How Does a Bias-Shifting Tube Tremolo Work ?
Вставка
- Опубліковано 20 лип 2024
- The Bias-Shifting Tremolo is generally found in tube amplifiers with less than 20 Watts of output power. It relies upon a Phase-Shift Oscillator (PSO) to generate a low frequency (0.5 to 10 Hz) signal which is used to alter the bias of either pre-amp or output tube(s) in a rhythmic pattern, creating the volume fluctuation typical of the Tremolo effect. This video covers all aspects of the Tremolo circuit, from design and function of the PSO, addition of controls such as Speed, Intensity, and remote Footswitch, to the application of the oscillator output to the cathode of a 12AX7 (Fender Vibro Champ) and the grids of two 6V6's (Fender Princeton Reverb). It should be noted that the principles discussed apply to virtually all amplifiers with BST Tremolo's.
If you enjoy easy-to-understand videos covering vintage tube amplifiers, circuit design and modification, scratch-building of amps and effects, unusual electromechanical devices, antique jukeboxes, and a whole lot more, then please subscribe to my channel. You will gain immediate access to over 100 videos and will be alerted each time a new one is posted. Thanks for watching :)
To learn something, is to be given a gift that has no equal in life. It is worth more than gold. Thanks Doug! Again!
Paul Fabrique Wow, thanks for the very kind words, Paul. You are quite welcome :)
So many thanks for another great video. You've answered so many questions about tube amps. Your attention to detail is compelling.
+SiliconSet You're welcome, SS. Rusty and I are glad the videos are helpful :)
Uncle Doug, thanks so much for your videos. I'm always thrilled to see a new one when I look at my subscriptions. I always learn something new ever time I watch one of your videos too. Thanks again for your wonderful work! buddy
You're quite welcome, Buddy. Thanks so much for watching and posting such a nice comment :)
What sets you apart from other so called tutorials is your easy to grasp explanations with schematic diagrams and examples of real circuits from well known amp circuits. Rusty may be up for an Oscar one of these days! I can't wait for part 2. Thank you for sharing. Again Thankz!
You're quite welcome, ATL. Rusty and I really appreciate your kind appraisal and are glad to hear that you enjoyed the video. Keep some popcorn handy for part 2 :)
You seem to anticipate my next question every time... thanks for the video on the mysteries of tremolo!
You're welcome, Charles. Glad the videos are helpful and of interest :)
Uncle Doug! Good to hear from you again! Love your videos. Thanks.
Likewise, Patrick. You're quite welcome. Glad you enjoy the videos :)
Love watching your (Rusty too!) videos. Will never get tired of these. Please keep them coming!
Thanks so much, Alex. Rusty and I will do our best :) ^. .^
You have taught me heaps thanks and a very creative idea of involving rusty too
+Ethan Yong You're welcome, Ethan. Rusty is actually the brains behind our partnership.....in fact, he's the one who chose to include me :) Seriously, we're glad the videos are helpful and informative.
THANK YOU so much uncle doug. You've done it again!
I get very excited when my you you tube feed, lists that "Uncle Doug" has uploaded a new video. Great video as always Uncle Doug. Thanks a lot.
You're welcome, Dan. Great to hear from you. Glad you liked the video :)
Great to see You back with new videos! And congratulations on over 11,5k subscribers! Cheers
Thanks so much, Nusior. It's great to hear from you. Rusty is out in the workshop as we speak, working on videos for our second million views.....at least that's what he told me :)
thank you so much. i just re-built a 1962 Fender tremolux amp. this video was very helpful. once i understood how the tremolo circuit works, i was able to get it working perfectly. i referenced your video for adding a bias adjustment to the 1962 Fender Princeton. once i got the bias just right all the pieces fell into place. the tremolux is now tremendolux! your videos saved me alot of time. Thank you again. your hard work is truly appreciated.
You're welcome, Joe. It's good to hear that the videos were helpful in the completion of your project. Congratulations on a successful outcome.
You did it again Uncle Doug! Very nice vídeo. Tremolo is one of my favorite effects!
Thanks, Rafael. Glad you liked it :)
Brilliant. Thank-you a million times over.
Love it. Thanks Doug! It's always a pleasure to watch your videos. Looking forward to see your next video :-)
You're welcome, Julez. Thanks for your kind words. Glad you enjoyed it :)
Hi Rusty and Doug great to see you back. Thanks for the video and great explanation. It reminded me of the phase shifters used in PA systems to help reduce the feedback between the mic and the speakers. Take care and all the best to you both. Colin
You're welcome, Colin. It's great to hear from you. Part 2 was just posted :)
Izaak, your question did not have a reply capability, so I will reply here. There is no fixed phase shift value for an RC filter. The Phase Shift = arctan 1/ (2 Pi x frequency x RC) thus, it is dependent upon the frequency cutoff and the values chosen for the resistor and capacitor. In the Phase Shift Oscillator, the values chosen for the (third order) filter yield a total phase shift of 180 degrees. It's also possible to set up a fourth-order filter (with different component values) providing four 45-degree phase shifts to accomplish the same result.
Thank you Uncle Doug, for sharing your wealth of knowlege with us here. As one who learns well via a visual explaination rather than text books, I very much value and appreciate the time you put into your videos and the thorough means in which you explain this wonderful technology and how it works! All your videos are really well done. I have learned so much from you! I just want to say thank you! .... I also get a kick out of Rusty and his aloof ways.
+Steve White You're welcome, Steve. Nice comments like yours make it all totally worthwhile. Rusty and I are glad you find our videos informative.
Super informative as usual, and another tube amp mystery cleared up (at least for me) - thanks Doug
You're welcome, Pod. You weren't the only one to whom this was a mystery. I got quite a few requests for this topic. Please stay tuned for Part 2 :)
This is a great video Uncle Doug. Well done!
Thanks so much, GG. Glad you liked it :)
I studied tube circuits in the early 1970's when I was training to be an electronics technician at RCA Institutes in NYC. Since then I had a successful career as an engineer in the aerospace than pharmaceutical industries. Not much work with tube circuits in those years and much of the knowledge faded. Seeing your videos brings back the days when electronics was fun and simple-enough to understand most circuits. Thanks for bringing back that knowledge and those memories with your well-presented and interesting videos!
You're quite welcome, Anthony. Thanks so much for your very kind comments :)
What a great instructional video. Not to mention I got to follow along with my AA764 schematic - you know, "the one that (supposedly) never was." This stuff is great. This is one area in which I don't miss nugging through stuff in the "good ole days" at all. You're doin' great stuff sir - many thanks. :)
You're quite welcome, Wes. Rusty and I are really glad that you're enjoying the videos :) ^. .^
Uncle Doug, THANK YOU! With this video, I was able to fix a non-working tremolo on a 1966 Supro! (And get the speed into a workable range!)
That's great news, John. We're glad the videos are helpful :)
This is great! You are a fantastic educator! Thank you!
