much for your input and kind words. Best of luck :)
Thanks Doug. I finally know what the presence control actually does! I have heard countless explanations over the years but none of them imparted any actual knowledge.
Doug. I've watched SO many of your videos in the last days, and I must say: You Sir, are the single most effective teacher or professor I've ever had. And I've got two degrees in Education, so I know a thing or two of what I'm talking about! -Jonah H.
You´ve nailed it again! Thanx for another well explained basic Amp-lesson :) It is so easy to follow, as you are always make clear explanations on how things work! If you go on like this, some day every viewer could be able to do his/her very own Amp just because you have been a good teacher!
Thanks Uncle Doug, I followed your instructions and what do you know,, it works.. A 5k ohm in series with the 20k pot and now I have an adjustable negative feedback loop.. Thank again Scott
Hi Uncle Doug. Once again I find your videos useful. Had missed this point about a presence control with a capacitor to the variable negative feedback (which I allready have). Just tested a 0.1uF in there (a 5f1) to get a sense of it, and yes it tightened up bass and got me intrugued. So this will be implemented. I always scroll the comments section of your videos to see if some people have some bright ideas or experiences and often there are. Gonna do more testings. Autumn Greets. Jon
oh, btw, the cap I tested was 1uF. so that is why bass seemed tighter i recon, will see more when I test it with different values and a pot. Have fun with your projects.
Although i am not a "tech," i truly enjoy your videos. I play and like vintage amps especially fender. I own a 63 super bf and a 64 tuxedo bassman as well as many others. I started watching your videos several months ago and admire how you walk us through the finer points and explanations. This has opened up a new interest for me in discovering and enjoying the vintage technology. When i meet a fellow gearhead there is now so much more information to parlay. Kind of like "car guys" i guess. Looking forward to more videos and again, thank you for your channel. "Toh"
Welcome aboard, Mick. Rusty and I really appreciate your kind words. Please check out all the instructional videos I have already posted (there are a bunch), and hopefully some will be of interest. Meanwhile, we'll get back to work, planning the next one :)) ^. .^
Bah, Uncle Doug! I've just buttoned my Princeton back up having installed the variable bias pot and added a 1M resistor to the tremolo circuit (best mod ever!), and now I'm going to have to open her up again to do this! As with all the other people who have commented, I've found your videos invaluable. My amp is an A1164 kit from Tube Amp Doctor and shortly after building it with no confidence about what was going on inside she blew her power transformer. This forced me to learn what was going on inside the amp so that I could diagnose the issue. (I think it was ultimately caused by a combination of excessive plate current and voltage, and using the 230v tap instead of the 240v tap. In Australia 230v should be fine buy my mains varies from 230v to almost 250v.) Thanks to your videos I know feel way more confident about the amp I built :)
Hi Uncle Doug, today I installed a 10k pot as suggested in one of your earlier posts below (given that the standard negative feedback resistor is only 2k7 in the Priceton). It does make the amp go into overdrive more readily from a volume of 6 upwards, but has little-to-no effect below that. Is this because 10k is not a high enough value maybe, or does the effect only really take place when the amp is cookin'?
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Greetings Uncle Doug! First, thank you very much for your series of videos that have helped many musicians and electronics aficionados understand how this beautiful gift of science sweets our lives! I have a PRS MT15 tube amp, a modern amp but like you’ve said “they all work on the same principles”. Preamp works as follows: V1 - input 12ax7 -> to V2 - lead gain 12ax7 V3 - lead gain 12ax7 OR V4 - clean gain 12ax7 -> to V6 - effects loop 12ax7 (always on) -> to Power Amp The power amp It has a match pair of 6L6WGC tubes with a 12ax7 phase inverter, and corresponding Presence potentiometer. Also a switch that controls the output with 15 or 7 Watt options. Output transformer has an 8 Ohm and 16 Ohm option. The problem is: While amp is powered on and standby off, a subtle noise is present at the speaker (30 watt 8 ohm 10 inch) with a 50 Hz frequency (Germany : 240 AC 50 Hz). Manipulating the Presence knob affects the hum sound, while still at standby off. Your experience and vast knowledge is requested: Is this something common in tube amps? If not, could you point me out to a circuit phase where I could test and possibly find an issue? In advance, thank you very much! Best regards, Gus P.S. Clean channel is very low compared to Lead channel (which volume potentiometer seems useless to control such beast).
I wish I could magically solve the problem, Gus, but there are many, many possible causes for 50/60Hz hum. You could try moving the wires around with a wooden stick, or try switching tubes. If the volume control doesn't affect the hum, then its source is after the volume control in the circuit. Good luck.
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Thank you very much Uncle Doug and Rusty! I’ve gone through the cable test, if you’ve got time you may like to go through the videos: ua-cam.com/video/trlbdxFL9gU/v-deo.html ua-cam.com/video/sCR8a8G0rF8/v-deo.html
I love your videos. My 16 year old son and I watch them together. We have been building guitar amps together. Thanks for the very clear and entertaining tutorials.
Thanks again for the great video. Your video convinced me to try out a "presence" addition to my champ 5f1. But I didn't have a 10k resistor handy, so I put in a 4.7k to the feedback loop from a 50k pot, the middle pot connection to the speaker jack...and then I put a 0.05 uf cap from the pot to ground (I didn't have a 0.1uf cap handy). I really like the control, and up until a volume at 7, it is wonderful. Past 7, it fattens up, and the control works nicely. After a volume of about 9, the volume doesn't increase any more, but is quite loud!, and has some neat variation in growl with the "presence" controller changing/cleaning the sound nicely. At 11, the with NFB at it lowest, it distorts in a unique way, the peaks are compressed in an exaggerated way, and almost sounds like it's cut out at the beginning of the strum: with NFB at its highest, it cleans up noticeably, but it still has a very compressed sound at the initial moment of strum...All the while, no longer increasing my volume since 9. I find it very interesting, and pleasing, but should I bother putting in that 10k instead of 4.7k, and the 0.1uf instead of the 0.05uf? Your opinion matters to me. Thank you.
You're welcome, Rik. Different circuits require different component values, and if you're as satisfied as you seem to be with the presence control you have fabricated, I would say stick with it. If you're feeling adventuresome some day, you might experiment with different resistors and caps, but it may be that values you had on hand were the exact right values for your circuit. Congratulations on a very successful modification.
Thanks for another clear explanation. I appreciate the way you break down the function of each portion of the circuit during the explanation, including both the electrical consequences and the effect on the sound.
Thanks so much, CNS. I taught school (math and science) for many years, and found that this incremental approach worked best. It's like cutting a steak into small, bite-sized portions.....rather than trying to cram it down your throat in one big piece. Although, I fear that Rusty would probably enjoy either method equally :))
I'm green on building amps but being a mechanic this makes sense to me - looked at the other video to the end and I'm adding it to my amp I'm building. Thank you!!
Excellent video. This is dead on with how my Louis Electric KR12 is affected by each of these controls. Very fun and effective for shaping tones and bloom with each of these pots. Keep up the great work! TB
Good explanation. The presence control is similar to the tone control on most guitars You have a variable pot (250 or 500K) and a capacitor in series (0.01uf, 0,022uf or 0.047uf to rolloff the high frequencies to ground. This circuit with the inductance of the pickup is a LRC (Inductance/resistive/capacitive) frequency filter.
It doesn't function the same at all. The presence varies the ratio of HF that is the feed back loop. You hear LF distortion alot more than HF distortion so allowing the HF to bypass the feed back loop gives you a more harmonic full top end.
I had wondered many times about what is and does presence work. I wasn't even close to guessing what it was. I had trouble thinking it added highs and thought maybe it might oscillate the highs or something.... I never would have guessed it actually suppressed the lower frequencies.... it sure doesn't sound that way to my ears.... Thank you...
Howdy. An addition to my previous comment. Most relevant for Hi-Fi enthusiasts. Because the neg. feedback arrives delayed to the summing point using secondary (global) neg. feedback there is a possibility for blocking distortion using strong neg. feedback, and therefore, strong drive. It may be checked, with an oscilloscope, whether the end tube grids go positive feeding the amp. with a square wave signal. If that happens the driver stage Slew Rate must be reduced. The reduced driver Slew Rate provides time for the feedback to act. Regards.
Thanks Uncle Doug, that was an interesting vid, I have been looking for ways to adjust tone in my circuits without having to include an additional tube for a tonestack. Using caps and pots and NFB's and treble and bass boosts are pretty cool. Cheers
Outstanding Explanation. Love your content! Thanks For your Time and Effort. I am a Component Level Repair trained Electrician, Retired. These explanations are huge as a refresher for skills that I haven't used in decades. I am also a Owner of Three Awesome Vintage Amps from the Sixties and Early Seventies. Looking for a Dual Trace and a Variac. Just for entertainment sake. I always enjoyed Electronic repair. Quite satisfying to properly troubleshoot and repair something. 🏴☠🎸😎
This is the best explanation I've seen for these amp circuits. You're really helping a lot of people that want to better understand their amps. Maybe you could use a oscilloscope to show the signal as it passes through the amp? Show how the filter capacitors shape the signal. Just a suggestion.
Thanks for your nice comments, Ray. Please see my video series on Power Supplies for oscilloscope views of AC ripple, smoothing by filter caps, tremolo signals, etc.
Uncle Doug I'll watch the power supply video. glad you mentioned it. I used my scope a lot servicing electronics. it could be very useful in certain situations. I regret selling my shop equipment. There's times I'd enjoy working on tube equipment again. Watching your videos is a good substitute.
Thanks, good uncle! This is hands down the best explanation of both negative feedback and presence control I've ever seen. I'm seriously considering building this into my 5E3. Switchable of course :-)
You're welcome, Necro. By all means, give it a try. It's a nice, easy-to-install addition to any amp.....and, if you're unimpressed with the results, you can always remove it easily. Let me know how it turns out :)
Thank You for your diligent response..the cap of .1uf is one end to ground the1st & 2nd lug are connected together as in a boost or cut but on the 3rd lug there is a 4.7k resistor connected to the wiper and the other end to ground. The potentiometer is a 5k which is average on Marshalls but this addition started to show up on the later models. The .68uf cap was usually the cap used on the 200 watt Majors (1968) and this is where the ''One Wire Mod'' was introduced, consisting of a 1 Meg Ohm resistor on the Volume1 (Normal Channel) output was rewired into cascading Volume2's (Bright Channel) input resistor or so I believe in theory these respond as a Volume and Gain when plugged into the Bright but if you plug into the Normal only that channel responds breaking the Gain's chain but very Loud and Bold which I use as such and call it the''Loud & Bold Mod'' on the 2 channel amps which is minus the Master!! I rebuilt so many amps in my day and a number of Marshall's but I was not privileged until today to be able to ask anyone or thought to do so. Thanks again Uncle Doug!
Good heavens, Nelson, you definitely know a lot about Marshall tone circuits !! Thanks for sharing all this information with us. I will definitely refer all such inquiries to you in the future.
Guitars and Amps Hola from San Diego Ca, Another fine video UD, here is an old video of my collection, a 66,67 and 72 princeton included.Original tune with my wife on vocals and me on guitar.The collection has grown since, you know, G.A.S.
OK, I just got back from watching the video, and was completely impressed. A bunch of really nice guitars, a wonderful collection of Fender amps, and a terrific background tune. Your wife's voice is better than about 90% of the female singers I hear on the radio, and the guitar playing was excellent.....very professional, with great tone. I wish you lived nearby and could do my amplifier sound checks. I'm not just being polite here....the video was very impressive. Great job!! Thanks for sharing it :))
I appreciate you teaching things like this as I am about to build an amp like this...good to get this information so I won't have to go back and mod ..I would like to install a reverb unit in mine...I have plenty of room in my chassis for tubes caps wiring pots and the extra transformer plus I have room in the cabinet for the spring reverb unit. .again I do appreciate what you do for us
Subscribed! This video was great. It's great how you explain what effect each component etc will have on the tone of an amp rather than just talking electronics like most tutorials. Thank you. I'll watch them all now. :)
As usual uncle doug your vids are very good Without boredom just Very infomative. You teach with excellence. Love Your sarcasm to about all the bad workmanship out there. Thanks again. Between you and D Lab on UTube The differences in thaught on cirquit Designs is interesting Stay well. Hope to see more of your vids in the future I'll be implementing the info into the repair of my old 60s blackface champ amp I baught for 100 $ back in 1996 at a little music store Back when I lived in the state of KY. The man offered it to me so I baught it. Still use it today. I didnt know it was worth 800$ at the time I dont care. I wanted it for practice and other uses like in church so it is now modified for more utility I added a tremolo cirquit And some other things But no modern solid state junk,all tube I like the sound of tube Electronics full warm Tones. See you again On utube Dwight
You make it all so simple and clear. I have a multipurpose Newcomb amp that could possibly benefit from this mod. As always, Thanks! BTW, you've got your 100'th video anniversary just around the corner. I'll have to open a bottle of champagne or at least a cold beer to watch that one :)
Thanks, VN. By all means, experiment with the Newcomb and keep us posted on how it turns out. You're right about the 100th video anniversary coming up soon. When I first started out, it seemed like it took forever to reach 10 videos.....and I never thought I'd ever reach 50 subscribers or 1000 total views, but things are rolling along pretty well now. Like you, I'll be forced to celebrate the milestone. Maybe Rusty and I will split a cold Coors Lite :))
Very informative and a really interesting modification to the NF loop. I've been focusing on your 5F1 scratch build video series. I've manage to build my chassis, although my design will be a 5F2-A Princeton. I will be posting updates on my Facebook page. Thank you for the Harbor Freight tip on the shears! I ended up purchasing the pistol grip pneumatic with a 20% off coupon which really takes you for a ride! Thank you for all you do Doug.
