1967 Marshall JTM50 Plexi - What does CRUNCH tone look like?
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- Опубліковано 26 лют 2021
- In this video we'll respond to some recent criticism that has ben leveled against me and take a look at this 1967 Marshall JTM50 Plexi head on an oscilloscope, and we'll talk a minute about distortion and what the "angry" Marshall crunch looks like in graphic form.
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CORRECTIONS:
My formula for calculating RMS is wrong here. I just did a quick search for the formula prior to this video and the source was in error. The real formula is
RMS Power (Watts)
= (Vpeak * .707)^2 / R
So that means the total peak power is about 64W and the RMS is closer to 26W.
Also at 6:27 is say "watts" but mean "volts".
Sorry for any confusion.
kool ...heyhey Brad , found a guy here @ Ca , to repair my old Musicman HD 112....thanx again for your interest , Nice to know yer there
Robert J
@@RJDCR Hey! I fixed my buddy's HD100-112... His trouble was that the board was fried, the traces were flaking off and the board itself had become conductive. It was fried beyond repair so I fired up the CAD program, re-traced the schematic and designed a replacement board and had a batch made (min. order was 5 anyways)... If ever you need a new board for yours, I have a few spares handy!!
@@G60syncro kool , thank you
Your formula above in this comment is correct, but you must have pushed a wrong button.
Wrms= (Vpk * .707)^2 / R or Wrms= (Vpk / 1.414)^2 / R
= (32 * .707)^2 / 8
= 22.6^2 / 8
=512 / 8
= 64Wrms or 128Wpk
Wpk = (Wrms * 2) or Wrms = (Wpk / 2)
@@natb9121 I was taking the 20V peak from my pre distortion sine wave. Plug that in and see what you get. The 64W figure was using the 32V estimate of the peak after distortion, which isn't a figure we can really trust in reality. It's just an estimate.
I have been designing tube high fidelity amplifiers for nearly 50 years, yeah I'm old.. Instruments are a vital and essential part of both engineering and repair diagnostics, but NEVER discount the actual sound. I always argue that it takes a good set of tools AND a good set of ears.
I think you’re right, and I would benefit from better use of the tool. (That’s what SHE said.)
@@TheGuitologist 🤣🤣🤣🤣🤣🤣🤣🤣🤣🤣🤣
Yup - how it sounds is FAR more important that how it looks on a scope for a guitar amp. The anal audiophile aficionado may disagree however 🤣
@@Pilotltd ah ... so that's where they are putting their test probes? Sorry to lower the tone :-) ... (audiophile: no we're good ... more probing)
I'm 74 and still do not know what I want to be when I grow up. In the meantime I have formed a power trio to do my 24 tunes. A much younger bass player and drummer. I needed players with a full tank. lol I am on my 3rd Marshall 50 watt Plexi hence the visit :-)
I think it would be cool to see the scope once in a while.
After clicking the video notification, I expected to immediately land on a picture of the Guitologist giving his critics the finger.
From working in a high volume repair facility the only thing we used was a load, scope and generator. Plugging into speakers was frowned upon. You didn't want to aggravate your co-workers.
That's the only good reason to do it that way, and it's easier to measure voltage, amplitude, etc by sight. you can compare waves, etc. But I really prefer listening. You can pick up so much by training your ear to hear problems.
@Busted Junk Studios - No seasoned musician with a good knowledge of tube amps & how they sound play & feel relative to original manufacturer specs with original components & component values from original parts manufacturers... would ever take their amp, especially tube amp to a mass repair facility like you worked at. Repairing an amp without listening thru a speaker with trained musical/gutarist-type ears who know how they originally sounded tonally, how they break up, where they break up, what type off clipping sound/etc... is ludacris and tube gear blasphemy.
DagwoodMMA Agreed, Amplifiers are really an art form, at least they can be. I purchased me first tube amp back in 1974 and will attest that Amplifiers are as individual as guitars. In addition to tone there is also feel and how they react to playing inputs.
Some players are just not at the point of experience to realize how to use or take advantage of the characteristics of an Amplifiers design.
@@hkguitar1984 until I got my own valve amp I wasn't able to understand what you say. I get it now. But I think @buster junk studios is keeping it real. They repair to spec. It is not possible for them to replicate the personal nuances of all the possible customers. So use science to calibrate - which is how it is done at the factory.