Thanks !! Glad you enjoyed it, Jeremy :)
I just semi-fixed a Bronco that's been dead for a year ir more. Then.... the tremolo/vibrato circuit didn't work. Perfect video to trouble shoot that circuit. Exactly the same circuit as the Princton. I'll give it a go tomorrow. Thanks, your videos are great.
Thanks, Dennis. Best of luck with your repair project :)
Uncle Doug ..... he-yea, the Bronco's "vibrato/tremolo" is working again... BUTT... I wish it had more depth... if possible.
Any ideas you have would be greatly appreciated. Happy New Year and thanks-a-lot.
Uncle Doug, The vibrato/tremolo seems very tube sensitive?? Works fine with a new JJ, less depth with a different JJ. Won't run at all with a couple of different used MESA tubes and Chinese 12AX7. Very odd.
Great to see you back with more videos Uncle Doug! I've been recently trying to get the tremolo section working on a clone of a Univox 45B but without success. (It's a push pull amp using 6BM8 's in the power section and I just love it's tone). The original circuit uses half a 12ax7 and it just doesn't seem to work, I might have to use the fender tremolo circuit design instead.
Greetings, Colin. It's great to hear from you. Sorry you're having tremolo trouble. The values of the three oscillator loop caps have to be completely correct, and oscillation will vary depending on the tube....some work better than others. Also, looking at the schematic, the "IMC" value should be around 1Meg, I believe. You might check it as well as the status of the footswitch jack. Best of luck :)
your videos are absolutely fantastic. thank you very much!
Vohs Norred You're welcome, Vohs. Glad you enjoyed them :)
Doug, I never shift the bias. First I blame the bias, then I shift the blame.
+John Bond That answer is a bit shifty, John, but definitely unbiased :)
He’s obviously going through a phase
sorry to be so off topic but does someone know of a method to get back into an instagram account?
I was stupid forgot my password. I would appreciate any tricks you can give me
@Damon Dallas instablaster ;)
@Collin Abdullah Thanks for your reply. I found the site on google and Im trying it out atm.
Seems to take a while so I will get back to you later when my account password hopefully is recovered.
Good video, as always, lots of information in it, with superb camera work of Rusty.
Seems he took on some weight over the holidays, maybe his new cat-friend will keep him on his toes!
Greetings and thanks for the kind words, TM. It's always good to hear from you. Rusty tends to put on a pound or two in the winter, but he assures me that it is "solid muscle". Our winter tennis ball games are somewhat limited due to bad weather, so I tend to think it's mostly due to a lack of exercise. Look for a slimmer, even more enthusiastic (if that's even possible) Rusty in the Spring :)
Superb as always.
Thanks so much, AO.
hi uncle Doug i enjoyed this video on the tremolo circuit thank you for sharing it !!!!
You're welcome, German. It's good to hear from you. Glad you liked the video :)
So precise just watt the Dr ordered! And another circuit where as the amp needs a footswitch to operate,.. the grounding shield and each hi pass RC has it's own function was very nicely put together that it can only make sense! Thanks again my good friend, my best to Rusty and yes by all means this gathering of thoughts definitely gave me the explanation(s) and more that I needed, also it's really interesting now that of that Ampeg pedal of less than 8 pieces, it was a variable resistor that by changing resistance (and not impedances) it would shift into different effects,...from amplitude to frequency and what's in between those effects at unity gain sure were unique, and as that in an amp say my 60watt '70s Univox it's already looking to be easy enough and a good lesson in Dougish Values! Later!
Nelson DeJesus You're welcome, Nelson. Rusty and I are glad the videos were helpful. Best of luck with your projects :)
Great explanations as usual!
+overdriven56 Thanks so much, OD. Rusty and I really appreciate it :)
Outstanding, i still don't get it all but i am closer and your diagrams are excellent. Thank You so much, D
You're welcome, Dennis :)
I had an AH-HA moment during this Uncle Doug. I have always had trouble understanding and explaining the differences of Tremolo, Vibrato and Reverb. Now I got it AH-HA!
That's good to hear, Curtis.
Thank you again. Just about to put the iron to the chassis of a Princeton because of your GZ34 recommendations and I was wondering how the tremolo works. Thank you.
You're welcome, Joe. Good luck with your project.
I loved your teaching. Thank you.
Thanks, LC
This was a fantastic video. Thanks.
You're welcome, Darron.
the Negative Feedback Loop video was definitely as help in following this one.
Glad to hear it, Curtis.
Another great video !!!
Mark Jordan Thanks, Mark :)
Great Vid uncle doug!! shade some light on that tremolo principle!! Thanks again!
alllx49 You're welcome, AL. Glad you liked it :)
I stuck with it....finally understanding what the hell is going on.I had to learn the language first, as with all things. Thanks for all your patient explanations Uncle Doug!
So true, PW, as with any new skill, one must first master the definitions and learn to speak the language.
The second half of the oscillator, is a cathode follower. It serves to separate the oscillator from the rest of the circuit, so that the oscillator is not influenced by it .. ;feeding thus the signal in a more proper way
Thanks for your input, TVC :)
I found it, Uncle Doug! Woohoo!
I just fixed my fuzz pedal the other day after it had been broken for a while. I must have rewired it 3 times and it still didn't work. Like an idiot, I had changed out one of the pots for no reason and it was dead on arrival. It took a couple of days but I finally decided to just put the old 1k fuzz pot back in and it worked again lol. D'oh!
I'm glad you solved the problem, JC :)
Thanks! Great video :)
So wait, Doug, my 2015 Fender EC "Vibro-Champ" (4 to 5 watts from a single GT 6V6 power tube, two GT 12AX7's and a 5YE GT tube rectifier. Anyway, it would seem that according what you have just taught us at 14:50 and a little after that my amplifier has tube tremolo and not tube vibrato. But judging by the name on my amplifier it has "Vibrato", not tremolo.
BTW my amp works like this: the tremolo only starts when you strike a note, meaning that it is never in a low volume wave oscillation of the tremolo cycle when you play a note. All the strummed notes always are emitted at the actual volume output that the volume knob is set too! It is only during the time period after striking a note does the oscillation begin. This is done right in my book! Not all S.S./Digital amps do it that way and when playing on them the music is not emitted the way it's entered into the amplifier input. You can strike a note during a low point in the signal oscillation at from the very instant of striking that note the emitted output volume is low, this changes the music amplified to our ears to such a point that I find that method unacceptable.
It has a rotary (rate) on/off switch (that can be controlled via a standard on/off Fender foot switch), that has a variable sweep from (0)-(1) through (1)2. The (0) setting is off and the "1" setting is the slowest setting. My standard setting is "1", although recently I am experimenting with different speeds. I learned to keep it on any new setting increase for a while, to allow me time to become used to the faster tremolo oscillations.Also to just change the speed a little at a time, say 1/2 of an numerical increment increase on the 1 through the 12 possible selections. , not even going from 1 straight into position 2, but rather from 1 to position 1.5, then after my ears have made the adjustments sound normal , then I can go to the 2nd. position of 12, and so on and so forth. I can indeed learn to adjust my playing to accommodate the faster tremolo or vibrato emanating from my amp and I enjoy the change of pace (ha -ha I made a funny! get it speed/change of pace-oh never mind!).