You're welcome, Eric. I built and posted a video on a 5F2-A Princeton several years ago. It has all the tone and charm of a Fender Champ, but with a tone control :) Good luck with your build.....and your new shears. Be careful of the gauge of metal you try to cut. I had to replace my blades a few months ago.
Great video Doug. Subscribed!! Say G'day to Rusty! I had one just like him! (18yrs) Thanks for making even less time in my day now I've discovered your vids! :)
+Gavin Moar You're welcome, Gavin, and thanks for subscribing :) Rusty is 17 years old and still chases the tennis ball for as long as you're willing to throw it. Like me, however, his eyes and ears don't work quite as well as they once did. We hope you enjoy the videos.
Excellent video as always! You do it up with class! I think you're on target with adding a feedback pot on the Twin; you'll have the best of both worlds and have a little more control over the tone...Looks like Rusty didn't make it onto the set in time :)
Thanks, Frank. I think the idea of adding a Resonance Control sounds intriguing also....it was in response to GuitarGeek's comment (below). Rusty was there, but he was busy operating the camera and couldn't appear in most scenes ^. .^
Thanks Doug and Rusty for the video. Always baffled me what a presence control did in reality on a circuit level. Really enjoyed your recent tone control video too. I will post comments there soon. Take care . I think you mentioned you once were a teacher, you still are. What got you into tube amps and electronics.
You're welcome, Mix. It's great to hear that the videos are helpful and appreciated. I started restoring vintage jukeboxes about 35+ years ago and wanted to be able to do all the work myself. This required that I develop the ability to repair the sound systems, which are quite similar to those in vintage guitar amps. Once I developed an interest in vintage guitars and amps, the jukebox-related knowledge was a great foundation. I have posted a few videos of my restored jukeboxes, which may be of interest. Thanks again for your input.....it is always appreciated :)
thank you again, Uncle Doug, this is exacly where I'm at on my fathers APS-4 amp from a Gulbransen 1960 organ.I'm useing the leslie and amp with my guitar. I won't do any thing to the amp just yet, till i learn more!
Walter Stahl Sounds good, Walter. It's nice that you are finding new uses for a "family heirloom". Best of luck with it, and with your modifications, should you choose to make them.
Awesome, very clearly explained and easy to understand. Is this the same general principle by which all presence controls work on all amps? Seems like it would be, why reinvent the wheel? Thanks!
Hi Uncle Doug, in the Bassman 6G6-B , the presence was generated in the tone stack.A 350K pot with an extra 70 K tap. The signal came from the bass pot if I remember well? My blond Basmann is made that way. Kind regards,Joseph
Greetings, Joseph. I checked the schematic and it is as you say, except it appears to originate with the Treble pot in the Normal Channel (only). I would view this as a way to fine tune the treble frequencies closer to midrange. Thanks for your input :)
Awesome again. Would love to see your explanation about the how and why of amp stage coupling whether reistance, capacitance, inductance or a transformer is used. I understand it in a basic way. But why its used when its used in a guitar tube amp not really sure.
+John Miner Thanks, John. This is not a topic with which I am familiar. The inter-stage coupling in the vintage amps I deal with is generally done in a cascading plate-to-grid chain with coupling capacitors to block the preceding tubes plate DC from reaching the following tubes grid. Perhaps a Google search would turn up more information. Good luck.
Inside my amp, I removed the Presence control and replaced it with a another potentiometer (250 Ohms, after few experimentations) which allows me to play with midrange frequencies as well as different other punchy sounds. It also controls the amount of the overall volume. But in my case, it is connected between one of the grids of the Phase Inverter and the 8 Ohms ouput jack.
As long as you are taking suggestions, I think an explanation of American versus British tone and circuits would be cool. Of course Fender versus Marshall. We hear these terms in marketing and blog slang but what does it mean? So nice you are able to reply to most comments. When you get more views than Taylor Swift it may get harder to do.
An interesting suggestion, DS. I must confess that I am completely unfamiliar with Marshall circuit design, but I can download some schematics and try to figure out what the significant differences are (if any). I try to respond to every comment, but (for some reason) YT doesn't alert me to some of them, and it would be a nightmare to have to check through all the comments for every video to try to find them. Meanwhile, Taylor better watch out.....we're catching up with her :))
Uncle Doug DS, The first Marshall amps were copies of the old Fender Bassman, which was never really a bass amp to begin with. Years back I got to hear what I thought was an old Bassman. The owner of the amp, a guitarist in an excellent ZZ Top cover band explained that it was an original tweed covered Marshall (Yes, Tweed!). I can't imagine what that beauty is worth now, but man could she sing! It would be cool if UD could find some secret to the British tone but I would think, in the end, the choice of tubes (EL's vs. 6-6's) the extra gain stages plus possibly less negative feedback along with closed back 4X12" cabs all contributed to the British vs. American sound.
@@UncleDoug I'm amazed at how the channel 2 sounds now on that one, great job indeed. And to adress the options, i'm adding the presence control, but without control, as it will go as a resistor and a cap, since i'm not into making more holes to the chassis. Or well, that's the idea, as soon as i find the right values for the tone to be good at both, low and maximun volume.
Thanks Uncle Doug! I always thought that I was creating a "presence" control that was just pegged in perpetuity when I DC'ed the NFB loop of an old Hammond (AO-44, AO-35, AO-fill-in-the-blank), but since an actual presence control is only rolling off very high frequencies from the loop with a high value cap, I suppose I'm sort of making a presence/resonance/gain/boost control that is, of course, maxed out! My AO-44 is like that, having pulled the input transistor and the low-pass filter stuff it was driving, I also pulled the NFB loop. I use it mostly as an output section behind my Frenzel (Jim Frenzel is a guy in Texas who builds and sells a lot of good PTP-wired stuff at really good prices, has an ebay store and frenzeltubeamps.com) preamp, or sometimes I use it for bass, with a little compression up front, and drive a single 15" black widow in a dead 1x15 solid state peavey combo. It works really well for both, and if I plug a guitar straight in, it also does an incredible almost-clean that I like for reggae-plucking with four fingers and then digging in with a thumb-pick. I am completely in control of the dynamics without touching the volume knob on my guitar. I do believe I will be subscribing now, this is a class I should definitely be taking.=)
Wow....thanks for all the info, Adam. It sounds like you are a capable ampliophile, who definitely should subscribe. I will check out the Frenzel Tube Amp site. Best of luck :)
Uncle Doug Done.....actually, I think I subscribed right when I was making that comment. And YES you should DEFINITELY check out Jim Frenzel's stuff. EVERYONE READING THIS SHOULD. He's often "backlogged" so it took longer than it was supposed to for my preamp to arrive, but a pair of 12ax7's in a pretty classic bassman/plexi (the preamp section of bassman or plexi, obviously) type circuit, point to point, gain+MV and 3 band eq, and I paid a discounted price of 200 bucks, but I think it's normally 275.....and that is obscenely cheap for what it is, I think. I've been looking around the web lately for more stuff to do to my "Yayo-FourtyFour" (nickname I gave it) lately, and I'm wondering if you know anything about the way an Orange "shape" control works, and if it would be possible to put one in an AO-44 that is basically stock, just with the input hot going straight to the grid of the first ECL86. I mean,m there's only two preamp stages, one's a driver, but I really only use it as a clean amp anyway, for separate preamps or on the extremely rare occasions that I want to get the sound of a particular pedal. I intend to try to build a 3x12ax7 high-gain overdrive with just gain and a trimpot on the output, and if the 44 had a really versatile tone control like the "shape" control on the Rockerverb and the Dark Terror, it would be perfect. . . . . Any ideas?
Adam, I looked at the Orange Dark Terror "Shape" control and it appears to be fairly straightforward, utilizing capacitors and resistance (like all passive tone controls) to allow selection ranging from "sharp" to "mellow". It resides between V2B and V3B of the circuit. If you wanted to add this tone control to the AO44, simply recreate the circuitry and insert it in the AO44 circuit, between the two stages of the 6GW8. How well it will work, I don't know, but like all experiments, you can simply try it and see. Let me know how it turns out.
Excellent explanation!! So clear, I can probably guess how a Resonance control works. Just like Presence in the feedback circuit, only with a capacitor that filters low frequencies?
These videos are wonderful, Uncle Doug. Can I call you Uncle Doug? It seems a bit familial and whatnot. Anyhoo, these videos are great. I'm a software guy. I've built an LA-2A and 1176 and some pre-amps. But I was just being a solder-monkey and following instructions. I've watched lots of videos on electronics, but how many videos can you watch about Ohm's Law? I get it, Ohms Law, for the hundredth time. But there's so little in between that and actual useful discussions. Your videos fall at just the right spot for someone like me. Something I always wondered about, but it never seemed to be explained, is why there are all these places where there's a resistor to ground. I always wondered what the point of that was. In this video, you note that basically you are shunting particular frequencies to ground by adding a cap. That sort of lit a bulb for me. Are basically all of those places where there's just a resistor to ground essentially 'relief valves'? I.e. do they just insure that any voltage above a certain level has a way out of the circuit before hitting the next component(s)?
I have posted videos on the specific purposes of resistors throughout the amp circuit, Dean. The purposes vary from location to location, i.e. tone, current, and voltage modification.
Oh, OK, I missed those. I'll take a look at them. I've honestly learned more in the last week or so watching one of your videos every night as I go to sleep than I have in all of the previous time I've spent on this. One question I had, and I'd post on the appropriate video if I could remember which one it was, is the issue of DC and AC being able to flow on the same wire. When that happens, is it sort of like a carrier frequency in radio, where the DC effectively just offsets the AC by its own amplitude and so the AC is no longer cycling around the zero point? If that's true, do things like transformers not really care where the crossing point is, as long as it's oscillating? Or, if it's true, are there places where that offset would matter? If you have a video on that of course just point me to it. There are a lot of them, so it's not always obvious which ones to look at for specific things like that. The other, stupendously obvious but overlooked by me, thing that I sort of realized (hopefully it was a correct realization) is that A/C doesn't 'flow' but DC does, right? A/C doesn't move from one place to another through the circuit, it just oscillates back and forth, while DC is positive or negative and does flow from place to another. Is that basically the correct way to look at that?
Yes, you can think of AC as waves of oil floating on top of smooth water (DC).....but they never mix. You're analysis of the difference between AC and DC flow is technically correct, but for most beginners and intermediate students of electrical circuits, it's probably easier to simply have the AC flow through the circuit, happily creating high voltage and warming filaments with low voltages.....so we generally don't belabor the point ;)
So transformers really don't care where the crossover point of an A/C signal is, just the amplitude of its oscillations, I would guess? Or do you have to remove that dc offset from the signal before it hits a transformer?
you say at 1:25 that the cathode and grid are out of phase. i thought that a signal taken off the cathode was in phase (non-inverted) with respect to the signal on the grid...just like a cathode follower. i believe it's the signal taken from the secondary of the output transformer that's out of phase with the signal on the cathode of the second stage.
You are correct, Mr. B. Sometimes in my quest for simplicity, I leave out some details. In this case, it is the signal sent to the cathode by the OPT that is out of phase with the grid, not the cathode itself. This is why, if you connect your NFB loop to the wrong OPT lead (i.e. the one that is in phase with the grid), the loop becomes a positive feedback loop and the amp will oscillate loudly. Rusty and I thank you for your input and will add a note to the video correcting this misconception.
Uncle Doug hi uncle doug, you're absolutely right about using the wrong OPT lead and oscillation....that's the difference between positive and negative feedback (regenerative vs degenerative).