@@TheGuitologist alas you are not the QC of the original manufacturers. And even if you were likely you bias the sound to your preference.
To me the best approach would be to be as skilled as you are to set up an amp to personal taste - but honestly that's unlikely to be possible for most users and the rest might get Darwin awards in the musical category.
Measurements are most likely, on average, to yield most consistent output in every sense of the world.
Wow! This is Master Class 102. For those of us who don't know about ACTUAL RMS output, and what bias does to the waveform and output this gives us so much to study. I really appreciate you taking the time to explain all this, professor. Cheers.
Thanks for helping to inspire the video!
@P J Pachasa Jr thats what I understand. but once things start to melt, you could take out a transformer or other high dollar parts. AC/DC biases their Marshall’s so hot that they blow up every other gig.
Did a quick service job on a bandmate's JCM800 Bass Series 50w a couple of years ago. Un-modded the amp by removing a few tacked-on resistors and cleaned the pots, jacks and tube sockets. Biased the EL34s at 70% MPD. Clean RMS output was 35W, just like on the JTM :) Put my bass through it at the next band rehearsal, worked great and had enough headroom.
Great info Brad,
Love hearing you respond to criticism in clear and intelligent fashion and explaining why you do what you do.
I have learnt heaps from you over the years and really appreciate it.
All the best from Downunder Mate
👍✌😎
I love the analysis between sound and “scope”
Very insightful
Thank you Brad, a great follow up that totally wasn't needed but is very much appreciated.
Awesome follow up video, brad! Thank ya! This reminds me of my 85’ jcm800 50watt head. Same square light and two metal toggle switches. The knob/ control layout is the same too but I only have two inputs, hi and low.
I love the use of the scope! Thanks, Brad
You're really good at this. I know very little about electronics, but its always pleasure to watch someone who loves his job - and does it well.
Brad, 0.707 is only used for RMS power calculation for a perfect sine wave. Once distortion is added, it becomes a difficult calculus calculation involving all the harmonics that were added.
THIS^
Difficult for some folks. ..........
That's why he says the number he said.
I can do inverse convolutions in my head
Easiest way to deal with RMS on a distorted waveform is to use a "True RMS" meter - ((Vrms)^2)/R. Just make sure you're staying within the frequency range where the meter is accurate. Also wanted to point out that in your measurement, you're mearing "zero to peak", not "peak-to-peak". You were doing it right, but I'm not sure as it came across.
I like the addition of the scope !
That would be great too have a refresher on the old scopes , just getting back to it . Got my variac the other day, now I need a scope , just going to take time to make sure I've got everything I need. Great work on the Marshall ,I've never seen a model like that one . No wonder it's rare . 🎸
I highly recommend Uncle Doug’s Oscilloscope tutorial series and Blueglow Electronics new series he’s doing. Both are excellent!
@@TheGuitologist Right on Brad ,I watch both .So I need to check it out .
Thanks for all your vids too ,they're great entertainment and informative .
I like you letting us hear what it's doing. Makes instant scence. I've learned alot jst watching..I blew nothing about tube Amps and never dare buy one...but now I want one there not as scary as they used to be .also other topics are great .I enjoy it
Been awhile since I been on your channel, sorry about that! I love what you do you have been helping me with all sorts of things for years now. I am having a problem with a fender bassbreaker 15 tryin to figure it out and your channel is my go to for amp repair.
What's the Bassbreaker doing?
I have one and it's been nothing but problems. I'm just gonna cut my losses
The fender Bass breaker just so happened to break more often than not.
That's why I wouldn't buy one even for $200 because so many broken bass broken I mean breaker amp
Great vid, Brad! Marshall's have their place, as you've shone.
You have taught me more than my classes ever could have.
I'm gonna rebuild a Crate V100 and build amps from scratch do to you...
Thank you....
I love when You take one more step to explain! Greetings from Sweden!
Thats the Lemmy oscillation on the bass control, don't worry the 4-15" Marshall cab smoothes that right out.👍
Thanks for showing the oscilloscope waveform!! I have a background in electronics and computers so that is what I know. Analog vs digital. It was weird trying to figure out the waveforms for guitar amps. All the troubleshooting videos showed troubleshooting without a scope! I just couldn't figure out why they would not use such a valuable tool!! Listening to the function generator for any length of time would quickly get on my nerves. But... It would be nice to hear a demonstration of what each waveform sounded like. I missed this video when it came out as I wasn't 't subscribed to your channel at that time.. Keep up the great work! Its both educational and entertaining.