Do I understand this correctly? MY amp has Tremolo? Or, is Fender just using the "Vibro-Champ" name on an amp that has tube tremolo?
I thank you ahead of time for any considerations that you may provide regarding my questions.
BTW, Those schematics are so beautiful. I am an engineer and an artist and even studied violin under Nina Chemolve a Julliard School of music graduate who graduated in the 5th. position/ranking in her class and a musician. I was a musician for most of my life, since childhood. I was also an artist since childhood. So I would love to make a few paintings of schematics. Simple black on white backgrounds just like all the schematics I used to prepare while I was working as a draftsman, which BTW opened the door for me into engineering. I retired as a Sr. Eng inner from NYS/OGS. It all started when I starved while preparing a Fresco Mural painting for the Suffolk County Dept. of Labor, for the Commissioner. He got impeached-possibly they used my research/visits to all the facilities that were using "CETA" people as trainees. They used me to get to him. I give the man credit, not may men these days even envision murals like he did. He was an absolute dictator who made his immediate senior team comprised of Deputy Commissioners literally jump 1/2" off the floor when he barked out an order.
I remember when the President of the Long Island McArther Airport interviewed me to obtain his permission to walk around his secure Jet Engine Facilitates- he came right out and said I was a "SPY" for the commissioner. My retort was that I took a burnt/used wooden match from his ash try (he smoked a pipe) and drew an airplane right before his very eyes on a piece of paper right upon his desk. He remarked that, yes, indeed I am an artist and gave me permission to enter the safeguarded spaces. This was a very adventurous job as I walked into many military facilities and even historical facilities that are and or had been in the past involved with the CETA Training programs, all the way back to the 1930's WPA Projects.
Well, after the commissioner was impeached the leaders of the Dept. of Labor said it looks like I might starve to death as a starving artist and offered me a 10 month CETA training course in Electronic/Mechanical Drafting, (one reason why I encourage you to open a school-as in the training literal saved my life. I accepted and graduated and finally had a career that would allow me to "eat" and have a stable life! I married and raised two children. I stayed with it and I drew up countless schematics for lasers and also the H.V.A.C Industry, through different employers. I got to the point where I was requested to make original schematics, not just making some other Engineers drawings look and be legally presentable once stamped by a P.E. I loved my engineering career, more than my art (scandalous)! I loved designing and that was the most wonderful time in my life.
Doug, you are a most excellent technical instructor a real asset to all of us who have found you. Knowledgeable, courteous, humorous and diligent in presenting thorough explanations of your selected topic/lesson. I hope you do open a Technical Institute. Look into the CETA government (Federal) programs that can pay the 100% tuition for your students, as opening and administering and educating students shall be costly. The CETA program should cover enough - 100% tuition, and remember to set the tuition rate to cover 100% of your expenses/operating costs and of course if you desire you can add a profit, after all that's the way the world does things. Although I know your main desire is to educate and I can find it in my heart if you do not desire a profit.
-Peter
Greetings, PD, and thanks for the very nice comments. Those who wish to help support our advertising-free channel can become Patreon patrons, pledging a contribution of their choice for each video we post. Re the difference between "Tremolo" and Vibrato", the terms are often used interchangeably, even by Fender, but they are both quite different. Regardless of what the effect is called on your VC, it is tremolo, because it is based on volume fluctuation, rather than frequency fluctuation (as it is in Vibrato). Very few tube amplifiers offer true Vibrato, but Magnatone is, to me, the front-runner with several true vibrato amplifiers. Watch some YT videos to see the difference between T and V....it is quite evident.
Great stuff.
Thanks, RH. It's good to hear from you :)
Hi Doug-I built a 2x6L6 AB-763 circuit on a Hoffman Circuit Board with this type of tremolo and it did work originally but has stopped working. You said this circuit is typically applied to 20 watts and under. I'm thinking this circuit is very picky about the output tube bias. Your video was very informative and I appreciate your teaching us with such clarity.
You're quite welcome, Earl. I'm not sure why a working tremolo would cease to function, unless something changed in the oscillator tube and/or oscillation circuit. If it worked at first, then it should be repairable. Good luck.
Uncle Doug I’m not the most savvy diagnostics tech but am continually learning as I go. Your videos have been very beneficial to me and I am grateful for all of your efforts to teach us out here in UA-cam Land.
@@Earlvis You're quite welcome, Earl. We're glad the videos are helpful.
Wow. Thx very informative.
You're welcome :)
Great stuff Uncle Doug! Have the popcorn ready to roll for the next vid ! Rusty is really "laying down" on his part of these videos maybe the new kitty will help! LOL!
Thanks so much, TD. Part 2 is almost done, and the kitty is indeed featured in his own "musical" video. I think you'll like it.....pets, oscilloscopes, photoresistors, and all sorts of surprises :)
Hey, thanks for these vids. Another really awesome explanation. May I request a video tracing a schematic and showing it's relation to a wired amp? Cheers
You're welcome, OK. Thanks for your kind words. I'll keep your suggestion in mind for a future video.
I'm a player, not a fixer ... but I like this stuff. I guess this applies also to an early '60s blonde Tremolux ... Mupricin for those kitty bites and scratches. Lecithin caps (1200 mg./day X 6) to clean out your arteries.
This kind of trem produces an outstanding sound, BTW. Great explanation. Even EYE understood some of it!! But my guess was that one of those cap/resistor units would be replaced by another pot to change the amount of in phase/out-of-phase, as an intensity control. Missed on that guess!
That's good to hear, TT. Best of luck with your projects.
Hey UD, great work yet again! Actually very timely as I'm considering putting output-tube bias-shifting tremolo in an amp. The last couple I've built are along the lines of the Valco Tremotone with a 6SQ7 acting on the first preamp tube. At any rate, I was under the impression that when you switched the tremolo on, it actually connected the circuit to ground. Then the switch was opened to silence it. You seem to show the opposite about the 8:00 mark. It's entirely possible I have also sucked up too many solder fumes...
Upon further review, yes, I need to invest in a good extractor system.
Uncle Doug You were right the first time. I've built several hand wired Vibrochamps. The tremolo does not have to have the foot switch plugged in order to function. Simply turning the intensity clockwise starts the oscillation. A great way to prove that grounding shuts the trem off is to simply plug a patch cord into the vibrato jack then touch a wire from tip to shield.
Uncle Doug By the way hello Uncle Doug. I haven't checked into Ampaholics Unanimous lately. Doesn't mean I'm not watching though. After watching this one I just had to chime in.
Greetings, Pisola....it's always good to hear from you. We missed you at our AU meetings :)
Uncle Doug
UD, I think I had the right thought and the wrong spot. You were indeed correct the first time the way it's drawn. Please check my math on this -- the Fender circuits (similar to what you have drawn) do shunt the voltage in the resistor-cap filter loop to ground, effectively turning the tremolo off. The circuits I've built are more like those used by Supro, Ampeg, Gibson and others, where the switch goes between R1, R2 or R3 and ground. Those circuits don't operate if the switch isn't closed to ground. Sorry about that.