MisterB4x Thanks again, Mr. B. I, like lots of amp builders (I assume) have suffered with the deafening consequences of attaching our NFB loop to the wrong OPT lead. It's always a 'shock' to discover that your precious, home-built masterpiece is functioning more like a police siren :))
Uncle Doug I nearly always had been (and will be) fooled with the "begin/end" dots of the transformer´s winding schematics. So it´s way faster to just solder the leads to the transformer-taps (as nearly all europe made transformers got no leads, as they got some taps on the transformer instead) and see (=hear) if my guess was right ;)
Nice video. You should maybe have said a word or two about how different amps have different NFB. Some amp are, lets say so, stable that their tone wont break up even with the NFB totally snipped. To get a nice range on the NFB a big potentiometer can be used. With a pot of 1M or so a range can be found. With a measurement of the interesting resistances, low level NFB and high level NFB, a resistor and a smaller pot can be used to control the NFB. Well... this is how usually tinker with the NFB loop of amps I come across.
I wonder, did Leo arrive at 820ohm for many of Fender amps Neg feedback resistor by ear or did he rely on a formula of sorts? I have been playing with different values but havent settled on anything. If the effect of changing the resistor is eq I would guess it could never be ideal for every guitar as guitars seem to have different natural peaks and valleys at certain frequencies.
Thanks Doug for doing this video. I found it informative, concise and to-the-point. It was well explained. I am wondering if you could say something about the "resonance control" and how it could be mixed with the presence control circuitry you described in your video. I could not find any schematics on the topic...
Greetings, Julian, and thanks for the kind words. In the video description I offer the following note: To turn the NFB Loop into a Resonance or Depth Control, add a .0047uf capacitor in series with the NFB resistor. To make it variable, add a 1Meg pot in parallel with the capacitor with one leg of cap. connected to the center (wiper) lug of the pot and the other leg connected to either of the (outer) lugs of the pot. For this to function, the Presence Control (if you had one) would have to be "off" so no high frequencies were being sent to ground. Here is a schematic: www.regiscoyne.com/tech/resonance/
Uncle Doug Thank you. I could not follow the explanation straightaway but it has become clear now that I have seen the schematic. I still don't understand why you can't have a presence control and a resonance control together working as a sort of tone stack circuit if that makes sense...
There probably is a circuit that would allow both to function, Julian. Perhaps a search would turn one up. The Presence control functions in between the Midrange and Treble control and the Resonance functions in between the Midrange and Bass controls. From what I understand, the effect of these additional controls is most evident at high volume levels.
hi i appreciate yours vidéos and you speak very clearly and youre a good teacher for a french like me ,id like to know how about what voltage of power supply and plate voltage on tubes and cathode voltage cause shematic dont mention nothing about this 5f1 .thanks
Thanks, Kubock. Simply buy a power transformer designed for the 5F1 circuit, follow the schematic carefully, and your voltages will most likely be within acceptable parameters for each tube. Good luck.
Very cool. I know this is an old video but I wonder if you could change the presence control to cancel some of the highs in the negative feed back. Instead of running a capacitor to ground run a resistor to ground after your negative feed back potentiometer? Very interesting stuff.
This is a great topic in addition some years back I was 1st introduced to the idea of the NFB as the would be designers aggressively sought out the overall purpose of the NFB and what and how to if not just alter for sensitivity per other schemes existing but how to achieve possibly a 60s Marshall/Park Plexi amp performance. After reading countless lectures and getting the understanding I noticed there were some presence controls with an added resistor to the .1uf capacitor and if I'm not mistaken up to .68uf was used!!. Is this resistor a form of a (?) preset in filtering along with the 5k-10k presence pot? I also settled in on a cascading use known as a 1 resistor mod using a 1megohm resistor on a '73 Model 1987 Marshall #1 by my first recollections and immediately noticed it sounded and responded just like the term used as a ''Brown Sound'' made popular by the designers of the Van Halen mods! On another I did bypass the NFB resistor altogether in one instance and it brought the Marshall amp#2 to a whole new level and I even removed the .68uf cathode capacitor and the .0022uf bright capacitor on the tonestack's 2nd stage tube which increased the preamp's character to immense proportions.Incredibly powerful. But as my question stands Please if you will the added resistor on the presence control knob it's not just there to alter the capacitor's performance so what is it's due purpose??!! Thank you!
Greetings, J56. I am not familiar with the circuit you are describing, but one of our viewers definitely is. Please see the detailed response from Nelson DeJesus, which should be posted nearby (probably above). I never cease to be amazed by the knowledge and experience of the folks out there in UA-cam Land :)
A musician friend asked me to convert his Great Grandmother's early 1940s Westinghouse tube console radio to a guitar amp. I tried the "It's easy to use an old tube radio as a guitar amp" route. The radio was not working, so I trouble shot it, recapped it and replaced three defective tubes. With the radio now working, I wasted a bunch of time trying to use the "phonograph" input for guitar. After much trial and error I came to the conclusion that the idea is largely bunk. Instead, I gutted the chassis, saved only the new can capacitor which was a 20/10/10 @450V and told my friend I would build him a Tweed Champ circuit in the chassis, retaining the same knobs and controls arrangement, with the dial lamps as pilot lamps. He picked out a new 8 inch 4 ohm speaker and away I went on my first amp build. I decided to include a tone control and when looking at late 50s schematics, I realized I was building a 5F2 Princeton! I referred to a number of your videos for instruction and decided to include a Negative Feedback control as I had a spare knob available on the radio panel. I fired up the amp yesterday, corrected a couple of my mistakes and tried it out. Whoa, that thing sounds just like it rolled right out of 1956. The NFB control was the most amazing thing about the whole project. With the control, and a Stratocaster, I can seemingly vary the gain from pretty clean (6K resistance) to very pleasing overdrive at 56K resistance. With a humbucker guitar, I can get over the top overdrive! The tone also changes subtly. I am really happy with the amp and am excited to let my friend (who is a professional musician) try it out. Without all the help from your videos, this project would have been a non-starter. I am pleased to show my gratitude with a Patreon subscription. Thanks again Uncle Doug and Rusty! Scott President, CEO and Janitor at Scotty's Workshop.
Wow, Scott, this sounds like a great build. All of your modifications make good sense and I'm glad the NFB control worked so well for you. To me, the NFB is actually much more useful and effective than a tone control (which is actually redundant with the guitar tone control). You're welcome for whatever assistance our videos provided. We commend you on this very successful project and sincerely appreciate your sharing it with us.
Thank you for your very insightful videos! How come you don't you need a diode on the negative feedback loop to make sure that the signal goes FROM the output transformer TO the cathode instead of the other way around?
You're welcome, Dj. The secondary of the OPT has higher voltage and much higher current (multiple amps), capable of driving a speaker.....while the cathode has very low voltage and extremely low current (milliamps). Thus the OPT signal, being more powerful, flows toward the cathode, rather than the other way around.
As always great video. you lost me at the end. but at least i have something to consider.(voltage spliter in a NBL) i'm new to this and i'll figure it out. thanks for the homework.
I will be posting a Phase Inverter video in the near future. Re a "solid state" rectifier, you simply use diodes instead of the two rectifier tube plates. Since they only conduct in one direction, they will divide the AC into a positive (B+) and a negative (to ground) Direct Current. It's a cheaper and more efficient way to get the DC your amp circuit requires.
I just did this mod to my monoprice 5 watt amp and it gives it a good amount more gain. People have done this before but you gave me the idea to put a resistor in line so you don't eliminate the negative feedback loop totally. I am thinking about lowering the value of the resistor and see how it does.
Theoretically, you could have a whole tone stack integrated into the NFB loop, correct? Would that yield a dramatic effect, or would it be something that falls under the Law of Diminishing Returns?
+Connor Nepomuceno I would think that an NFB loop could be modified to provide tone control, Connor. I have no idea how dramatic it would be, however. Perhaps you should experiment with the concept and let us know what you discover.
Say I wanted to add a Presence control to a AB763 circuit. The NFB voltage divider uses awkwardly small values (820R/47R)... Would adding a typical Presence circuit work properly? By typical circuit, I mean a 25k pot + 100n cap in parallel with the 47R resistor. Or, should I increase the voltage divider values to something like 82k/4.7k and adjust the PI tail resistor to compensate? Hope my question makes sense. Glad to see you're making regular videos again! Cheers :-)
In my experience, the only way to effectively add such a modification is by experimentation. Test your theories and see what works. Today I altered the tone characteristics of a mid-60's Gibson GA15RVT from shrill and grating to warm and pleasant.....by experimentation with different capacitors in different locations in the signal chain. I suggest that you try the same method with your project. Good luck.
Another gem, Uncle! Is the negative feedback source from the OT secondary out of phase with the signal at the driver triode grid because the signal from the 6V6 to the OT is out of phase with the driver triode (each triode inverts phase)?
Thanks, Jeff. You can count the phase reversals as the signal passes through each stage of amplification, allowing you to see why the NFB signal is sent where it is.
Hi Uncle Doug, as always thank you for the great explanation and everything you do to explain things in detail. I don't know if this has been addressed yet, but I am a little perplexed. If you have a NFB resistor value of 2700 ohms or 2.7k like on an AA764, and you change the value to 10K in series with a 50k pot. How does that go below the original value when it is already 3-4 times higher on the lowest setting? Am I missing something? I would think that a 1k or 1.5k resistor in series with the 50k pot would allow adjustment above and below the original values. I am just wondering, since I am in the process of building my first amp, an AA764 with both NFB and a presence control. Any response would be greatly appreciated 🙏 Take care!
Greetings, Brad. Naturally, you tailor the NFB values to suit the circuit. In the case of a AA764 PR, you would use values like those you suggested, so you could adjust from much lower to much higher than the regular NFB resistor.
Hi Uncle Doug: Thank you for your reply today. I built a mojotone 5e3 deluxe and it came out great. I do find it breaks up a little too much. I'm trying to decifer what pin of the 12ax7, pin 7?, of tube to come off of with a resistor to create the negative feed back loop. I also assume the other end goes to the negative speaker terminal. I have a photo of my chassis, how can I send it along for your reference. Thanks again, Bob.
+starlingave You're welcome, Bob. There is a wealth of information on this subject on the Internet, i.e. www.tdpri.com/forum/amp-central-station/69-why-doesnt-tweed-deluxe-have-negative-feedback-loop.html It appears that the most common way to install the NFB is a 22K resistor between the speaker jack and the cathode of V1b. Good luck.
Hi Uncle Doug, I have a silver faced 1972 Twin Reverb, which I have black faced to Ab763 circuit. I had added a 25 k pot to see how much resistance I liked and it came out to about 15k on the pot by itself in series with the 820 ohm existing resistor. Question 1, should I just put in. 15k resister to give me the amount of reduced negative feedback, 2, I would like to get the bass up a little, like a resonance control, should I use the suggested .0047uf cap in series with the 15k resistor. 3, what frequency blocking roughly would I get with that cap, thanks again for your very informative videos, Bob
+starlingave I tend to like the flexibility of an adjustable NFB, Bob, but you could simply replace the pot with a fixed resistance if you wished. Please see the formulas in my Tone Stack video to calculate the frequency cutoff with that cap/resistor combo......or you could do what I do and simply try it and see how it sounds. Good luck.
This is a good tinker mod in between projects.Since it’s reversible, I was thinking about adding a NF to my MOD 102+ kit (nice little 8watt champ-like amp with an el84 in v2) just to see the effect. I don’t think it’ll improve much but maybe it’ll relieve some of the hum the amp has at high volumes when I use a long plate 12ax7 in V1. A vintage Sylvania long plate 12au7 cleans it up and smooths out distortion nicely but the gain goes down.I’m just curious more than anything. P.S. If u ever decide to use sponsors, ‘Great Courses Plus’ has a very practical electronics course. I think you’d approve! Can’t beat the value at $20 a month for 24 lectures. That also includes about a million other subjects. Idk how sponsors work on UA-cam but I just thought they’d be a good fit for your channel.
Thanks for the ad suggestion, William. I'll keep it in mind. You can make the NFB loop either switchable on/off, or variable with a pot. See my Homebuilt Champ from Spare Parts videos to see how. It does make a big difference.
@@UncleDoug I added the NF to the 102+ MOD kit. It did make a big difference. Mainly in volume. More of an off/on thing than adjustable. So I replaced the pot with a switch. The amp is based on a champ but has some differences in output tube (el84) and push/pull pots for extra tone options on bass&treble and the volume too. It seemed to work better going into pin 8 rather than 3. It really had a lot of feedback with the pot dimed going into 3. It IS pretty handy with it on as the amp has great overdrive but it’s too loud to take advantage of in home settings/recording. Is there any great problem with #8 cathode pin? Seems fine. Day later..figured out the pot wasn’t getting grounded in the chassis hole I cut(tad too large) fixed that and feedback stopped on pin three but still better results on 8.I think the tone controls must be the reason? A 4.7k resistor seems to elicit more of a result in this particular amp,as well. I’m still figuring this stuff out ;-). Thanx.
thx Uncle Doug! i watch your videos again and again as my knowledge of tube-amps increases over time :-). 2 questions : is it wise to use a bypass cap (lets say a low value to enhance the treble on the plate output) in combination with a precense control in the nfb loop? or is this a not common thing to do (using a bypass cap in the cathode in combination with a presence control)?