I think you got a great critisism. You are a great teacher. You were pushed to do better. And in this video you did.
You create great videos!
Hi Brad, love the videos! Another good tip is to connect your Fluke multimeter to the output of the amp and set it to read AC volts. The Fluke meter shows true RMS without having to calculate anything. To calculate watts, just square that RMS voltage from the Fluke and divide it by the load resistance (most likely 4 or 8 ohms). Using the scope, you can run the amp just up to clipping for the first measurement (maximum clean watts) and then run it up to max output to measure full output wattage. Either way, the Fluke meter will read the true RMS voltage. Keep up the great work!
So cool to see the scope. I dnt know anything about most of what you do but I love to see it..dies the scope do the same thing as a heart monitor. Looks simular... thanx for the content love the channel...hopefully see u in some guitar channel live chats .lol always awsome to see ya there too..like robert bauer's chat. I always smile to see you pop in the chat.. stay well my freind thanx again👍👍👍
Cool old classic...nice job sorting it out for your channel followers..👍
The oscilloscope is very very interesting !!!!
I’ve heard from several sources that one of the most important pedals you should have on your board is a good EQ pedal, because you can change and sculpt your tone in a wide variety of ways to your hearts content just by making subtle adjustments to the EQ.
There is something to be said for that. I'm getting close to opening some viewer mail by the way.
You are my source for guitar amplifiers, great stuff
I understand everything you say... is stuff I don't need to know, but I'm glad you do, along with every guitar tech I've paid not nearly enough to.
In my old days when i was troubleshooting i almost never needed to reach for my DMM because if you have a scope on your bench then all you have to do is connect to your voltage ref/common and can instantly check multiple voltages, power and signals at an instant. At the bench i only used the dmm for continuity and resistance/impedance checks
cool amp & interesting video - have heard of asymmetrical clipping in relation to the classic overdrive pedals from the past, never really knew what it meant until now.
I'm not sure I prefer this method of troubleshooting. But I really appreciate it because, as I am learning more and more about all of this stuff. I do not own a Scope yet, at some point I will get one. That being said. I suppose it's going to be up to what each specific viewer comes here for. I started off watching videos like yours, specifically because I love seeing different amplifiers. Now that I'm taking a larger interest in service and repair, it has become something else.
Anyway, pretty friggin cool amp!
Nah, clearly his quantum flux was deviated by the inverse phantom Atlantian dodecahedron squared. Anyone could see that.....(btw, sounds killer, Man).
:D:D:D
Reminds me of my senior year high school English teacher who’d give you a star on the blackboard if you did something bad. And if you were really bad you’d get a circled star. And if you were really really bad you got a star circled and square rooted. 😆✔️⭐️⭕️
I think the co-phasing loop became asynchronous with the bass modulator that created an enharmonic audio stream that was in contention with the tone stack parametrics.
Being an electronics dummy, I enjoy seeing how the analytics tools work. And the sonic results. Thanks.
Used to have an old bass amp 2xEL84 at about 13W power. It was designed to not produce distortion, linear volume and just 3 gain stages. When cranked the thing was producing a nice crossover distortion - usually it's unpleasent sound but this one was gnarly in great way, hooked to G12 K100 it sounded great.
Most people that watch the channel have no ideal what the scope is for it what it does or even how to read it,I enjoy the way do things I enjoy hearing the amps you repair ,I've learned alot of things from you're repairs you do my father was electronics engineer and could repair just about anything from televisions to amps,so I know things from my father as well such as diode and capacitor testing ex..and can go through things myself if I have schematics for the equipment I'm working on and what each diode or capacitor ex. is supposed to read that's how you learn ,but most of the people that watch you're repairs have no ideal what or how the scope works so how can they criticize you in anyway when they dont know from the start even what an ohm meter is or does ,it blows me away that someone would even reply that ,anyway the videos are spot on dont change the way you do something just because post a comment on something such as that ,anyway take is easy brad
When I saw the O-scope in your last video I thought my browser had jumped to an episode of Mr. Carlson's Lab. None the less it was interesting to see you use one.
I always enjoy your videos, Brad. A couple of thoughts on this one are below and I've included a bit of info for those who don't do this for a living and/or haven't been at it for decades. If anyone finds any error with my info, let me know and I will re-check.