Another amazing tutorial, thank you. The "Charles Bronson style" hand acting was superb as is Rusty's camera work. :)) If I understand the pop quiz question correctly, feeding the signal to both 6v6 grids would cancel each other out. If I'm wrong, please stamp my meal card "no dessert". :))
Thanks so much, Andrew. It's great to hear from you....one of few who recognizes my hand-acting skills (the pathos of my performance in this case tastefully accentuated by kitten scars). Re the Pop Quiz, no gold star yet (Correction: see explanation below). Think about the frequency of the oscillator signal.
Uncle Doug I watched again, and with your hint, think I might have it. Hearing a 10Hz signal would be a bit difficult.
Uncle Doug ***** I think you've got it... Human hearing is supposed to go as low as 20Hz, though everyone's ears are different. Even so, 10Hz and lower is quite a stretch... I proly woulda guessed it with some thought, but I read threough comments first. Aw well, next time, I suppose. Looking forward to the Optoisolator/Lamp-LDR video, Doug! Thanks for another great installment.
Thanks so much for your excellent answers, Andrew and Britt. Actually, now that I've thought about it, Andrew's first answer also makes sense. The music signals applied to the grids of the PR 6V6's are of opposite phase, due to the (cathodyne) phase inverter, and reinforce each other in the OPT. The oscillator signals applied to the 6V6 grids are in phase and therefore would indeed cancel each other in the OPT...which is the way that audible noise is reduced in the output of push-pull amps. The signal to the cathode of the 12AX7 in the single-ended VC, is not heard simply because it is subaudible.
hey uncle dave. i have watching your videos and been trying to get myself back and beyond.my ampeg r12r b has been my inspiration to get back into guitar amps.we have corresponded about this amp and i am trying to take in as much info as i can. right now i have taken the chassis out and am trying to identify and draw out the circuit before i do ANYTHING.it has been 40 years since my devry days and i have been working the trades for my livelihood but always into music. built many electronic kits.i just want to thank you for taking me back to school and making it interesting. you had mentioned that i could test the op xfrmr in circuit. i have found a thread on how to deal but wondering if you have touched this subject on any of your videos. again thanks for all you do. sean
Greetings, Sean. Glad the videos are helpful. Yes, I have demonstrated how to test OPT's in several videos. Check my Impedance Ratio video for some helpful info.
Beautifully explained. I now feel I can add tremelo to my Champ clone. Am I right in thinking the reason so many trems don't work in old amps is because the old capacitors have decayed?
Thanks Uncle Doug and Rusty.
Thanks, Mick. You'll have to add an oscillator tube to accomplish your goal, so be sure your power transformer can handle the additional current demand. There are multiple causes for tremolo failure, and the change of value in aging oscillator loop capacitors is one of the most common.
Perfect timing Uncle Doug. I am just gathering the parts to build a 6G2 Brownface. This really helped me understand the "why's" of the trem circuit.
Could you persuade Rusty to let his cat get in the next video ? Or do you think he won't want to share the lime light ?
Greetings, BF. It's great to hear from you. You project sounds like an excellent one. Here's an article that you may not have seen that might provide some insights and inspiration: www.vintageguitar.com/8931/the-fender-princeton/ I'm glad the video was helpful :) Re Rusty's "pet", he is very proud and would probably allow the kitty to share some of his screen time. If so, the little rascal will appear in Part 2.
Uncle Doug, I appreciate you for all of the knowledge you have distributed over the years! you are an asset to all of us ampaholics out here. I have a problem with a tremolo that I want to run by you. I am working on a GA17RVT Scout Giibson. I put a virtual center tap from the heaters to the cathode of the 6AQ5 output tubes like I had seen in another video. As soon as I did this the tremolo quit working. Even though I removed it the trem still doesn't work I have checked all the parts and can't find any that are bad. the 6EU7 PI/tremolo tube checks good as do both of the output tubes . If you have any thoughts on this I would greatly appreciate it. thanks in advance.
I responded elsewhere, Jon. Best of luck with this.
Hello Uncle Doug, thank you very much for this and the other videos, the explanations are among the best I could find anywhere on the Internet. Great that you could took the time to produce these and share it with all of us.
I have one question please: On a vibrochamp schematic, I noticed that there is a 47 Ohm resistor added to the cathode bias section of the 2nd part of the 12ax7 preamp tube. Is there any particular reason why this resistor is added? Is it because of the tremolo? Thanks..
You're quite welcome, Ivan. I really appreciate your very nice comments. The 47 Ohm resistor forms a voltage divider (with the 1500 Ohm cathode bias resistor) for the negative feedback loop from the output transformer secondary, Ivan
Pop Quiz: Two reasons you can't hear the Tremolo's oscillator through the speaker when the amp is idle.
Reason 1: The oscillator's output of 0.5-10Hz is below the low-frequency cut-off point of the human ear. I've read you can't really hear sounds below approx. 20Hz. This mainly applies to the single-ended amplifier. The tremolo's output should still reach the speaker in a single-ended amp(you can probably see it on a scope) but you just can't hear it. If you increased the frequency of the oscillator, you could probably hear the hum on the output easily.
Reason 2: In the push-pull amplifier the tremolo's oscillator is being fed into the grid's of the 6V6 tubes in phase. This will still produce an oscillating current in the primary of the output transformer, but since the current will be pulling equally in both directions on the primary, none of signal will be transferred to the secondary and therefore the oscillator signal will never reach the speaker.
Congratulations, DU, you get an A+ in Tremolo 3101. The oscillator frequency is subsonic and (due to being applied in-phase to both output tubes) cancelled within the output transformer. Well stated and well done :)
Thanks. I design all kinds of SMPSs normally, but these vintage tube designs are awesome. I have some old pieces of test equipment that are entirely tube powered. Even a kit-built oscilloscope from 1968 that is all tubes, well, except for a small handful of power diodes. The most interesting part of the scope though is the small tube powered SMPS that generates the acceleration voltages for the CRT. Really enjoy watching your videos. I've learned quite a lot about how tubes and tube circuits function.
You're welcome, DU. It's great to hear that the videos are helpful. Thanks so much for your input :)
Thanks a lot Uncle Doug! Great video as always! I've half of a 12AX7 left in my amp but no space for the two additional chicken heads:(
Murrat Osmani You're welcome, Murrat, and thanks for the kind words. Keep looking for space for the chicken heads.....your amp needs a tremolo :))
Great video Uncle Doug, like always :-). I have a VOX JMI AC30/6 top boost head (1966) on my bench with a failing vib/trem channel. Thx for your excellent explanation About tremolo, it will help me a lot. Is there anywhere in your video series some explanation how vibrato (circuit) works?