You're welcome, Stefan. First, I believe that bypass caps in traditional values, like 10 to 50uf, enhance all frequencies.....not just treble. Second, I see no reason why both bypass caps and NFB/Presence Controls cannot happily coexist within the same circuit.
great video. also love that you include your dog. I love 4 things in life. God my family rescuing dogs building amp and pedals. plus my 5th love smoking weed but I do that while doing the other 4 haha. great videos though, I love that you include your dog.
Uncle Doug, I am grateful for the knowledge you share. I perform in situations often with an orchestra where I need a clean amp, then the next day it can be classic rock and blues. I've decided to try a Tweed Super amp for both purposes. So, I believe the variable negative feedback idea may be of value to me. On this amp (5F4) the resistor in the feedback circuit is 56k. in your video, you stated that you were going for a range of about 50% to 300% of the 22k resistance in your Champ amp. So, would I perhaps use something like a 25k resistor and a 125k pot? I figure this would bring my range to 25k to 150K roughly maintaining the 50% to 300% ratio. Could I try a 250K pot? (much easier to get a matching alpha 250k pot) I've been a professional musician for over 40 years now, I guess I'm trying to recreate that magic I felt when I first cranked that tiny tube amp back in the late 60'S.... :)
+Jason Goldsack Sadly, I have never personally encountered a Traynor amp, Jason, but would love to have the opportunity some day. I understand that they are quite nice amps.
I will have to send you some pictures and see if I can send you a schematic. BTW. I would live a video on how a tremolo circuit and the circuit that sends a signal through a spring reverb. I have both on my amp.
I have already posted two videos on Tremolo and one on Reverb. The Reverb video is on another channel (I had not started mine yet). Do a Google Search for "Fender 6G15 Clone" on the ElPaso TubeAmps channel.
Excellent video, as always! Question: why is the signal for negative feedback typically (always?) taken from an OT tap (as opposed to a pre-OT signal, for example, the connection between the anode of the 6v6 and the OT)?
excellent, thank you very much 👌🏼🙏🏼 It could both be added as modifications (variable NFB and presence ctl.) to an old Champ, later Champ/VibroChamp or even "recent" Champion 600. Nice, interesting and enlightening.
Thanks again for another wonderful video Uncle Doug! This was a topic that was like voodoo magic to me. You've made it understandable even for a non techy person. Cheers!
@@UncleDoug Dear Sir, I have a Blues Jr III that I converted from PC board to turret board using a conversion kit. There are several things that I would like to address on it. The volume/gain control gets loud/distorted very quickly rather than gradually, the master volume gets loud very quickly, and I'd like to get a bit more highs. Would you be willing to, for a fee, have a look at my schematic and provide any recommendations to adress these issues? I know you probably get inundated with requests and have your plate full of stuff you're working on now but better I ask and you say no than not ask at all. If I had someone near me I thought I could ask for help I would. Thanks again for all your wonderful videos and thank Rusty for his contributions 😁. Cheers!
@@romannumeral5547 Greetings, Roman. I am currently taking a break from amp repair and have never done personal consultations.......so I'm really not the guy for this project. You might consider reducing the values of the volume and master volume pots if loudness comes too soon. Have you tried cranking up the treble on the guitar tone pot? Best of luck with your tuneup of the BJ.
@@UncleDoug Thank you for your reply Uncle Doug. No problem, you don't know if you don't ask. I've got an audio taper pot I plan to use to replace my linear master volume pot so that might help. This amp has a lot of gain so I may try a 12AY7 in place of the 12AX7 in V1 position. Thanks again and take care.
Great video... quick question. I f I understand correctly the effect of the "presence control" will be affected by the amount of negative feedback used? Would it make sense to have two pots? Or would that make too many variables in the circuit?
Thanks, J-F. One pot will do. Yes, the frequency range and amount of out-of-phase signal returned to the cathode of a pre-amp tube will determine which signals are suppressed.
I'm building a Princeton Reverb amp from scratch. The kit provides a 2.7K NFB resistor. I assume applying your formula of a resistor of around 1/2 the original resistance and a pot that provides a maximum of 3X the resistance would result in a 1K resistor and a 10K pot. That combo would produce resistance from 1K to 11K. Does that sound like a good plan? I want a clean sound but not a lot of breakup. Thanks for getting me thinking about varying the NFB.
I think a great video idea would be going over a mystery to me ( and hopefully others) and that is how guitar signal and electricity are combined in a tube in both preamp (triode) and power amp examples. as a layman "paint by number" builder of pedals, it's a mystery as to how this occurs. complicating matters is new knowledge from your vids- that amps use both rectified and AC voltage to achieve this. confusing. sorry this paragraph is the best way I could think of asking short of hitting you with about 20 pm's.
You're idea is excellent, Mel: An overall idea of how amps work. We start out with an analog input....the wiggling of the guitar strings....and end up with a (much greater) analog output....the wiggling of the speaker cone....BUT there are all sorts of mysterious, invisible electrical events going on in between to make it happen. Rusty and I will start putting together a video which (attempts to) explain those "mysterious" events. Thanks for a great suggestion !!
Thaaaanks for sharing this. So, can I just cut the NFL cable inside my 57'Champ Reissue to test how it sounds with all those lost frequencies otherwise cancel led safely? My training it's been changing a couple of pick ups so far but I'd use rubber gloves, plastic clamps etc to manipulate the innards of the Champ circuit... Massive thanks for this informative video!!
You're welcome, Ramon. You are wise to take precautions when working on your (potentially lethal) amp chassis. Be sure to drain the filter caps before touching the circuit (see my biasing videos for instructions if you don't know how to do this). Yes, you could disconnect the NFB loop in your '57 Champ (R12 @ 22K), but I don't think you'll like it. This amp is designed to break up quite early, especially with humbuckers, and the lack of a NFB loop will probably make the tone excessively "dark", distorted, and noisy. If you do it, let me know how it turns out. Here is a link to the schematic and layout: support.fender.com/schematics/guitar_amplifiers/57_Champ_schematic_Rev-B.pdf
Thanks for the response. Sadly as things turned out, I had to sell it. But I hope I'll get a replacement next year. I'll stick to my handwired VOX AC-15 for now. Best regards!
hi if we measure negative feedback for the result ,where you put your scope is it at the input of the grid of output tube or on the output transformer, so its makes a difference,this is less good at the output transformer .thanks
I have never used a scope for this purpose, KF, simply because it's easier and more effective to tune the circuit by ear. Use a recorded input (or have someone play) into the amp and experiment with different pots, caps, resistors, and settings to achieve the desired tone effects.
hi, nice video, thanks alot. I was wondering if the negative feedback can be connected this way in a amp that has a long tail pair phase invertor.I mean, connecting it on the second triode's cathote, before the signal goes to the phase invertor tube.
NFB loops are often used with LTP phase inverters, but the return line is connected to the cathode and second grid of the PI tube itself. Here is a diagram: robrobinette.com/images/Guitar/Deluxe_Models/6G3_Schematic_Signal_Path.gif
but would it change something to go against the standard and connect the negative feedback to the cathode of the triode that is just before the long tail plate phase invertor.(assuming this triode is not a cathode follower and that there is no capacitor to ground on the cathode ). just like the second triode in the fender pro junior.
You could try it and see......but if it worked well in that configuration, I would imagine that Fender and other amp builders would have wired it that way. Also, recall that the NFB signal has to be fed to a cathode in which the signal is out of phase with it.
Wow. In 9 minutes, you completely explained the function and basis of the Fender Presence control. I have been twisting that knob for a long time, and could tell that it brightened the sound, but now I understand why and therefore, how to use it better. I know that these are now very old videos, but thank you for taking the time to make them and for your incredibly clear explanations.
You're quite welcome, Jon. Glad to hear that the video was helpful :)
Once again Uncle Doug has led me out of darkness into the “presence" of the….anyway, thank you, and keep ‘em coming!
You're welcome, GB :)
You and me both!
I forgot to mention, your videos remind me of the training I use to get when I spent a lot up at McIntosh Laboratory in Binghamton New York. I would go up there quite a bit over a 25 year period and learn from the engineers up there. One of my favorite teachers, was my uncle Gordon J Gow, the president and co founder of McIntosh back in 1949. Thanks again, Bob
+starlingave Good grief, Bob, any comparison to engineers at Macintosh is high praise indeed. I am not worthy.....but thanks just the same :)
Another great video and a great mod! Once in a while we see people who say to completely remove the NFB loop, and that is not a good idea. At low gain levels, it's not that apparent, but when you start pushing an amp, you can quickly turn the amp into mud. Low frequency overload. Removing the NFB loop makes an amp sound terrible and it's counterproductive. Leo used NFB in order to control the gain so that we could hear all that wonderful chime. I've been using an adjustable NFB for quite some time after I came to appreciate its purpose. I typically use a 3k resistor with a b50k linear pot. With this setup you are able to carefully dial in some great breakup. I was pleased to see that you use it, too! I also add the Presence Control. Try auditioning a few different cap values or use a switch to select from several different values so that you can dial in the low frequency control. This really comes into play if you're a Blues player. In a Triode, the electrons flow from the Grid to the Plate and from the Cathode to the Plate. When you use NFB, the triode acts like a mixer. The grid contains your guitar signal and the cathode "mixes" a low amplitude guitar signal from the output transformer; just like you said, the cathode signal subtracts, (not adds) the opposite of amplifies, the incoming signal from the grid. The Plate in the triode sends this mixed tamed signal on to the power tube(s). NFB subdues the signal in that triode. The neat part about the Presence Control is that you can subdue the lower frequencies, that causes farting and blocking distortion, which sounds terrible, and allow the (lower amplitude) higher treble frequencies to run wild. Again, selecting from various Presence capacitors to dial in the low frequency cutoff is recommended. Furthermore, guitar pickups will reach all the way down into an good tube amp. Use a higher value presence capacitor for a humbucker equipped guitar - 0.1uf, 0.047, because they produce more low end harmonics. Use a lower value capacitor for a single coil equipped guitar - 0.047, 0.022, 0.01 because their low frequency output is lower. Adding a 3 or 4 pole rotary switch that allows you to select the Presence capacitor is a great mod. Try it and while you're strumming a chord, select different cap values. I think you'll be pleasantly surprised. Thank you Doug for providing us a forum to share our experiences! We are all blessed to have you around...
You're welcome, Mark, and thanks for your very detailed and informative review of the applications and values of NFB. We are fortunate to have a contributor of your knowledge and experience.
Rotary switch, great idea. Thanks!
Uncle Doug I added the variable NFB loop to my 69 Princeton. It has really opened up my amp and made it so much more versatile. Before the modification, to get the sound I wanted, I was either too loud for the audience in the front row or (if I was in a small room) playing so quite I couldn't hear my amp on stage. Now, after the mod, I can adjust the amps volume to the level I need, then adjust the NFB. I also play with a pedalboard and they sound even better now. Thank you Uncle Doug for another great video
Greetings, Donny. I'm glad to hear that this modification worked so well for you. It really makes these videos worth producing when viewers (like you) apply the information in useful ways. Thanks so much for your input and kind words. Best of luck :)
Thanks Doug. I finally know what the presence control actually does! I have heard countless explanations over the years but none of them imparted any actual knowledge.
I'm glad our video was helpful and informative, Joe :)
Doug. I've watched SO many of your videos in the last days, and I must say: You Sir, are the single most effective teacher or professor I've ever had. And I've got two degrees in Education, so I know a thing or two of what I'm talking about! -Jonah H.
Thanks so much for your very nice comment, Jonah. It is particularly meaningful considering your background :)
You´ve nailed it again! Thanx for another well explained basic Amp-lesson :)
It is so easy to follow, as you are always make clear explanations on how things work!
If you go on like this, some day every viewer could be able to do his/her very own Amp just because you have been a good teacher!
Thanks so much, TC. I really appreciate your kind appraisal. Rusty and I thrive on praise.....and a nice, cold beer every once in a while :))) ^. .^
Finally I can start to understand how the NFB works.
Thanks. Great videos I have learned a lot.
Thanks Uncle Doug,
I followed your instructions and what do you know,, it works.. A 5k ohm in series with the 20k pot and now I have an adjustable negative feedback loop..
Thank again
Scott
You're welcome, Scott. I'm glad it worked for you.