Note that measuring a clipping waveform peak value or peak-to-peak does not result in a true rms value. That approach must be done with as close to a sine wave output as possible. With a true, clean sine wave the rms value is 0.707 but as the waveform clips more, the value increases until a true square wave reaches 1. Once this is the case, the peak voltage is the same as a DC voltage and its absolute (peak) value is used to determine the power output as is.
When I check power output of an amp, I plug into a dummy load. There is another bit of speaker cable also going to a couple of terminals on a strip near the scope. One of the scope probes is almost always hooked up to these. I also put an extra jack in parallel with the input to the dummy load so I can plug a cable into it and plug the other end into a meter set for rms AC voltage readings. I adjust the output of the amp to just on the edge of clipping and take note of voltage indicated by the meter. Square that voltage and divide by the impedance (most often 8 ohms, but verify it anyway) to get the rms power.
20 volts into 8 ohms is 50 watts, and 28.28 volts into 8 ohms is 100 watts.
Need to determine what voltage you need to look for when looking to see if an amp delivers what it should? Multiply the impedance and the power rating and take the square root of that number to get the rms voltage needed to deliver that power rating into that load.
e.g.: voltage for 100 watts into 16 ohms?
100 X 16 = 1600
the square root of 1600 is 40 volts
Are we sure?
Try squaring the voltage and dividing by the impedance to double check:
40^2 is 1600
1600/16 = 100.
50 watts into 4 ohms?
50 X 4 = 200
the square root of 200 is 14.14 volts
Are we sure?
14.14^2 = 200
200/4 = 50
yup.
Also remember that the value of 14.14V is half of the 28.28V required to deliver 100 watts into 8 ohms as noted in paragraph 4. This is a further indication that the 14.14 value is correct.
From the way the signal is fluctuating, I think the filter capacitors may have a problem, even if they are new. There is also a good chance the lower capacitance values were part of the original design for a few reasons, such as cost, and space available. Another would be to intentionally have the high voltage be pulled down at higher output, which compresses the signal, resulting in a slower decay of the output. This remains an approach used in many tube amps to this day. If the caps check OK, there's not too much hum, it sound OK, and behaves the way the owner likes, that's fine too. Larger capacitors would reduce the compression, and while some prefer that, others do not. To each their own.
The capacitance and the ESR (Equivalent Series Resistance) of filter caps should be checked. The ESR of a capacitor is a resistance to ground, and as it increases with age, less of the ripple voltage is bled off to ground. This results in a 120Hz ripple in the supply and looks like the fluctuations shown on the scope.
Even for those who have a meter that can measure capacitance, a device to test ESR is equally important. There are 2 testers I like. The TC-1 and the MESR-100. The TC-1 is $20-$30 on Amazon (search for "tc-1 tester"). The MESR-100 is $55-$70 on Amazon (search for "MESR-100"). These take the doubt out of capacitor testing by checking capacitance and ESR.
The TC-1 can auto-detect and test various components. Just connect the component and press the button. Just be sure caps are discharged first or your brand new tester could be cooked!
The MESR-100 is primarily for testing capacitors, but can also be used to locate shorts in something like a string of 20 op-amps or a bank of output transistors. It can measure resistance down to 1 milliohm and the closer you get to the shorted component, the lower the resistance. Again, be sure to completely discharge the power supply or it could be "good-bye tester".
On a side note, when the meter is not plugged into the second input jack for the dummy load, I run another cord from it to a current dropping resistor of somewhere around 150 ohms or so, and then back to a speaker input. This results in the dummy load taking most of the power while a small fraction of it reaches the speaker so I can hear what it sounds like, but at a much reduced level to avoid irritating co-workers and rattling things off the shelves.
Nothing wrong with how it sounded or how it looks on the scope. Good job 👍
Thank you! Love your videos!
brad would like to see u do a hi voltage rms measurement with a bias probe on the 13 or so jerks that disliked this video...keep doing your great work.
I have to say that your skills are geared to making these amps sound great. A scope is good for visualisation of signals
. Good for diagnosing component failure and measurement of levels, You need your ears to know what sounds good when tweaking something. Keep up the good work on dialing in those sounds , that we all love from a cranked guitar amp.
Great work Brad very Educational 5*****
I think displaying the amp and the way it sounds is a way better format for the videos (in my opinion). Keep it up man.