Thanks, Stefan. I'm glad the tremolo videos were of interest. I have never addressed how true vibrato circuits work, mainly because they are very uncommon, quite complex, and I have never personally worked on one (as found in some Magnatone amps). Fortunately, there are lots of Internet sites that address and explain them. Good luck.
thx Uncle Doug. in the mean time the problem is solved. It was a defective plate resistor from the oscillation tube. but thx for respons like always.
Thank you
You're welcome, JV.
as always thanks for the effort you make these video's with... i bot 2) ao29 hammond.
one has a black transformer 360 volts dc...
the silver one is 35-0-350 volts and also powered a rack of tone generators.
so i bot 6 more, 3 of each type. the silver transformer has enough to run a pair of 6L6's
and the black transformer at least a pair of 6v6's... going to fabricate them as mono blocks. brian
+brian foley I am familiar with the power transformers that come on AO-29's and they are quite stout. It sounds like you made a great buy, since even mediocre power transformers now cost between $65 and $100 each. Congratulations on some great acquisitions.
Here is a like for Rusty! Thanks for the video!
Thank you, Nico :)
Thanks for the video. I've been curious how these really work, particularly the oscillator. Would that be the same as the freq oscillators in old tube radios such as superhets? Is there any functional advantage to use 3 vs. 4 RC filters to adjust the phase shift? I can see where a 3 step network ought to be cheaper and less complex due to fewer components. And, can you say if there is preferred location for the speed control on the resistors or would it be depending on which allows a shorter connection to the controls on the face plate? Thanks again! Wonderful as always.
You're welcome, Bob....thanks for your kind words and good questions. Radio transmission uses oscillators (to produce a carrier wave). Radio oscillators operate at much higher frequency than audio oscillators, i.e. RF rather than AF.....at least a thousand-fold difference.
Yes, I think the 3-stage filter is probably cheaper and easier to produce than a 4-stage.....and probably less prone to failure (fewer parts). Fender tends to place the speed control pot on the first RC stage, while Vox uses the second stage. Even the 3rd stage could be made variable as long as the loop gain is adequate. The lead length would probably not be of significance since no music signal is present.
Uncle Doug this is an awesome and incredibly helpful video. I noticed you said the Tremolo effect is turned off by setting the resistance of the speed control very high. I'm wondering why the Depth control on the 5F11 circuit wouldn't also eliminate the Tremolo effect. Seems like if the wiper is all the way left (low position of the 250k pot), it would allow maximum Tremolo oscillation to the grid of the 6V6GT's while having the wiper all the way right (high position of 250k pot) would allow the oscillation to all escape through the 25 uF capacitor on the bias line and not effect the grid of the 6V6GT's at all. If this were correct then you could turn off Tremolo by either turning the Speed or Depth pot's all the way down. What am I missing?
Thanks
In theory, I doubt that all of the 2 to 5 Hz oscillation would "escape" through the 25uf cap. Recall that capacitors are frequency dependent, and this is an extremely low (subaudible) frequency. In practice, I can tell you that stopping the oscillation with the speed control will cease all tremolo effect, while reducing the intensity to minimum will reduce it to a very low level, but generally not eliminate it.
Uncie Doug, another fascinating video. I'm having trouble understanding the fender champ tremolo. How does connecting the cathode of the tremolo circuit affect the cathode bias of the preamp tube?
+Jared Zecchel That is how it works, Jared, by altering the bias of the second 12AX7 triode. When the bias is elevated, the volume increases, and when the bias is reduced, the volume decreases.
Thanks, very informative video. It helped me trying to understand the tremolo circuit in a Silvertone 4750 organ amp I am trying to convert for guitar use. The tremolo in this Amp is similar yet different from your example. It appears to only use 1/2 of a 12ax7 and feed out of its cathode to intensity control. I want to add a speed control this amp does not currently have one. Is it important to use Reverse Audio pots for this? Your video did not mention Reverse audio pot use in tremolo circuits.
Thanks,
Aaron.
I don't use reverse audio pots, Aaron. I have never found a situation where one was needed. Good luck with your project.
Hi Uncle Doug! It's another great video I must say. I haven't been a big fan of tremolo effect until now. I decided to search for some other tube tremolo circuits and I've found one which used to be part of Fender Pro-Amp (6G5), Super-Amp (6G4) or Concert (6G12). These look more complex than those you have shown in your video. I'm little confused of the signal path because it doesn't go from plate to grid but it goes from plate to plate and than it doesn't continue from cathode in these circuits. Or am I wrong? What is the purpose of the second portion of tremolo tube? Thank you for your response.
The Brownface Fenders had a harmonic tremolo that was very complex. Here is a great site that explains them: carlscustomamps.com/types-of-tremolos-in-tube-amps/
Very nice art work Uncle Doug. I cant draw a straight line. Have you had a chance to work on a new video. I enjoy the so much. Thanks so much.
Thanks, Monty. Yes, a new video is underway.
Hi Doug, excellent video! Question about the Vibrochamp style of applying bias wiggle tremolo to a preamp tube. Assuming that the power transformer can handle it, could you insert this type of tremolo circuit into almost any type of guitar amp? Maybe a 5E3 or perhaps in a Deluxe Reverb in place of the Photoresistor 'roach' style?
Thanks, Kley. Yes, as demonstrated in my recent video on the Bell & Howell projector amp, you can add tremolo to virtually any amplifier, including a 5E3 Deluxe. Since the 12AX7, which is generally used as the oscillator tube, draws so little current, there is little risk of harm to the power transformer.
Uncle Doug Nice, that was very helpful! I am nearing completion of a scratch built AB763 style build. I took inspiration from a Doug Hoffman kit that uses a Fender 6G16 bias wiggle style tremolo - but am exploring some other options. The bias wiggle applied to the output tubes is proving to be a little weak and requiring a rather cold bias to produce optimal tremolo. So I plan on using the Vibrochamp preamp style tremolo possibly in its place. Pretty fun stuff!
The earlier in the signal chain that you apply the tremolo input, the stronger its effect. Before you abandon your present tremolo circuit, try applying it to successively earlier points (i.e. cathodes) in the signal chain......
Uncle Doug Thanks for the tip!
You're welcome.
I love your videos! One suggestion though: while I realize you are trying to keep your audience from leaving to watch pole dancing videos, can we get a bit more of the mathematical underpinnings of the designs you present? For example, you state in this video that by choosing the right values of the resistors and capacitors, you can get 3 60-degree phase shifts. I see below that your reply to one comment mentions that phase shift = arctan(1/wRC), but it would be good to mention this is in the video too (and the fact that phase shift varies with frequency, although I assume by using the right values you can minimize this effect across the pertinent frequency range).
Thanks. I appreciate your input, Tim, but I can assure you that the mere mention of the term "arctan" will send 90% of my audience streaming for the exits. I present math concepts when necessary, but honestly didn't feel it was needed in this presentation.