Hi Uncle Doug. Once again I find your videos useful. Had missed this point about a presence control with a capacitor to the variable negative feedback (which I allready have). Just tested a 0.1uF in there (a 5f1) to get a sense of it, and yes it tightened up bass and got me intrugued. So this will be implemented. I always scroll the comments section of your videos to see if some people have some bright ideas or experiences and often there are. Gonna do more testings. Autumn Greets. Jon
Thanks, Jon. We're glad the videos are useful :)
oh, btw, the cap I tested was 1uF. so that is why bass seemed tighter i recon, will see more when I test it with different values and a pot. Have fun with your projects.
Although i am not a "tech," i truly enjoy your videos. I play and like vintage amps especially fender. I own a 63 super bf and a 64 tuxedo bassman as well as many others. I started watching your videos several months ago and admire how you walk us through the finer points and explanations. This has opened up a new interest for me in discovering and enjoying the vintage technology. When i meet a fellow gearhead there is now so much more information to parlay. Kind of like "car guys" i guess. Looking forward to more videos and again, thank you for your channel. "Toh"
Thanks for sharing this with us, Ivan. We're glad the videos have been helpful.
Uncle Doug- I am a recent subscriber. You do an excellent job explaining how things work. I look forward to more of your instructional videos.
Welcome aboard, Mick. Rusty and I really appreciate your kind words. Please check out all the instructional videos I have already posted (there are a bunch), and hopefully some will be of interest. Meanwhile, we'll get back to work, planning the next one :)) ^. .^
Bah, Uncle Doug! I've just buttoned my Princeton back up having installed the variable bias pot and added a 1M resistor to the tremolo circuit (best mod ever!), and now I'm going to have to open her up again to do this!
As with all the other people who have commented, I've found your videos invaluable. My amp is an A1164 kit from Tube Amp Doctor and shortly after building it with no confidence about what was going on inside she blew her power transformer. This forced me to learn what was going on inside the amp so that I could diagnose the issue. (I think it was ultimately caused by a combination of excessive plate current and voltage, and using the 230v tap instead of the 240v tap. In Australia 230v should be fine buy my mains varies from 230v to almost 250v.)
Thanks to your videos I know feel way more confident about the amp I built :)
You're welcome, Tom. We're glad to hear that the videos are helpful. Best of luck with all your projects.
Hi Uncle Doug, today I installed a 10k pot as suggested in one of your earlier posts below (given that the standard negative feedback resistor is only 2k7 in the Priceton). It does make the amp go into overdrive more readily from a volume of 6 upwards, but has little-to-no effect below that. Is this because 10k is not a high enough value maybe, or does the effect only really take place when the amp is cookin'?
Greetings Uncle Doug!
First, thank you very much for your series of videos that have helped many musicians and electronics aficionados understand how this beautiful gift of science sweets our lives!
I have a PRS MT15 tube amp, a modern amp but like you’ve said “they all work on the same principles”.
Preamp works as follows:
V1 - input 12ax7
-> to
V2 - lead gain 12ax7
V3 - lead gain 12ax7
OR
V4 - clean gain 12ax7
-> to
V6 - effects loop 12ax7 (always on)
-> to Power Amp
The power amp It has a match pair of 6L6WGC tubes with a 12ax7 phase inverter, and corresponding Presence potentiometer. Also a switch that controls the output with 15 or 7 Watt options.
Output transformer has an 8 Ohm and 16 Ohm option.
The problem is:
While amp is powered on and standby off, a subtle noise is present at the speaker (30 watt 8 ohm 10 inch) with a 50 Hz frequency (Germany : 240 AC 50 Hz). Manipulating the Presence knob affects the hum sound, while still at standby off.
Your experience and vast knowledge is requested:
Is this something common in tube amps? If not, could you point me out to a circuit phase where I could test and possibly find an issue?
In advance, thank you very much!
Best regards,
Gus
P.S. Clean channel is very low compared to Lead channel (which volume potentiometer seems useless to control such beast).
I wish I could magically solve the problem, Gus, but there are many, many possible causes for 50/60Hz hum. You could try moving the wires around with a wooden stick, or try switching tubes. If the volume control doesn't affect the hum, then its source is after the volume control in the circuit. Good luck.
Thank you very much Uncle Doug and Rusty!
I’ve gone through the cable test, if you’ve got time you may like to go through the videos:
ua-cam.com/video/trlbdxFL9gU/v-deo.html
ua-cam.com/video/sCR8a8G0rF8/v-deo.html
I love your videos. My 16 year old son and I watch them together. We have been building guitar amps together. Thanks for the very clear and entertaining tutorials.
+Dave Mclaughlin You're welcome, Dave. It's great that you and your son can enjoy this hobby together. Rusty and I send you both our best wishes.
Thanks again for the great video. Your video convinced me to try out a "presence" addition to my champ 5f1. But I didn't have a 10k resistor handy, so I put in a 4.7k to the feedback loop from a 50k pot, the middle pot connection to the speaker jack...and then I put a 0.05 uf cap from the pot to ground (I didn't have a 0.1uf cap handy). I really like the control, and up until a volume at 7, it is wonderful. Past 7, it fattens up, and the control works nicely. After a volume of about 9, the volume doesn't increase any more, but is quite loud!, and has some neat variation in growl with the "presence" controller changing/cleaning the sound nicely. At 11, the with NFB at it lowest, it distorts in a unique way, the peaks are compressed in an exaggerated way, and almost sounds like it's cut out at the beginning of the strum: with NFB at its highest, it cleans up noticeably, but it still has a very compressed sound at the initial moment of strum...All the while, no longer increasing my volume since 9. I find it very interesting, and pleasing, but should I bother putting in that 10k instead of 4.7k, and the 0.1uf instead of the 0.05uf? Your opinion matters to me. Thank you.
You're welcome, Rik. Different circuits require different component values, and if you're as satisfied as you seem to be with the presence control you have fabricated, I would say stick with it. If you're feeling adventuresome some day, you might experiment with different resistors and caps, but it may be that values you had on hand were the exact right values for your circuit. Congratulations on a very successful modification.
Thanks for another clear explanation. I appreciate the way you break down the function of each portion of the circuit during the explanation, including both the electrical consequences and the effect on the sound.
Thanks so much, CNS. I taught school (math and science) for many years, and found that this incremental approach worked best. It's like cutting a steak into small, bite-sized portions.....rather than trying to cram it down your throat in one big piece. Although, I fear that Rusty would probably enjoy either method equally :))
I'm green on building amps but being a mechanic this makes sense to me - looked at the other video to the end and I'm adding it to my amp I'm building. Thank you!!
You're welcome, Greg. Good luck with your project.
Excellent video. This is dead on with how my Louis Electric KR12 is affected by each of these controls. Very fun and effective for shaping tones and bloom with each of these pots. Keep up the great work!
TB
baugus423 Thanks, TB. Glad you liked the video :)
Good explanation. The presence control is similar to the tone control on most guitars
You have a variable pot (250 or 500K) and a capacitor in series (0.01uf, 0,022uf or 0.047uf to rolloff the high frequencies to ground. This circuit with the inductance
of the pickup is a LRC (Inductance/resistive/capacitive) frequency filter.
It doesn't function the same at all. The presence varies the ratio of HF that is the feed back loop.
You hear LF distortion alot more than HF distortion so allowing the HF to bypass the feed back loop gives you a more harmonic full top end.
Thanks to Doug and Rusty for a very informative explanation of NFB loops and presence controls
You're welcome, John :)
I'd shock myself right off the planet, but I love learning what you teach.
Thanks for watching....and for being careful, RG :)
These are the most informative videos about tube amps on YT. .
Thanks so much, Joe. Rusty and I really appreciate your kind words :) ^. .^
Uncle Doug
And I appreciate your expertise as well, Doug. Thanks again.
I had wondered many times about what is and does presence work. I wasn't even close to guessing what it was. I had trouble thinking it added highs and thought maybe it might oscillate the highs or something.... I never would have guessed it actually suppressed the lower frequencies.... it sure doesn't sound that way to my ears.... Thank you...
Passive tone controls, like Fender's, cannot add or increase ANY frequencies, Dee.
Howdy. An addition to my previous comment. Most relevant for Hi-Fi enthusiasts.
Because the neg. feedback arrives delayed to the summing point using secondary (global) neg. feedback there is a possibility for blocking distortion using strong neg. feedback, and therefore, strong drive. It may be checked, with an oscilloscope, whether the end tube grids go positive feeding the amp. with a square wave signal. If that happens the driver stage Slew Rate must be reduced. The reduced driver Slew Rate provides time for the feedback to act.
Regards.
Thanks for your very detailed analysis, Eugene.
Plain and simple. Your are an amazing teacher Uncle Doug.
Wow....thanks, Jim :)
Thanks for the simple synopsis of these circuits and controls. I'll be adding both to my next design.
You're welcome, Nic. Good luck with your build(s) :)
Great teachers can change the world, you sir are great teacher...with a great sideman. Rusty, king loves your video's as well.
Wow....what a nice compliment. Thanks so much, F86.....and I will be sure to let Rusty know that King sent his regards :))
Thanks Uncle Doug, that was an interesting vid, I have been looking for ways to adjust tone in my circuits without having to include an additional tube for a tonestack. Using caps and pots and NFB's and treble and bass boosts are pretty cool. Cheers
You're welcome, Elams. Rusty and I are glad you enjoyed the video. Thanks for watching and best of luck with your tone adjustment project :)
Outstanding Explanation.
Love your content! Thanks For your Time and Effort.
I am a Component Level Repair trained Electrician, Retired.
These explanations are huge as a refresher for skills that I haven't used in decades.
I am also a Owner of Three Awesome Vintage Amps from the Sixties and Early Seventies.
Looking for a Dual Trace and a Variac. Just for entertainment sake.
I always enjoyed Electronic repair.
Quite satisfying to properly troubleshoot and repair something.
🏴☠🎸😎
Thanks so much, LW. Glad you enjoy our videos :)
This is the best explanation I've seen for these amp circuits. You're really helping a lot of people that want to better understand their amps.
Maybe you could use a oscilloscope to show the signal as it passes through the amp? Show how the filter capacitors shape the signal. Just a suggestion.
Thanks for your nice comments, Ray. Please see my video series on Power Supplies for oscilloscope views of AC ripple, smoothing by filter caps, tremolo signals, etc.
Uncle Doug I'll watch the power supply video. glad you mentioned it. I used my scope a lot servicing electronics. it could be very useful in certain situations. I regret selling my shop equipment. There's times I'd enjoy working on tube equipment again. Watching your videos is a good substitute.
Thanks, good uncle! This is hands down the best explanation of both negative feedback and presence control I've ever seen. I'm seriously considering building this into my 5E3. Switchable of course :-)
You're welcome, Necro. By all means, give it a try. It's a nice, easy-to-install addition to any amp.....and, if you're unimpressed with the results, you can always remove it easily. Let me know how it turns out :)
Thank You for your diligent response..the cap of .1uf is one end to ground the1st & 2nd lug are connected together as in a boost or cut but on the 3rd lug there is a 4.7k resistor connected to the wiper and the other end to ground. The potentiometer is a 5k which is average on Marshalls but this addition started to show up on the later models. The .68uf cap was usually the cap used on the 200 watt Majors (1968) and this is where the ''One Wire Mod'' was introduced, consisting of a 1 Meg Ohm resistor on the Volume1 (Normal Channel) output was rewired into cascading Volume2's (Bright Channel) input resistor or so I believe in theory these respond as a Volume and Gain when plugged into the Bright but if you plug into the Normal only that channel responds breaking the Gain's chain but very Loud and Bold which I use as such and call it the''Loud & Bold Mod'' on the 2 channel amps which is minus the Master!! I rebuilt so many amps in my day and a number of Marshall's but I was not privileged until today to be able to ask anyone or thought to do so. Thanks again Uncle Doug!
Good heavens, Nelson, you definitely know a lot about Marshall tone circuits !! Thanks for sharing all this information with us. I will definitely refer all such inquiries to you in the future.
Guitars and Amps
Hola from San Diego Ca, Another fine video UD, here is an old video of my collection, a 66,67 and 72 princeton included.Original tune with my wife on vocals and me on guitar.The collection has grown since, you know, G.A.S.
Wow, thanks for the link, TB. I will definitely check it out :))
OK, I just got back from watching the video, and was completely impressed. A bunch of really nice guitars, a wonderful collection of Fender amps, and a terrific background tune. Your wife's voice is better than about 90% of the female singers I hear on the radio, and the guitar playing was excellent.....very professional, with great tone. I wish you lived nearby and could do my amplifier sound checks. I'm not just being polite here....the video was very impressive. Great job!! Thanks for sharing it :))
More great info. I am really enjoying going over many of your older videos. Such a great teacher. Thanks so much.
Excellent video. Thank you very much for making these. I feel like my understanding of my amp has increased 10 fold
You're welcome, David. That's great to hear.