I think it would be cool to hear the amp and see the scope so we can visually see what’s happening as the tone changes. I would find that very interesting for sure.
Thanks for adding to a already cool video
Thank you very much for presenting JTM50 on the oscilloscope. Nice to see how it distorts gently. It is such a rare amp and you are the first one on youtube showing it inside. I have recently built this legendary amp on ClassicTone transformers (rip CT). And I am going to compare my one under the scope with yours original one! You provided me great material for comparison. Do not worry about haters, job well done. Greetings from Poland! :D
I’d love to see your video. Feel free to use clips if you need them.
@@TheGuitologist How it sounds ua-cam.com/video/CANviOlU24I/v-deo.html
Measurements: ua-cam.com/video/5b6qpnf90LI/v-deo.html
I had 55W rms without clipping on SS rectitier, later I added switch to GZ34 rectifiet, but havent measured power yet.
Where a scope really becomes useful is when you're tracing-out the audio signal path in a guitar amplifier or pedal, you can get-by with a multimeter and some deductive reasoning though, but sometimes some electronic faults can be hard to find unless you have some way of seeing how a circuit is behaving, that's where a scope really comes into it's own.
How's about both scope and speaker simultaneously? Could you advise how to safely connect a scope to the output?
It's interesting to see the notch difference with the mids and treble. Reminds me that EVH had a 50k mid pot in his plexi
Hi, i found an old oscilloscope in the garage which i apparently inherited long time ago. :D I am all for more oscilloscope stuff on your channel as it was fun to watch, even though i dont have much a clue about that besides some very basic physics courses. Teach us some more so i might have found a new toy for setting gain on my amps. ;)
I think there is a small mistake in the output power calculation. The correct formula is Prms=Vrms²/R with Vrms=Vpeak * 0.707, so output power of the marshall is (32*0.707)²/8= 63.98W which is closer to the 50W (clean) specified.
Sick video, as always. 🤙
Keep doing things your way, Uncle Doug goes into theory while he does his videos and shows how he calculates plate dissipation or any other technical fine points. The flow of your videos is easy to watch and in your zone.
Thanks for the feedback. I may end up doing more of a hybrid thing where I do a little of both ways. We'll see.
Was just thinking that too.
I use the scope with dummy load because I hate getting some rough sound or loud “testing” by surprise that scares the crap out of me.
I dig Uncle Doug, but when he gets really heavy into the theory and drawing everything out... he loses me a little bit.I love his repairs and his amp builds though. I like how Brad just does the repair and let’s you look over his shoulder and explain things as they need explanation. In all honesty though, I’m not a fixer or builder, just a player who likes seeing how this stuff is done and what amps look like inside. Cheers.
Maybe, just once in a while, you do it tutorial style (this amp is a delicious opportunity for that!), but I don't need it on every repair video, LoL AC/DC uses the same amp, though.
I agree with this. I think a lot of your audience are players who like to see how shit works. You're a killer player who shows how shit works and then plays through said shit. There are other by-the-book channels. This is our fuck-the-book channel lol
If you are asking for preferences, I like sound better, but it is interesting to see how the scope works. It was interesting to see how radical the bass got on the wave without the treble up. Makes me wonder what that would have been sounding like from the amp. I'm betting that you dialed out most of the bass because it was over-expressed in the amp. Great video.
Might be an interesting project to make an adapter for the scope similar to what you have for your tube sockets that sits between the jack and the lead so you can do both. Have a load through the speaker and still be able to observe the waveform.
It wouldn't be hard to set up a switchable load to switch from speaker to dummy.
good video brad, ive talked myself outa buying an oscilloscope many times, i think i just prefer the cap probe and speaker method for my junk i build
I can do everything I need to do with guitar amps without a scope. If I did more working where silence was necessary or where I was doing more hi-fi stuff or radios, I'd probably get better using a scope real quick.
Your work us always great! Don't let a little criticism bother you, it means someone is paying attention!
Thanks. It's no bother.
> The Guitologist
You can get some more information from the oscilloscope if you lengthen the time base and adjust the trigger level. The longer trace may indicate that the wobble on the sine wave is mains hum, and the longer trace may indicate that the trace with the extreme tone settings is mains hum.
You're one heck of a tech! I would like if you demonstrated using both speaker and oscilloscope; the sound is immediate gratification, however, the oscilloscope shows more depth of information.