Hey the best to you first of,..the info applied will come in handy in adjusting and tailoring the timing of a tremolo circuit being there's custom kits where it's a drop in and play but to their preset. Here there's an explanation that at least coincides with a general understanding that can be added onto foregoing an actual degrees and potentials for a mishap.Thanks! On my 1969 Ampeg GV-22 a rare composite of a Class A but in a Solid State format, not,.. but I wished (MAYBE) a GZ34,..the duplicated Tremolo circuit design comes in the format of a rich quality ''B'' series Hammond Organ, Ampeg produced amazing little effects on their 60s amps for accordians,Old ighty-Eights,Harmonica, and my favorite the guitar,..even better than the ole Magnatones, Reverb with a tail when the cut-off's engaged and a separate cavernous Echo, secondly the ''Hammond Styled Tremolo'' also with a tail, with a separate Vibrato as noted by the rocker switches and man there's a definite difference,..the Ampeg Gemini series amps are some of the best Tremolo amps made but with the ''Superlative'' GV-22 40 watter 6L6s there's no equal as` an all around,.. your formatted review brings it to a practical insight that kept me from a misguided attempt of an filter cap upgrade, some reason with the replaced newer filters the effect would cut out and the footswitch had to be rest in order to get going again gotta feeling the circuit's anomaly is presently dependent on a bias that rejects extreme changes figure in the older resistors,bypass caps, tubes 6CG7/6FQ11(?) , RCA7025, and the Audio filter electrolytics, it's in the timing, by all means please alert me if you think otherwise, Thanks again Uncle Doug the best to you and urs and our Buddy Rusty the Wonderdawg!!
Thanx Uncle Doug and Rusty, I was just working on a similiar circuit!
What is the 25 uf 25v Cap for, it is an electrolytic, thanx
You're welcome, GH. That is the cathode bypass cap for the first 12AX7 stage (the phase shift oscillator tube). It increases the gain within the tube.
Nice video like always Doug! Dat tail lol
BTW Can you make a video specially about Vox AC30s? I heard that original Voxs differ alot from Fender/marshalls and have very unique and strange designs in it thanks to Dick Denny.
Thanks from me.....and Rusty's tail :) I will definitely keep the Vox AC30 in mind for a future video. Perhaps a comparison with a similar Fender amp.
Thanks mate
Doug, I think you made a small mistake here. At 6:16 the phase shifts at each RC network should be 60, 120 , 180 degrees instead of 120, 240 , 360 degrees, But I still understand your explanations, Good work !
You're ignoring the initial 180 phase shift from the grid signal to the plate output. Then, when you add 60 degrees to it in succession, the result is 240, 300, and finally 360 degrees. The incoming signal from the loop must be in phase with the signal on the grid for oscillation to occur. If your statement were true, the 180 degree out-of-phase incoming signal would nullify the grid signal and no oscillation would occur. Source: www.valvewizard.co.uk/trem1.html
Doug, good stuff! Curious why one amp uses the second half of the 12AX7 (Champ) and the Princeton does not. It would seem the cathode follower is needed to provide more current to be able to move the bias on the single ended preamp tube via its cathode bias (higher current node) vs. the grid bias approach on the power tubes in the PR (low current node). What are your thoughts on this?
I think that the circuit did not require the second triode, so it was left unwired. The simplest explanation is generally the right one.
Uncle Doug, thank you for your great videos. They are entertaining an very informative. One thing I don't understand about the tremolo is how does the oscillation start? When the tube is idle there is no signal on the grid. If the current through the plate is steady there shouldn't be any ac signal coming from it either... What makes the dog start chasing its tail?
There is always electrical noise in any circuit. The moment the oscillation circuit is ungrounded, the (almost sub-audible) noise starts the oscillation.
@@UncleDoug Thank you very much! That makes sense. And thanks again for your channel. It encouraged me to get back to my old hobby when I was a teenager. Now, I am 60 and started playing with tube amps again🙂 I got Sano amp from the 60-x that I am fixing and also converting its stereo input into 2 guitar inputs - one of these channels with 2 stage 12AX7 modifying into a distortion channel. An interesting feature of the amp - it has a separate amp with its own 8in speaker for the reverb. My other project is the guts from Lowrey tube organ turning into a guitar amp, using the organ's vibrato and possibly the tone tabs, also making the foot pedals working to play along and keeping some of the manual's keys for the guitar tuning. This whole contraption (all of the organ's innards) will be wall mounted in my house.
All this and more after watching your channel! 🙂
Thank you!
@@bobmead8184 Glad to hear that the videos are helpful, Bob. Good luck with your projects.
@@UncleDoug Thank you!
Hi Uncle Doug! Any chance you can come up with a video explaining how optocoupler tremolos in Fenders work?
Yes, indeed, Marco. We have posted such a video. Please check our play list.
Uncle Doug Yes sir I just saw it this morning! Thank you and I now understand it fully. Thank you!
You're welcome :)
Brilliant as always! Can the cathode biasing version be inserted in the 6G15 circuit? I'm thinking not since I haven't seen it done and it seems like something a lot of people would like to own!
You may be thinking of the "Revibe", JB. Do a search and read about it.
@@UncleDoug So I'm not the only one that thinks this is a good idea. It looks to be well worth the price but, since my serious income production days are long gone, coupled with vast amounts of time on my hands, I'm more in the "Uncle Doug" DIY camp these days. I think this would be a great video project (hint hint) if you agree and no patent issues you'd have to deal with exist.
At any rate, thank you so much for sharing your vast knowledge in such an entertaining way!
It would be an interesting project, JB. Weber himself says that the build is "challenging, even for experienced techs".@@Jazzboful
you`re god. I swallowed all your vids in less than a week! Thanks a lot for this! My best wishes from Argentina
Thanks so much, Gonzalo. Rusty and I really appreciate your very kind words and are glad you enjoyed the video(s). Our best to you :) ^. .^
Uncle Doug Hey uncle! Did you saw this?
www.synthtopia.com/content/2015/01/29/korg-noritake-introduce-futuristic-nutube-vacuum-tubes/
I have seen this ad, Gonzalo, and am very interested to find out how they sound in an amplifier circuit. It's hard to believe they can duplicate the nasty tone of a pair of over-driven, saturated 6L6's :)
In one of your videos, I seem to remember you talking about the difference in tremolo and vibrato. I believe in one of your builds, you showed the circuit of a true vibrato circuit. Can you tell me which video that was? I would like to put a true vibrato circuit in my own build. Thanks so much.
Good luck, Geoff, because it requires three tubes and a whole lot of wiring. See the Fender Brownface Concert video. Magnatone had a true vibrato, but it was based on components that are no longer available. You an also check out the Weber ReVibe on the Internet.
Uncle Doug-I've been looking at your Tremolo videos, and have been wanting to build a replica of the 6G15 three knob reverb circuit married to a 6G12 three tube tremolo seen in the Fenders, but instead of the 6G12 circuit I'd like to have the Vibrato circuit that Magnatones use. Have you seen anything like that or know of a schematic? I really like the true pitch shifting effect found in the Magnatone, and I don't know of any other amps that use it. Is that circuit more complicated, or why don't more builders use the pitch shifting Vibrato vs. the volume shifting tremolo? Thanks as always.