I appreciate you teaching things like this as I am about to build an amp like this...good to get this information so I won't have to go back and mod ..I would like to install a reverb unit in mine...I have plenty of room in my chassis for tubes caps wiring pots and the extra transformer plus I have room in the cabinet for the spring reverb unit.
.again I do appreciate what you do for us
For some guidance in the addition of the reverb effect, please see my series on the Supro Tremo-Verb. Good luck.
Subscribed! This video was great. It's great how you explain what effect each component etc will have on the tone of an amp rather than just talking electronics like most tutorials. Thank you. I'll watch them all now. :)
adam stevenson Thanks so much, Adam, and welcome aboard. Rusty and I hope you enjoy all (111) of our videos :)
Thank you for the Presence explanation; it is counterintuitive and I did not understand it before.
You're welcome, Glenn.
As usual uncle doug your vids are very good
Without boredom just
Very infomative.
You teach with excellence. Love
Your sarcasm to about all the bad workmanship out there.
Thanks again.
Between you and
D Lab on UTube
The differences in thaught on cirquit
Designs is interesting
Stay well. Hope to see more of your vids in the future I'll be implementing the info into the repair of my old 60s blackface champ amp I baught for 100 $ back in 1996 at a little music store
Back when I lived in the state of KY. The man offered it to me so I baught it. Still use it today. I didnt know it was worth 800$ at the time I dont care. I wanted it for practice and other uses like in church so it is now modified for more utility I added a tremolo cirquit
And some other things
But no modern solid state junk,all tube
I like the sound of tube
Electronics full warm
Tones. See you again
On utube
Dwight
Thanks for your interesting comments and kind words, Dwight.
Nice explanation. Discovered you channel recently, and I really enjoy learning from you - Great Stuff!!
Thanks,Bill. We're glad you enjoy our videos :)
You make it all so simple and clear. I have a multipurpose Newcomb amp that could possibly benefit from this mod. As always, Thanks! BTW, you've got your 100'th video anniversary just around the corner. I'll have to open a bottle of champagne or at least a cold beer to watch that one :)
Thanks, VN. By all means, experiment with the Newcomb and keep us posted on how it turns out. You're right about the 100th video anniversary coming up soon. When I first started out, it seemed like it took forever to reach 10 videos.....and I never thought I'd ever reach 50 subscribers or 1000 total views, but things are rolling along pretty well now. Like you, I'll be forced to celebrate the milestone. Maybe Rusty and I will split a cold Coors Lite :))
Very informative and a really interesting modification to the NF loop. I've been focusing on your 5F1 scratch build video series. I've manage to build my chassis, although my design will be a 5F2-A Princeton. I will be posting updates on my Facebook page. Thank you for the Harbor Freight tip on the shears! I ended up purchasing the pistol grip pneumatic with a 20% off coupon which really takes you for a ride! Thank you for all you do Doug.
You're welcome, Eric. I built and posted a video on a 5F2-A Princeton several years ago. It has all the tone and charm of a Fender Champ, but with a tone control :) Good luck with your build.....and your new shears. Be careful of the gauge of metal you try to cut. I had to replace my blades a few months ago.
Thank you again Uncle Doug and Rusty...learning so much.
You're welcome, Is :)
Thank you, Doug; keep up the great work.
You're welcome, Brad.....we will :)
Great video Doug. Subscribed!! Say G'day to Rusty! I had one just like him! (18yrs) Thanks for making even less time in my day now I've discovered your vids! :)
+Gavin Moar You're welcome, Gavin, and thanks for subscribing :) Rusty is 17 years old and still chases the tennis ball for as long as you're willing to throw it. Like me, however, his eyes and ears don't work quite as well as they once did. We hope you enjoy the videos.
Thanks for these great simple explanations, Finally i understand this.
You're welcome, TDJ. I'm glad the videos are helpful :)
Excellent video as always! You do it up with class! I think you're on target with adding a feedback pot on the Twin; you'll have the best of both worlds and have a little more control over the tone...Looks like Rusty didn't make it onto the set in time :)
Thanks, Frank. I think the idea of adding a Resonance Control sounds intriguing also....it was in response to GuitarGeek's comment (below). Rusty was there, but he was busy operating the camera and couldn't appear in most scenes ^. .^
Great video! Thank you so much for sharing! You've cleared up many things I had questions about..
You're welcome, Joey. That's good to hear.
You are an excellent tutor - many thanks for your videos.
Thanks, John. I'm glad the videos are helpful :) ^. .^
Thanks Doug and Rusty for the video. Always baffled me what a presence control did in reality on a circuit level. Really enjoyed your recent tone control video too. I will post comments there soon. Take care . I think you mentioned you once were a teacher, you still are. What got you into tube amps and electronics.
You're welcome, Mix. It's great to hear that the videos are helpful and appreciated. I started restoring vintage jukeboxes about 35+ years ago and wanted to be able to do all the work myself. This required that I develop the ability to repair the sound systems, which are quite similar to those in vintage guitar amps. Once I developed an interest in vintage guitars and amps, the jukebox-related knowledge was a great foundation. I have posted a few videos of my restored jukeboxes, which may be of interest. Thanks again for your input.....it is always appreciated :)
Thanks Doug That was very informative
You're welcome, Eric. I'm glad you liked it :)
Put a nfb in the last amp. nice addition when you wire it up correctly. Thanks. Happy Thanksgiving.
+Joe Carson Same to you, Joe.
thank you again, Uncle Doug, this is exacly where I'm at on my fathers APS-4 amp from a Gulbransen 1960 organ.I'm useing the leslie and amp with my guitar. I won't do any thing to the amp just yet, till i learn more!
Walter Stahl Sounds good, Walter. It's nice that you are finding new uses for a "family heirloom". Best of luck with it, and with your modifications, should you choose to make them.
Awesome, very clearly explained and easy to understand. Is this the same general principle by which all presence controls work on all amps? Seems like it would be, why reinvent the wheel? Thanks!
Hi Uncle Doug, in the Bassman 6G6-B , the presence was generated in the tone stack.A 350K pot with an extra 70 K tap. The signal came from the bass pot if I remember well? My blond Basmann is made that way.
Kind regards,Joseph
Greetings, Joseph. I checked the schematic and it is as you say, except it appears to originate with the Treble pot in the Normal Channel (only). I would view this as a way to fine tune the treble frequencies closer to midrange. Thanks for your input :)
Uncle Doug Indeed UD, so it is. Thanks again for your SUPER videos.
Kind regards,Joseph
Awesome again. Would love to see your explanation about the how and why of amp stage coupling whether reistance, capacitance, inductance or a transformer is used. I understand it in a basic way. But why its used when its used in a guitar tube amp not really sure.
+John Miner Thanks, John. This is not a topic with which I am familiar. The inter-stage coupling in the vintage amps I deal with is generally done in a cascading plate-to-grid chain with coupling capacitors to block the preceding tubes plate DC from reaching the following tubes grid. Perhaps a Google search would turn up more information. Good luck.
Inside my amp, I removed the Presence control and replaced it with a another potentiometer (250 Ohms, after few experimentations) which allows me to play with midrange frequencies as well as different other punchy sounds. It also controls the amount of the overall volume. But in my case, it is connected between one of the grids of the Phase Inverter and the 8 Ohms ouput jack.
Good grief, Paul. You should patent this circuit design.
As long as you are taking suggestions, I think an explanation of American versus British tone and circuits would be cool. Of course Fender versus Marshall. We hear these terms in marketing and blog slang but what does it mean? So nice you are able to reply to most comments. When you get more views than Taylor Swift it may get harder to do.
An interesting suggestion, DS. I must confess that I am completely unfamiliar with Marshall circuit design, but I can download some schematics and try to figure out what the significant differences are (if any). I try to respond to every comment, but (for some reason) YT doesn't alert me to some of them, and it would be a nightmare to have to check through all the comments for every video to try to find them. Meanwhile, Taylor better watch out.....we're catching up with her :))
Uncle Doug DS, The first Marshall amps were copies of the old Fender Bassman, which was never really a bass amp to begin with. Years back I got to hear what I thought was an old Bassman. The owner of the amp, a guitarist in an excellent ZZ Top cover band explained that it was an original tweed covered Marshall (Yes, Tweed!). I can't imagine what that beauty is worth now, but man could she sing!
It would be cool if UD could find some secret to the British tone but I would think, in the end, the choice of tubes (EL's vs. 6-6's) the extra gain stages plus possibly less negative feedback along with closed back 4X12" cabs all contributed to the British vs. American sound.
Magnificent upload sir,God bless you a lot.
Thanks so much, AMIT.
Great info. This should help me reducing the excesive low frequencies when i push my princeton to maximun volume
Yes, or you can install a choke across the speaker lugs.....as was done with the Silvertone 1474 (see video).
@@UncleDoug I'm amazed at how the channel 2 sounds now on that one, great job indeed. And to adress the options, i'm adding the presence control, but without control, as it will go as a resistor and a cap, since i'm not into making more holes to the chassis. Or well, that's the idea, as soon as i find the right values for the tone to be good at both, low and maximun volume.
Thanks Uncle Doug! I always thought that I was creating a "presence" control that was just pegged in perpetuity when I DC'ed the NFB loop of an old Hammond (AO-44, AO-35, AO-fill-in-the-blank), but since an actual presence control is only rolling off very high frequencies from the loop with a high value cap, I suppose I'm sort of making a presence/resonance/gain/boost control that is, of course, maxed out! My AO-44 is like that, having pulled the input transistor and the low-pass filter stuff it was driving, I also pulled the NFB loop. I use it mostly as an output section behind my Frenzel (Jim Frenzel is a guy in Texas who builds and sells a lot of good PTP-wired stuff at really good prices, has an ebay store and frenzeltubeamps.com) preamp, or sometimes I use it for bass, with a little compression up front, and drive a single 15" black widow in a dead 1x15 solid state peavey combo. It works really well for both, and if I plug a guitar straight in, it also does an incredible almost-clean that I like for reggae-plucking with four fingers and then digging in with a thumb-pick. I am completely in control of the dynamics without touching the volume knob on my guitar. I do believe I will be subscribing now, this is a class I should definitely be taking.=)
Wow....thanks for all the info, Adam. It sounds like you are a capable ampliophile, who definitely should subscribe. I will check out the Frenzel Tube Amp site. Best of luck :)
Uncle Doug Done.....actually, I think I subscribed right when I was making that comment. And YES you should DEFINITELY check out Jim Frenzel's stuff. EVERYONE READING THIS SHOULD. He's often "backlogged" so it took longer than it was supposed to for my preamp to arrive, but a pair of 12ax7's in a pretty classic bassman/plexi (the preamp section of bassman or plexi, obviously) type circuit, point to point, gain+MV and 3 band eq, and I paid a discounted price of 200 bucks, but I think it's normally 275.....and that is obscenely cheap for what it is, I think. I've been looking around the web lately for more stuff to do to my "Yayo-FourtyFour" (nickname I gave it) lately, and I'm wondering if you know anything about the way an Orange "shape" control works, and if it would be possible to put one in an AO-44 that is basically stock, just with the input hot going straight to the grid of the first ECL86. I mean,m there's only two preamp stages, one's a driver, but I really only use it as a clean amp anyway, for separate preamps or on the extremely rare occasions that I want to get the sound of a particular pedal. I intend to try to build a 3x12ax7 high-gain overdrive with just gain and a trimpot on the output, and if the 44 had a really versatile tone control like the "shape" control on the Rockerverb and the Dark Terror, it would be perfect. . . . . Any ideas?
Adam, I looked at the Orange Dark Terror "Shape" control and it appears to be fairly straightforward, utilizing capacitors and resistance (like all passive tone controls) to allow selection ranging from "sharp" to "mellow". It resides between V2B and V3B of the circuit. If you wanted to add this tone control to the AO44, simply recreate the circuitry and insert it in the AO44 circuit, between the two stages of the 6GW8. How well it will work, I don't know, but like all experiments, you can simply try it and see. Let me know how it turns out.
Excellent explanation!! So clear, I can probably guess how a Resonance control works. Just like Presence in the feedback circuit, only with a capacitor that filters low frequencies?
Thanks, BR. Yes, if you eliminate the low frequencies from the out-of-phase feedback signal, then no low frequencies will be neutralized.
These videos are wonderful, Uncle Doug. Can I call you Uncle Doug? It seems a bit familial and whatnot. Anyhoo, these videos are great. I'm a software guy. I've built an LA-2A and 1176 and some pre-amps. But I was just being a solder-monkey and following instructions. I've watched lots of videos on electronics, but how many videos can you watch about Ohm's Law? I get it, Ohms Law, for the hundredth time. But there's so little in between that and actual useful discussions. Your videos fall at just the right spot for someone like me.