Can you provide sound and scope output at the same time?
Hey Brad do you know how I can get a good reasonably priced Tone Bender and do you think they'd work well with an AC30 and Tele, 335, and Strat? Weird question I know but I trust your opinion.
Can you run a speaker and the scope as well? It would be interesting to see what happens to the signwave and the sound at the same time while adjusting the knobs.
Love your work Brad. 👌
I like both ways, I will say it can get a little boring with just the scope. I watch d-labs, who is a freaking genius, but he doesn't know how to play guitar and usually uses the scope to show what it can do, he does, sometimes has someone play. But I enjoy watching ALL of your videos, whether it's amp repair or spf or just venting. I agree with a lot of your views on things. I do appreciate you showing how to use an oscilloscope, I've never used one so that was pretty cool. Thanks man.
Love Terry and love D-Lab also. I like to HEAR an amp screwing up because it's more of an audio-visual experience for the audience and also it's how most guitar players know something is wrong with their amp - they HEAR something wrong. It's more tactile, I guess.
I only use my scope when I have to, I agree that troubleshooting is more fun than the alternative of not using your senses!!!
I would assume that the former complaint made about the crossover distortion being not so pleasing is merely subjective. That being said, for educational purposes, could the “crossover” issue perhaps be remedied by sending the signal from channel 2 back to the phase inverter circuit and adding resistance??? I’m picturing as if it would work, because I’m a noob. Just trying to learn as much as possible about solving these little “issues”. Any advice on this idea?
I like to hear what the Amp is doing first. Using the scope also will help find and discover issues. Both would be the best of both worlds.
I have a fender pa 100 I would love to have you mod the different inputs. Be a cool vid series I think too!
Scope or speaker, with the demo you include there is always a sense of what the amp sounds like.
Simply one of you tube’s best channels. “You don’t fuck the future Paulie, the future, it fucks you”. Sometimes the future is amazing.
We still love you
Do your thing Brad. If you need the scope, use it. I like your “plain old videos “
I say use both, speaker and machine. It would be cool to see and hear the point of break up and overdrive at the same time. Seeing it visually while hearing it would be cool. I'm just getting into pedal building and amp building projects so this is super great timing for all of this stuff.
Could you adjust the scope ("zoom" or trigger) to better see the waveform when you turned up the bass?
I'd suggest slowing down the sweep so you could see the lower frequency.
Uncle Doug did some vids with a Scope, (including one where a mistake was pointed out to him - which he analysed and corrected in a follow up), but generally does not , in the view that most folk looking at their own acquisitions wont have a scope to hand, or trained as such how to use them. If we get vids with them, its a bonus, and we learn together. If none, no worries.
Very interesting. It just so happens that I have a 1987X reissue on the bench after a major failure and I'll be interested in checking my results against yours. It should improve my scopeology.
I'd like to see that too.
13:00 I don’t know shit about electronics or sine waves or oscilloscopes but seeing the change in the wave by Brad changing the settings and levels really explains better to me how amps are tricky bitches!!
How long does it typically take you to repair an amplifier? I've had one of my amps at this local tech shop and it's been there close to 3 months...
That’s typical. There aren’t many of us and lines are long. A friendly reminder or inquiry might hurry things along a little.
Did you change the dummy load ? It was 16 ohms on the previous video (AFAK)
I wanna hear the before and after. Do what you do.
A little of both, but I like to hear you play.
It is always practiced to set the bias with a clean sine-wave, and least crossover notch. Then you take the ac voltage output, square that, and divide by the output impedance. That # is your wattage.
That would be the PEAK wattage. I set bias using 70% of plate dissipation as a standard. Never used the method you describe. But maybe I'll do a video soon where we try it one way, then try it another way and compare how close the two are. There is a THIRD way to bias an already familiar amp and that is to bias to the overall wattage dissipation of the entire amp. We might try that too. Thanks for the idea!
I just recently traded for an oscilloscope, I've always used a speaker simply because I didn't have an oscilloscope. So I enjoy seeing this method because I'm still learning. I thought just using signal and speaker( small 5w amp) was the poor boys way of testing/tracing. so maybe add some scope work into some future videos, it would be interesting. I also notice you have two scopes one old(like mine 30MHz) and one newer model. what are the differences? I didn't think you needed much range for audio. do you have any knowledge about the self biasing systems on some of the newer Bugera amps, and how they work?