You need to watch my video on the Panoramic (Magnatone) amp, WS. Everything you need to know is provided in the video, including the reason that so few people are successful in building one.......the key components, Varistors, are very difficult to obtain.
Very nice video sir. You use the Vibro Champ as a sample to describe the tremelo function. But doesn't vibrato control the frequency of the signal, while tremelo is a varying of the amplitude? Or is this just an amplifier name "terminology" confusion? I would think Fender would have called it a Tremo Champ.
+Harry Thanks, Harry. You are correct. Fender (and many other amp builders) erroneously named their amplitude-modulating tremolo as if it were a frequency-modulating vibrato. True vibrato is rare in guitar amps and personified most outstandingly by Magnatone. Another good source of vibrato effect is the Leslie speaker often converted for guitar use.
Doug. I understand the loop that sets up the oscillation, however, what I cannot understand is where the initial signal at the plate comes from? You penciled in a sine wave (at the plate) and showed how the sine passes through the 3 HPFs, but I cannot see what produces that initial sine wave or signal that gets fed through the HPFs and begins the oscillation. Is there some property that makes a 12ax7 self oscillate?
The oscillation can be triggered by the operation of the tremolo footswitch, turning on the tremolo intensity switch or pot, or any electrical disturbance that affects the oscillation loop, which, if properly designed, is "anxious" to begin oscillating . Some trem circuits have small signal insertions that "kickstart" the oscillation.
@@UncleDoug thanks doug. Love your videos.
I kept looking for this question because this was exactly what I was wondering.
I think I have got it, the larger tremolo signal shifts the smaller guitar signal up and down the bias slope, cyclically sliding it aternately between the steep part and the flat part of the slope.
A viable theory, WC. It's interesting that the Tremolo signal does not seem to increase the volume of the music signal significantly, but does reduce it significantly. Perhaps the excursion into the "flat part" of the slope effectively shuts down the tube response (to music signal), while movement along the "steep part" of the slope has a much less dramatic effect.
Question : isn't negativ feedback preventing the trem to work at its best?
I mean, too much nfb will make the trem really weak
Hi Uncle Doug, when the intensity pot is to the right the tubes are biased by the -34vdc when the pot is set to the extreme left the oscillation goes to the grids, but what voltage is being applied? Is the -34 totally out of the picture? Thanks for your excellent videos!
The -34VDC should be applied to the grids, Alvaro.....but the tube bias varies in response to the tremolo.
If you've already answered this in one of the Princeton Reverb videos please let me know (I've watched several and have not yet found an answer to this question). On the schematic (shown at 10:14), what is the function of that .02 uf capacitor to ground coming out of the tremolo circuit? I see the 220k plate resistor connected to point B, and if my learning is going well enough that I'm not embarrassing myself, I think the .1 just before the Intensity knob is a coupling cap (please correct me if I am mistaken about any of this). But in-between I see that 1 Meg to .02uf. Again, unless I'm embarrassing myself here, what I've learned so far leads me to believe that's a low-pass filter, and if my math and conversions from microfarads to farads were correct, the cutoff frequency would be 7.95 hz. My question though is what is that for? What is it filtering out?
The broader reason for asking (other than I'm all-in on genuinely trying to learn this stuff now) is that in the amp I told you about last week that I am going to replace the cracked tone slope resistor on with the other capacitor I asked for advice about whether or not to replace (and will do so), the capacitor in question is that exact .02 uf cap at that identical point in the circuit. It's not a Princeton Reverb literally but the guy basically built a Princeton with a beefed up power section from what I can tell. I traced the entire circuit and component values to figure that much out. On the amp, the tremolo is pretty weak right now. If a cap drifted/went bad on that filter, it just made me wonder what exactly that part of the circuit/RC filter on the Princeton schematic is there for, coming out of the tremolo section. Realistically if I can't get it up and running all by myself I'll surrender to Nashville Amp, but before I do, I'm enjoying the process of learning as much as I can as I go.
Anyhow, I got some parts from Antique Electronic Supply as you suggested (although I ordered from Amplified Parts and realized it's the same store) and intend on pulling the chassis out this weekend. You'll be happy to know I've built a discharge tool for the filter caps so I'll be safe. I did manage to completely fix a "pop" in yet another amp that was making a noise every time I plugged in a 1/4" guitar cable to the input. Still don't have a scope or proper tools to see noise, but through a process of patience, experimenting, and ruling out all the tubes, I found a little bit of resistance between chassis ground and the volume pot. Loosened the knob completely and cleaned the chassis on both sides with 99% isopropyl alcohol, put it back together and my meter read perfect continuity. Powered the amp back on and it was perfect. Left it on for about 2 and a 1/2 hours and often plugged back in. Still perfect. A day or two later it was still perfect. Couldn't believe I actually fixed it at all (I may have a confidence problem) but finally realized I must have, put the back panel back on and put it back in the amp closet! So slowly but surely I'm figuring a few things out at least. Any words of wisdom you have on that RC filter in that Princeton schematic will be taken to heart. I do love learning
Greetings, Sean. Congratulations on your continued technical education and success. I'm glad you already recognized the cap as part of an RC filter. By watching my video on RC Filters, you could actually determine the filter type and cut-off frequency. As to the purpose of the filter, recall that the tremolo oscillator output is strictly electrical (to modulate the bias of the 6V6's).....there is no music signal.....so this filter prevents the delivery of any "noise" to the output tube grids.....only the modulating DC voltage.
@@UncleDoug Thank you so much. I sincerely appreciate you getting back to me. That makes perfect sense to me now. The part that confused me earlier was that I had not been thinking of the tremolo oscillator as strictly electrical, but of course that makes perfect sense when you explain it that way and completely answers my question!
Hey @uncle doug. I was wondering if it is possible to take the vintage fenders with these tremolo stages and eliminate/modify them into another gain stage? With a added master volume I'd think it have lots of gain on tap. If at all possible, can you also make a video on how to do something like that as a possible amp mod to do to ones amp? Basically asking if we can convert a tremolo into another gain stage Thank you so much!
I would not suggest it, Robert. You can't simply add another stage to an amp circuit without altering all the other stages.......and the end results would not justify the time, effort, and expense. I speak from sad experience. Fender amps are known more for their tone and effects, not so much for volume. I guess that was Marshall's domain.
I'm going to take a spin at the pop quiz.... The AC signal that comes from the Oscillator is Sub-Sonic... below 20hz. So even if it made the speaker move, it's not audible. I would almost think they could have put a diode in there so there's only plus DC or Zero going to the bias circuit...
I'm getting closer to getting this stuff. You did well my friend. Now if someone would just make a slow-mo animation of the oscillator working, I really think I'd have it nailed.
Greetings, DK. It's good to hear from you. Thanks for your kind words. Yes, the oscillator signal is subaudible, and your idea about rectifying it is interesting, but remember that it must be a full AC signal to accomplish its goal of boosting and suppressing the tube bias. Rectification would neutralize its effect. There is a second reason that the oscillator signal to the 6V6's is inaudible, and would probably be so even if it were of a higher (audible) frequency. Hint: Think about the output transformer.