Something I always wondered about, but it never seemed to be explained, is why there are all these places where there's a resistor to ground. I always wondered what the point of that was. In this video, you note that basically you are shunting particular frequencies to ground by adding a cap. That sort of lit a bulb for me.
Are basically all of those places where there's just a resistor to ground essentially 'relief valves'? I.e. do they just insure that any voltage above a certain level has a way out of the circuit before hitting the next component(s)?
I have posted videos on the specific purposes of resistors throughout the amp circuit, Dean. The purposes vary from location to location, i.e. tone, current, and voltage modification.
Oh, OK, I missed those. I'll take a look at them. I've honestly learned more in the last week or so watching one of your videos every night as I go to sleep than I have in all of the previous time I've spent on this.
One question I had, and I'd post on the appropriate video if I could remember which one it was, is the issue of DC and AC being able to flow on the same wire. When that happens, is it sort of like a carrier frequency in radio, where the DC effectively just offsets the AC by its own amplitude and so the AC is no longer cycling around the zero point?
If that's true, do things like transformers not really care where the crossing point is, as long as it's oscillating? Or, if it's true, are there places where that offset would matter?
If you have a video on that of course just point me to it. There are a lot of them, so it's not always obvious which ones to look at for specific things like that.
The other, stupendously obvious but overlooked by me, thing that I sort of realized (hopefully it was a correct realization) is that A/C doesn't 'flow' but DC does, right? A/C doesn't move from one place to another through the circuit, it just oscillates back and forth, while DC is positive or negative and does flow from place to another. Is that basically the correct way to look at that?
Yes, you can think of AC as waves of oil floating on top of smooth water (DC).....but they never mix. You're analysis of the difference between AC and DC flow is technically correct, but for most beginners and intermediate students of electrical circuits, it's probably easier to simply have the AC flow through the circuit, happily creating high voltage and warming filaments with low voltages.....so we generally don't belabor the point ;)
So transformers really don't care where the crossover point of an A/C signal is, just the amplitude of its oscillations, I would guess? Or do you have to remove that dc offset from the signal before it hits a transformer?
you say at 1:25 that the cathode and grid are out of phase. i thought that a signal taken off the cathode was in phase (non-inverted) with respect to the signal on the grid...just like a cathode follower.
i believe it's the signal taken from the secondary of the output transformer that's out of phase with the signal on the cathode of the second stage.
You are correct, Mr. B. Sometimes in my quest for simplicity, I leave out some details. In this case, it is the signal sent to the cathode by the OPT that is out of phase with the grid, not the cathode itself. This is why, if you connect your NFB loop to the wrong OPT lead (i.e. the one that is in phase with the grid), the loop becomes a positive feedback loop and the amp will oscillate loudly. Rusty and I thank you for your input and will add a note to the video correcting this misconception.
Uncle Doug
hi uncle doug,
you're absolutely right about using the wrong OPT lead and oscillation....that's the difference between positive and negative feedback (regenerative vs degenerative).
MisterB4x Thanks again, Mr. B. I, like lots of amp builders (I assume) have suffered with the deafening consequences of attaching our NFB loop to the wrong OPT lead. It's always a 'shock' to discover that your precious, home-built masterpiece is functioning more like a police siren :))
Uncle Doug
I nearly always had been (and will be) fooled with the "begin/end" dots of the transformer´s winding schematics. So it´s way faster to just solder the leads to the transformer-taps (as nearly all europe made transformers got no leads, as they got some taps on the transformer instead) and see (=hear) if my guess was right ;)
Nice video. You should maybe have said a word or two about how different amps have different NFB. Some amp are, lets say so, stable that their tone wont break up even with the NFB totally snipped. To get a nice range on the NFB a big potentiometer can be used. With a pot of 1M or so a range can be found. With a measurement of the interesting resistances, low level NFB and high level NFB, a resistor and a smaller pot can be used to control the NFB. Well... this is how usually tinker with the NFB loop of amps I come across.
Thanks, Per, for your kind words and your detailed input :)
I wonder, did Leo arrive at 820ohm for many of Fender amps Neg feedback resistor by ear or did he rely on a formula of sorts? I have been playing with different values but havent settled on anything. If the effect of changing the resistor is eq I would guess it could never be ideal for every guitar as guitars seem to have different natural peaks and valleys at certain frequencies.
Thanks Doug for doing this video. I found it informative, concise and to-the-point. It was well explained. I am wondering if you could say something about the "resonance control" and how it could be mixed with the presence control circuitry you described in your video. I could not find any schematics on the topic...
Greetings, Julian, and thanks for the kind words. In the video description I offer the following note: To turn the NFB Loop into a Resonance or Depth Control, add a .0047uf capacitor in series with the NFB resistor. To make it variable, add a 1Meg pot in parallel with the capacitor with one leg of cap. connected to the center (wiper) lug of the pot and the other leg connected to either of the (outer) lugs of the pot. For this to function, the Presence Control (if you had one) would have to be "off" so no high frequencies were being sent to ground. Here is a schematic: www.regiscoyne.com/tech/resonance/
Uncle Doug Thank you. I could not follow the explanation straightaway but it has become clear now that I have seen the schematic. I still don't understand why you can't have a presence control and a resonance control together working as a sort of tone stack circuit if that makes sense...
There probably is a circuit that would allow both to function, Julian. Perhaps a search would turn one up. The Presence control functions in between the Midrange and Treble control and the Resonance functions in between the Midrange and Bass controls. From what I understand, the effect of these additional controls is most evident at high volume levels.
hi i appreciate yours vidéos and you speak very clearly and youre a good teacher for a french like me ,id like to know how about what voltage of power supply and plate voltage on tubes and cathode voltage cause shematic dont mention nothing about this 5f1 .thanks
Thanks, Kubock. Simply buy a power transformer designed for the 5F1 circuit, follow the schematic carefully, and your voltages will most likely be within acceptable parameters for each tube. Good luck.
Very cool. I know this is an old video but I wonder if you could change the presence control to cancel some of the highs in the negative feed back. Instead of running a capacitor to ground run a resistor to ground after your negative feed back potentiometer? Very interesting stuff.
Thanks, Chris. There are all sorts of ways to modify NFB loops.....and lots of room for experimentation.
This is a great topic in addition some years back I was 1st introduced to the idea of the NFB as the would be designers aggressively sought out the overall purpose of the NFB and what and how to if not just alter for sensitivity per other schemes existing but how to achieve possibly a 60s Marshall/Park Plexi amp performance. After reading countless lectures and getting the understanding I noticed there were some presence controls with an added resistor to the .1uf capacitor and if I'm not mistaken up to .68uf was used!!. Is this resistor a form of a (?) preset in filtering along with the 5k-10k presence pot? I also settled in on a cascading use known as a 1 resistor mod using a 1megohm resistor on a '73 Model 1987 Marshall #1 by my first recollections and immediately noticed it sounded and responded just like the term used as a ''Brown Sound'' made popular by the designers of the Van Halen mods! On another I did bypass the NFB resistor altogether in one instance and it brought the Marshall amp#2 to a whole new level and I even removed the .68uf cathode capacitor and the .0022uf bright capacitor on the tonestack's 2nd stage tube which increased the preamp's character to immense proportions.Incredibly powerful. But as my question stands Please if you will the added resistor on the presence control knob it's not just there to alter the capacitor's performance so what is it's due purpose??!! Thank you!
Greetings, J56. I am not familiar with the circuit you are describing, but one of our viewers definitely is. Please see the detailed response from Nelson DeJesus, which should be posted nearby (probably above). I never cease to be amazed by the knowledge and experience of the folks out there in UA-cam Land :)
A musician friend asked me to convert his Great Grandmother's early 1940s Westinghouse tube console radio to a guitar amp. I tried the "It's easy to use an old tube radio as a guitar amp" route. The radio was not working, so I trouble shot it, recapped it and replaced three defective tubes. With the radio now working, I wasted a bunch of time trying to use the "phonograph" input for guitar. After much trial and error I came to the conclusion that the idea is largely bunk. Instead, I gutted the chassis, saved only the new can capacitor which was a 20/10/10 @450V and told my friend I would build him a Tweed Champ circuit in the chassis, retaining the same knobs and controls arrangement, with the dial lamps as pilot lamps. He picked out a new 8 inch 4 ohm speaker and away I went on my first amp build. I decided to include a tone control and when looking at late 50s schematics, I realized I was building a 5F2 Princeton! I referred to a number of your videos for instruction and decided to include a Negative Feedback control as I had a spare knob available on the radio panel. I fired up the amp yesterday, corrected a couple of my mistakes and tried it out. Whoa, that thing sounds just like it rolled right out of 1956. The NFB control was the most amazing thing about the whole project. With the control, and a Stratocaster,
I can seemingly vary the gain from pretty clean (6K resistance) to very pleasing overdrive at 56K resistance. With a humbucker guitar, I can get over the top overdrive! The tone also changes subtly. I am really happy with the amp and am excited to let my friend (who is a professional musician) try it out. Without all the help from your videos, this project would have been a non-starter. I am pleased to show my gratitude with a Patreon subscription. Thanks again Uncle Doug and Rusty!
Scott
President, CEO and Janitor at Scotty's Workshop.
Wow, Scott, this sounds like a great build. All of your modifications make good sense and I'm glad the NFB control worked so well for you. To me, the NFB is actually much more useful and effective than a tone control (which is actually redundant with the guitar tone control). You're welcome for whatever assistance our videos provided. We commend you on this very successful project and sincerely appreciate your sharing it with us.
Thank you for your very insightful videos! How come you don't you need a diode on the negative feedback loop to make sure that the signal goes FROM the output transformer TO the cathode instead of the other way around?
You're welcome, Dj. The secondary of the OPT has higher voltage and much higher current (multiple amps), capable of driving a speaker.....while the cathode has very low voltage and extremely low current (milliamps). Thus the OPT signal, being more powerful, flows toward the cathode, rather than the other way around.
Thank you for your answer Doug! Very helpful indeed!
As always great video. you lost me at the end. but at least i have something to consider.(voltage spliter in a NBL) i'm new to this and i'll figure it out. thanks for the homework.
Thanks, Franky. I'm sorry that I lost you......if you have any specific question(s), I'll gladly answer them. Meanwhile, best of luck !!! :)
Can you show about Phase Inverter what it does and also Solid State rectivires,love your site even I can understand some of it THANKS
I will be posting a Phase Inverter video in the near future. Re a "solid state" rectifier, you simply use diodes instead of the two rectifier tube plates. Since they only conduct in one direction, they will divide the AC into a positive (B+) and a negative (to ground) Direct Current. It's a cheaper and more efficient way to get the DC your amp circuit requires.
I just did this mod to my monoprice 5 watt amp and it gives it a good amount more gain. People have done this before but you gave me the idea to put a resistor in line so you don't eliminate the negative feedback loop totally. I am thinking about lowering the value of the resistor and see how it does.
Lowering the resistance will increase the amount of NFB and result in more headroom and clarity, Steve.
@@UncleDoug thank you for the clarification, I will leave it be. I am happy with how it turned out.
Theoretically, you could have a whole tone stack integrated into the NFB loop, correct? Would that yield a dramatic effect, or would it be something that falls under the Law of Diminishing Returns?
+Connor Nepomuceno I would think that an NFB loop could be modified to provide tone control, Connor. I have no idea how dramatic it would be, however. Perhaps you should experiment with the concept and let us know what you discover.
Say I wanted to add a Presence control to a AB763 circuit. The NFB voltage divider uses awkwardly small values (820R/47R)... Would adding a typical Presence circuit work properly? By typical circuit, I mean a 25k pot + 100n cap in parallel with the 47R resistor. Or, should I increase the voltage divider values to something like 82k/4.7k and adjust the PI tail resistor to compensate? Hope my question makes sense. Glad to see you're making regular videos again! Cheers :-)
In my experience, the only way to effectively add such a modification is by experimentation. Test your theories and see what works. Today I altered the tone characteristics of a mid-60's Gibson GA15RVT from shrill and grating to warm and pleasant.....by experimentation with different capacitors in different locations in the signal chain. I suggest that you try the same method with your project. Good luck.
Another gem, Uncle! Is the negative feedback source from the OT secondary out of phase with the signal at the driver triode grid because the signal from the 6V6 to the OT is out of phase with the driver triode (each triode inverts phase)?
Thanks, Jeff. You can count the phase reversals as the signal passes through each stage of amplification, allowing you to see why the NFB signal is sent where it is.
Hi Uncle Doug, as always thank you for the great explanation and everything you do to explain things in detail. I don't know if this has been addressed yet, but I am a little perplexed. If you have a NFB resistor value of 2700 ohms or 2.7k like on an AA764, and you change the value to 10K in series with a 50k pot. How does that go below the original value when it is already 3-4 times higher on the lowest setting? Am I missing something? I would think that a 1k or 1.5k resistor in series with the 50k pot would allow adjustment above and below the original values. I am just wondering, since I am in the process of building my first amp, an AA764 with both NFB and a presence control. Any response would be greatly appreciated 🙏
Take care!