I have a 1974 super lead 100 that I would like restored to orig specs (it was modded) can you do that???
For me having a electronics background (now retired) and also being a fellow guitarist, the best electronics engineer on youtube and this not being a criticism of Brad in any way shape or form can be found at Mr Carlsons Lab who also works on guitar amps now and then but he don't play guitar, best leave that to Brad, who always has one of the best endings on his video's and is such a pleasure to listen to. I always found having a career in electronics a constant challenge having to keep up with technology to the point of what's the point as didn't enjoy it so much in the end as proved to be a thankless task. The best days were for me the old days with valves and big fat transformers rather than todays dreaded switch mode power supplies, class D amplifiers microprocessor controlled microprocessors and miniaturisation of the already miniaturised. I live in the past I hear you say, well you'll all get there eventually.
Be the man your kids want you to be.. No need to defend or explain. Do the best you can for yourself, your family and all those you come in contact with. I will keep watching and promoting.
I appreciate that.
That wobble on the sine wave when you crank it looks like a power supply/filter cap issue to me.
Yeah, it seemed like some kind of motorboating (output is affecting power supply which is affecting a prior stage). But I'm not sure why it would clean up when turning up the mid and treble unless they decreased the gain at bass frequencies.
It would be great to see the scope and hear it through a speaker at the same time........
I would have love to have heard it through a 60s/early-70s cab with some old 25 or 30w celestions.
60's Marshall cabs had 16 Ohm 75W Celestions - 4x G12T-75's.
Yes, more oscilloscope please! :)
Do both! Great stuff!
I prefer this type of content, amp repairs.
Anyway, yes, always interesting to see the signal (obviously when problems are presents)
On other hand, it could be interesting to listen to the oscillation you show with only bass setting ^^
Adding the scope sometimes would be cool.
Blackmore use to say that his bass knobs were always turned all the way down too on his Marshall majors.
That’s been my Marshall experience. 0 is extreme, but 2 or 3 is very normal to get that driving crunch with NO FLUB.
Thanks, Brad. I don't know how anyone could consider that a master scope class. Do what you feel like doing. It doesn't matter to me either way.
Oscilloscope & Variac is the king in working out amp problems
I found a early 70's Plexi with a matching cabinet from a pawn shop in 1990 for $500 ! Well the owner wanted $500 and I only had $375 to my name with a week until payday ! I offered him $350 without even turning it on and he laughed at me . So I went home and grabbed my guitar and tube screamer and some cables and went back and tried it out ! I remember with the volume at about 3 it was plenty loud but kinda thin and harsh sounding . The owner said it may need new tubes . I told him that I think these old Marshalls where meant to be cranked to really sound good and he said go ahead and crank it up ! Oh my God in heaven ! It may have been the loudest half stack I've ever heard and man did it sound great cranked up ! Like punch you in the gut when you hit a power chord ! I was in a cover band that played Ozzy and Megadeth and Pantera and I remember running through Crazy train and Mr.Crowley on that amp in the store . I finished my try out and told the owner my situation and he looked at the ground for a few seconds ............looked up at me and said give me $325 and get it out of here before I come to my senses ! Fast forward to @ 2000 and I was dating a beautiful lady and not in a band anymore ( to many problems with attention from scantily clad drunk woman and too many drummers on coke) so my beautiful girlfriend made me quit the band so I took my amp to a local music store and told the owner who I knew pretty well what I had and would he be interested in buying it ! He said sure he'd take a look at it but tried playing that whole it's and old amp and I have a store full of brand new Marshalls but what are you looking to get out of it ? I didn't really know what I had until I said $700 and he said the most I would give is $500 for it and that's after my tech inspects it . He had his tech come out and put it on an oscilloscope and then had me play it cranked wide open . He looked at the owner and said "Give him the $700 or I will ........ don't you dare lose this over a couple hundred bucks ! I walked out with my money thinking I had got one over on my local music store instead of the other way around ! Now it's 21 years later and I would gladly give $5000 for that same amp ! By the way within a month of me selling my Plexi ..............my girlfriend got a job in another city and left me ! D'OH !
My guess is that the 50W rating of the amp is basically what the amp will put-out when it is just on the verge of going into clipping distortion, it's a common way to rate the output of amplifiers.