Uncle Doug
Ok, here's my next guess... since this amp is Push-Pull, is it possible that the bias shifting signals cancel each other on the way to the output transformer?
Actually since they are in phase with each other, they would cancel each other within the OPT; but yes, you are correct :)
GREAT Video..... thank you.
Two questions i have wanted to ask for a long time.
1. Om both schems.
Is there no input signal to a Trem Oscillator.?
There is no guitar signal involved.
It just osculates its own signal that it creates at idle.?
2. On just the second schem with one triode.
It oscillates the signal from the plate, just like the two triode version above.
But is also uses the plate for the output signal that goes to the proceeding bias circuit.?
That seems rather counterintuitive..... if yo get my confusion.
Not exactly sure WHY i think it would be a problem, but i do.
Thanks Again 🙂
Thanks, M. 1.) It's my understanding that the oscillator loop signal is initiated by existing background noise in the circuit, then amplified. 2.) In a fixed bias circuit, there is a separate grid bias supply. The oscillator simply modulates the bias supply to create the tremolo effect.
In your diagram you drew showing the "on/off" switch for the tremolo, you didn't place a dc blocking capacitor between the plate of the tube and your switch. With the circuit as drawn, when the switch is closed, B+ will be shorted to ground and toast your power supply, power transformer, etc.
Swarf Rat Swarf, the diagram (at around 8:00) was drawn primarily to represent the phase shifting that occurs in the oscillator loop. It was not intended to be a schematic. You are correct that in the schematic of the actual circuit (at 9:29), the footswitch is insulated from DC by capacitors on both the plate and (biased) grid of the 12AX7. Thanks for your input.
Uncle Doug Can you explain in more detail this capacitors, I cant see why B+ will short to ground in the 9:29 circuit. Thank you for your amazing knowledge !!!!!!!
Greetings, Damian, and thanks for your kind words. If you look at the schematic, the plate (+270VDC) of the first triode of the oscillator 12AX7 is connected to the center pin of the footswitch, which is chassis-grounded when the switch is closed. The two oscillator caps, .02uf and .01uf block the DC plate voltage from reaching the switch and prevent a short from occurring. In my diagram (at 8:00), I didn't show any blocking caps and Swarf was asking about it.
I've been studying something similar myself.... and one question came
up... If in the oscillator, high pass filters are used... how is it
we get a SLOW oscillation?... since we're only passing highs?
Although the oscillator appears to use three high-pass filters, their purpose is not to filter, but to provide phase shift. The three RC pairs each provide about 60 degrees of phase shift, for a total of 180 degrees. Low-pass filters can also be used. Here is an excellent, basic explanation: www.learnabout-electronics.org/Oscillators/osc31.php
First I would like to thank you in advance for abiding my ignorance. And for the clear articulation of all things tube amp and canine domestication.
My question, where/how is the original oscillation generated? The tube seems to be all by itself with only an output to the circuit. If rusty is going to chase his tail, his head or tail has to be set in motion. The oscillation is self sustaining, yes, like if I play a note on the guitar then bring it in front of my amp it will ring out to the exuberance of my adoring fans. But what is the first cause? General instability?
An excellent question, Dr. H. From what I understand, the background noise in the amplifier circuit initiates the oscillation in the Vibro-Champ. It can take a second or two (sometimes a little more) for the oscillation to reach operating level. In amps with Tremolo and a footswitch, the abrupt activation of the switch "kickstarts" the oscillation almost immediately.
Hi Uncle Doug, hope you are doing well. While slightly off topic, I have a question that this video brought to mind. What happens to the fixed bias part of the circuit if you choose to convert to cathode biasing? Is the fixed biased circuit with the diode disabled? Does the tremolo then work by the same principles if the amp is converted to cathode biasing? Thanks in advance :)
I would advise against such a conversion, LHR. You would be fundamentally changing the performance characteristics of the amp. A hybrid form of biasing might be acceptable.....Fender tried this with several of their larger circuits.....but complete conversion is not a good idea, and would probably require some sophisticated circuit modifications to preserve the tremolo function..
Thanks!
@uncle doug,
Why is the NFB resistor changed from 22k (5f1) to 2.7k for the vibro champ? Im thinking of adding the tremolo to my 5f1. I have a 20k pot in series with the 22k resistor now and seem to like the amp better running closer to 40k resistance. Can this be left or do I need to use the 2.7k resistor instead?
Use whatever works best for you, RS.
I just bought a new 68' Fender Twin ReverbI reissue and I am getting motor boating in the tremolo speed circuit especially if I turn it up past five on the speed control. Is that typical? Thanks again for all the great vids.
Hearing a tremolo pulse is not really "motorboating", BC. This is a common issue and generally associated with the photoresistor-neon bulb unit. There are several Internet sites dedicated to the eradication of this problem. Here is one suggestion: The official Fender solution (Service Bulletin number 9)
"The ticking caused by the Vibrato is caused by improper lead dress.
It can almost be "cured" by connecting a .01 mfd 600 volt mylar capacitor
on the 10 meg ohm resistor in the vibrato circuit. this resistor is located
on the parts panel. Remove capacitor across the 10 meg ohm resistor (old
modification) if in place."
(That is, run the cap from the junction of the 10 meg resistor and the
optoisolator to ground.)
"If this does not produce the desired results, then the leads should be
dressed as follows and excessive lengths shortened.
1. Dress the leads to the vibrato speed and intensity controls away from
the tone controls and filter leads.
2. "Bunch" the leads to the components on the parts panel which connect to
the tube socket of the 7025 (12AX7) vibrato tube."
vintageamps.com/plexiboard/viewtopic.php?f=8&p=808085&t=75329
Or for the blackface PCB reissues, there is a guy called Fritz D Cat
that has a simple mod to cure the ticking tremolo. Similar to the #9
Service Bulletin, but geared twoards the PCB reissues: "...the cheapo
ceramic disk cap in between the opto-isolator and the electrolytic cap
is the mod. I used an 0.01 uF @ 1KV ceramic disk cap. It's soldered to
the north side of R45 and the south side of C24." see here: www.fenderforum.com/userphotos/index.html?recid=67321
Thanks so much for getting back to me Uncle Doug. I will try returning this new Twin as it was out of box with no manuals or cover. I will then test the tremolo in the new amp. If it still prevails then I'll recommend these fixes to the warranty technician. Thanks Again B
You're welcome, B. Good luck :)
Thank you Uncle Doug I will keep you informed of the fix for the tremolo problem. B
You're welcome. Good luck :)
Uncle Doug- How difficult would it be to put in this bias wiggle type tremolo into my ol 68 pro reverb ? I really prefer the bias wiggle of the smaller brown face amps and it doesn't look too difficult?? Has anyone ever approached you with this type of job
The problem with bias-modulating tremolos is that they work best in lower-wattage and SE amps. The idea of modulating the bias in high-powered output tubes, i.e. in a Pro-Reverb, is not a good idea.