Greetings, Brad. Naturally, you tailor the NFB values to suit the circuit. In the case of a AA764 PR, you would use values like those you suggested, so you could adjust from much lower to much higher than the regular NFB resistor.
Hi Uncle Doug: Thank you for your reply today. I built a mojotone 5e3 deluxe and it came out great. I do find it breaks up a little too much. I'm trying to decifer what pin of the 12ax7, pin 7?, of tube to come off of with a resistor to create the negative feed back loop. I also assume the other end goes to the negative speaker terminal. I have a photo of my chassis, how can I send it along for your reference. Thanks again, Bob.
+starlingave You're welcome, Bob. There is a wealth of information on this subject on the Internet, i.e. www.tdpri.com/forum/amp-central-station/69-why-doesnt-tweed-deluxe-have-negative-feedback-loop.html It appears that the most common way to install the NFB is a 22K resistor between the speaker jack and the cathode of V1b. Good luck.
Hi Uncle Doug, I have a silver faced 1972 Twin Reverb, which I have black faced to Ab763 circuit. I had added a 25 k pot to see how much resistance I liked and it came out to about 15k on the pot by itself in series with the 820 ohm existing resistor. Question 1, should I just put in. 15k resister to give me the amount of reduced negative feedback, 2, I would like to get the bass up a little, like a resonance control, should I use the suggested .0047uf cap in series with the 15k resistor. 3, what frequency blocking roughly would I get with that cap, thanks again for your very informative videos, Bob
+starlingave I tend to like the flexibility of an adjustable NFB, Bob, but you could simply replace the pot with a fixed resistance if you wished. Please see the formulas in my Tone Stack video to calculate the frequency cutoff with that cap/resistor combo......or you could do what I do and simply try it and see how it sounds. Good luck.
This is a good tinker mod in between projects.Since it’s reversible, I was thinking about adding a NF to my MOD 102+ kit (nice little 8watt champ-like amp with an el84 in v2) just to see the effect. I don’t think it’ll improve much but maybe it’ll relieve some of the hum the amp has at high volumes when I use a long plate 12ax7 in V1. A vintage Sylvania long plate 12au7 cleans it up and smooths out distortion nicely but the gain goes down.I’m just curious more than anything. P.S. If u ever decide to use sponsors, ‘Great Courses Plus’ has a very practical electronics course. I think you’d approve! Can’t beat the value at $20 a month for 24 lectures. That also includes about a million other subjects. Idk how sponsors work on UA-cam but I just thought they’d be a good fit for your channel.
Thanks for the ad suggestion, William. I'll keep it in mind. You can make the NFB loop either switchable on/off, or variable with a pot. See my Homebuilt Champ from Spare Parts videos to see how. It does make a big difference.
@@UncleDoug I added the NF to the 102+ MOD kit. It did make a big difference. Mainly in volume. More of an off/on thing than adjustable. So I replaced the pot with a switch. The amp is based on a champ but has some differences in output tube (el84) and push/pull pots for extra tone options on bass&treble and the volume too. It seemed to work better going into pin 8 rather than 3. It really had a lot of feedback with the pot dimed going into 3. It IS pretty handy with it on as the amp has great overdrive but it’s too loud to take advantage of in home settings/recording. Is there any great problem with #8 cathode pin? Seems fine. Day later..figured out the pot wasn’t getting grounded in the chassis hole I cut(tad too large) fixed that and feedback stopped on pin three but still better results on 8.I think the tone controls must be the reason? A 4.7k resistor seems to elicit more of a result in this particular amp,as well. I’m still figuring this stuff out ;-). Thanx.
thx Uncle Doug! i watch your videos again and again as my knowledge of tube-amps increases over time :-). 2 questions : is it wise to use a bypass cap (lets say a low value to enhance the treble on the plate output) in combination with a precense control in the nfb loop? or is this a not common thing to do (using a bypass cap in the cathode in combination with a presence control)?
You're welcome, Stefan. First, I believe that bypass caps in traditional values, like 10 to 50uf, enhance all frequencies.....not just treble. Second, I see no reason why both bypass caps and NFB/Presence Controls cannot happily coexist within the same circuit.
Thx Uncle Doug.
You're welcome :)
me inscrevi recentemente no canal do Uncle Doug e virei fã... obrigado pelas informações Doug!
Você é bem-vindo, Renato. Agradecemos seu tempo e interesse em nosso canal. Obrigado por subscrever :)
great video. also love that you include your dog. I love 4 things in life. God my family rescuing dogs building amp and pedals. plus my 5th love smoking weed but I do that while doing the other 4 haha. great videos though, I love that you include your dog.
Thanks, Jon. Rusty and I don't smoke, but we do share a love for your other four favorites :)
great video! on the twin it has double rectifier tubes thats interesting
Thanks, Russell.
Uncle Doug, I am grateful for the knowledge you share. I perform in situations often with an orchestra where I need a clean amp, then the next day it can be classic rock and blues. I've decided to try a Tweed Super amp for both purposes. So, I believe the variable negative feedback idea may be of value to me. On this amp (5F4) the resistor in the feedback circuit is 56k. in your video, you stated that you were going for a range of about 50% to 300% of the 22k resistance in your Champ amp. So, would I perhaps use something like a 25k resistor and a 125k pot? I figure this would bring my range to 25k to 150K roughly maintaining the 50% to 300% ratio. Could I try a 250K pot? (much easier to get a matching alpha 250k pot) I've been a professional musician for over 40 years now, I guess I'm trying to recreate that magic I felt when I first cranked that tiny tube amp back in the late 60'S.... :)
Greetings, AA. I responded to this question on my Facebook page. See you there :)
Have you ever working on the Traynor Amps? My 1969 Traynor Guitar mate Reverb tube amp is my pride and joy.
+Jason Goldsack Sadly, I have never personally encountered a Traynor amp, Jason, but would love to have the opportunity some day. I understand that they are quite nice amps.
I will have to send you some pictures and see if I can send you a schematic.
BTW. I would live a video on how a tremolo circuit and the circuit that sends a signal through a spring reverb. I have both on my amp.
I have already posted two videos on Tremolo and one on Reverb. The Reverb video is on another channel (I had not started mine yet). Do a Google Search for "Fender 6G15 Clone" on the ElPaso TubeAmps channel.
Excellent video, as always! Question: why is the signal for negative feedback typically (always?) taken from an OT tap (as opposed to a pre-OT signal, for example, the connection between the anode of the 6v6 and the OT)?
Because the signal has to be converted from high voltage low current, to low voltage high current to be effective in the NFB. This is done by the OPT.
excellent, thank you very much 👌🏼🙏🏼 It could both be added as modifications (variable NFB and presence ctl.) to an old Champ, later Champ/VibroChamp or even "recent" Champion 600. Nice, interesting and enlightening.
Glad you enjoyed the video, Fernando. Yes, you could add a Presence control if you wished.
Thanks again for another wonderful video Uncle Doug! This was a topic that was like voodoo magic to me. You've made it understandable even for a non techy person. Cheers!
That's great news, RN. You're welcome :)
@@UncleDoug Dear Sir, I have a Blues Jr III that I converted from PC board to turret board using a conversion kit. There are several things that I would like to address on it. The volume/gain control gets loud/distorted very quickly rather than gradually, the master volume gets loud very quickly, and I'd like to get a bit more highs. Would you be willing to, for a fee, have a look at my schematic and provide any recommendations to adress these issues? I know you probably get inundated with requests and have your plate full of stuff you're working on now but better I ask and you say no than not ask at all. If I had someone near me I thought I could ask for help I would. Thanks again for all your wonderful videos and thank Rusty for his contributions 😁. Cheers!
@@romannumeral5547 Greetings, Roman. I am currently taking a break from amp repair and have never done personal consultations.......so I'm really not the guy for this project. You might consider reducing the values of the volume and master volume pots if loudness comes too soon. Have you tried cranking up the treble on the guitar tone pot? Best of luck with your tuneup of the BJ.
@@UncleDoug Thank you for your reply Uncle Doug. No problem, you don't know if you don't ask. I've got an audio taper pot I plan to use to replace my linear master volume pot so that might help. This amp has a lot of gain so I may try a 12AY7 in place of the 12AX7 in V1 position. Thanks again and take care.
@@romannumeral5547 Linear pots don't work in the VC position, RN, for the reason you discovered. The audio taper will make a big difference.
Great video... quick question. I f I understand correctly the effect of the "presence control" will be affected by the amount of negative feedback used? Would it make sense to have two pots? Or would that make too many variables in the circuit?
Thanks, J-F. One pot will do. Yes, the frequency range and amount of out-of-phase signal returned to the cathode of a pre-amp tube will determine which signals are suppressed.
I'm building a Princeton Reverb amp from scratch. The kit provides a 2.7K NFB resistor. I assume applying your formula of a resistor of around 1/2 the original resistance and a pot that provides a maximum of 3X the resistance would result in a 1K resistor and a 10K pot. That combo would produce resistance from 1K to 11K. Does that sound like a good plan? I want a clean sound but not a lot of breakup. Thanks for getting me thinking about varying the NFB.
For a clean sound, your formula should work, Thorne. A raunchier tone would probably require a 25K pot. Good luck.....I hope it works well for you.
Great explanation. Thank you Doug
You're welcome, Rick.
I think a great video idea would be going over a mystery to me ( and hopefully others) and that is how guitar signal and electricity are combined in a tube in both preamp (triode) and power amp examples. as a layman "paint by number" builder of pedals, it's a mystery as to how this occurs. complicating matters is new knowledge from your vids- that amps use both rectified and AC voltage to achieve this. confusing. sorry this paragraph is the best way I could think of asking short of hitting you with about 20 pm's.
You're idea is excellent, Mel: An overall idea of how amps work. We start out with an analog input....the wiggling of the guitar strings....and end up with a (much greater) analog output....the wiggling of the speaker cone....BUT there are all sorts of mysterious, invisible electrical events going on in between to make it happen.
Rusty and I will start putting together a video which (attempts to) explain those "mysterious" events. Thanks for a great suggestion !!
Thaaaanks for sharing this.
So, can I just cut the NFL cable inside my 57'Champ Reissue to test how it sounds with all those lost frequencies otherwise cancel led safely?
My training it's been changing a couple of pick ups so far but I'd use rubber gloves, plastic clamps etc to manipulate the innards of the Champ circuit...
Massive thanks for this informative video!!
You're welcome, Ramon. You are wise to take precautions when working on your (potentially lethal) amp chassis. Be sure to drain the filter caps before touching the circuit (see my biasing videos for instructions if you don't know how to do this).
Yes, you could disconnect the NFB loop in your '57 Champ (R12 @ 22K), but I don't think you'll like it. This amp is designed to break up quite early, especially with humbuckers, and the lack of a NFB loop will probably make the tone excessively "dark", distorted, and noisy. If you do it, let me know how it turns out. Here is a link to the schematic and layout: support.fender.com/schematics/guitar_amplifiers/57_Champ_schematic_Rev-B.pdf
Thanks for the response.
Sadly as things turned out, I had to sell it.
But I hope I'll get a replacement next year.
I'll stick to my handwired VOX AC-15 for now.
Best regards!
You're welcome, Ramon. Best of luck !!
hi if we measure negative feedback for the result ,where you put your scope is it at the input of the grid of output tube or on the output transformer, so its makes a difference,this is less good at the output transformer .thanks
I have never used a scope for this purpose, KF, simply because it's easier and more effective to tune the circuit by ear. Use a recorded input (or have someone play) into the amp and experiment with different pots, caps, resistors, and settings to achieve the desired tone effects.
ok thanks
@@UncleDoug
hi, nice video, thanks alot. I was wondering if the negative feedback can be connected this way in a amp that has a long tail pair phase invertor.I mean, connecting it on the second triode's cathote, before the signal goes to the phase invertor tube.
NFB loops are often used with LTP phase inverters, but the return line is connected to the cathode and second grid of the PI tube itself. Here is a diagram: robrobinette.com/images/Guitar/Deluxe_Models/6G3_Schematic_Signal_Path.gif
but would it change something to go against the standard and connect the negative feedback to the cathode of the triode that is just before the long tail plate phase invertor.(assuming this triode is not a cathode follower and that there is no capacitor to ground on the cathode ). just like the second triode in the fender pro junior.
You could try it and see......but if it worked well in that configuration, I would imagine that Fender and other amp builders would have wired it that way. Also, recall that the NFB signal has to be fed to a cathode in which the signal is out of phase with it.