I'd like to just say that I appreciate the videos that you make especially on amp repairs Mr Stuart. You're channel is one of amp repair channels that watch. Kudos to you and goodluck on your channel!
When I taught troubleshooting techniques, I emphatically stressed the importance of history. Always assume someone else has seen the same problem. In his book, "Guitar Amps and Their Problems 2.0" Michael Fratus describes this exact failure. His book contains over 2500 amp repair cases. Every amp tech should have a copy. It just may prevent a sleepless night!
Well done! This video is more realistic in the sense that you take us along through the diagnostic process without a lot of heavy editing. This way those of us who do maintenance on our own amps can actually come away with something to remember for future reference. It reminds me of my high school days in electronics lab.
I have watched many channels, many techs online and have a real love of amps and the old-school valve tech. You sir are becoming one of my favourites! Your honesty and explanation of the full diagnostic process are proving to be not only enjoyable but really educational. Thank you. Please keep them coming.
@@stevelark9840 Yes Uncle Doug of course, he got me started with the idea of videoing my repairs. The Guitarologist is also pretty good. Also Psinoc Audio are great and very knowledgeable. There's also a woman (cab;t recall her name) who is a bit of a beginner but is doing good work out there her channel is Fazio Electric. I also enjoy Fran Blanche of 'FranLab'.
The only good thing about a truly baffling problem that you eventually solve, at the cost of great time and effort and brain cells is that you will never, ever forget what the fix was!
Your verbal tour... or dialogue on your diagnostics.... mirrors my own Internal logic circuit perfectly. Good job . I get a kick of the difficult ones also. Nicely done. I'm a bit relentless or compulsive on those sort of problems. (My coffee mug also says 'Grumpy Ol' Man'...very strange. So funny.) They must clone us somewhere. Hah. From the Sea Islands of S.C. USA. CHEERS !
Hello Stuart, thank you for showing this fault Diagnostics, alot of people may not understand how much effort it takes, I'm sure your other tech mates can help when something is not so obvious such as the PCB on Marshall DSL tsl amps that got me on my first fault finding mission. As a Electronics Technical Officer in Communications PBN and every other transmission device in telecommunciations that's 15 years experience before you progress to upper levels of building Networks World Wide. Cheers I enjoy your channel and respect the knowledge base and be so careful when dealing with Valve amps.
Hi I'm pleased you are enjoying them! Yes I'm quite careful. 400V DC is no fun - but was talking to a friend yestrday who regulaarly worked on 33Kv (!) equipment. Yowza!
Great Video, As it clearly shows that we sometimes do mistakes. I'm an amp tech and juste like you I had an issue on C11 on a Pro Junior too. Like you I spent hours finding the cause. Instead of your case, the amp was working fine until it behaves like a compresser-sustainer when pushed. The capacitor had no sign of wear or burn, and it blocked DC when testing, but the remaining 90% capacitance clearly showed that the lost 10% had turn into leak at high volume.
great job mate..so good to see your wizardry...one correction about all musos being great people ??? how many drummers do you know ha ha ha. not joking there a breed of there own..whats nice to know im a producer and when i go to bed I'm going maybe redo that bridge or remove it etc..your a perfectionist that means and dedicated..love it
I can tell you do this for the love of the challenge. We all develop our own strategies over time. And sometimes, hard to avoid confirmation bias. Being able to back up and go in another direction is what brings home the win. Aint it the truth, it is never as simple as a lot of videos represent it. Still watching here. :) Nothing like a good struggle to entertain us.
Yes I like to keep the struggle in the video if possible. All too easy to make yourself look like a brilliant fault finder. What you say about confirmation bias is SO true.I have stories....
Colored magic markers or Sharpies are handy for marking the male spade lugs on the board; it can be removed with alcohol, but I generally mark the thin end or edge of the connector so that the Sharpie colored marker can't be seen when you plug the female connectors on again. Of course, taking a picture with your smartphone is always a good idea.
Hi Mr Stuart! Love the channel, new subscriber here. I am a professional luthier/guitar tech and have owned and operated a guitar repair shop in Texas for the last 15 years… and as you mentioned, due to such a shortage of good amp techs around and the one I had been using retiring, I’ve decided to take my knowledge and experience with guitar/pedal electronics and expand it to servicing amplifiers as I get requests for it all the time. I’ve got what I believe could be an issue with my Fender Silverface Bassman 70 that brought me to this video: I believe it’s got a low output “problem”. I use quotes because it’s not necessarily a problem as is and what I mean by that is, that amp still sounds good it seems to me and I am actually liking that I can turn it up and it’s not killing anyone with volume, but I’m running a single 12” speaker rated probably around 80 watts and with the volume up it still seems way quieter than a 70 watt tube amp should be. My question then is, if there’s a problem in the amp causing low volume/output but it still sounds good (no weird distortion or anything) and I actually like that it’s not so loud especially for recording, could that be risking any future abnormal fault or failure of anything in the amp if I continue to run it that way? I would hate to ruin the tranny. Thanks for all the information you share, it’s much appreciated!
Hi. Great news on your getting into the amp repair business. Watch as many CZcams videos as you can as you can learn a lot that way. On you amp that's a tricky one. If you're going to start repairing amps you'll need to get yourself soe way of measuring the actual power the amp is giving. I do that with a dummy 8ohm load and I hook the scope across that. You can then work out the power (if you don't know how ti do that jut ask me). You're going toi need to start getting some test equipment together so a scope and a signal generatot are essential I'm afraid.
Thanks very much for sharing your work. It's the only vidéo i found that shows this issue and hiw to fix it. I have recently bought a hot rod from 1996, it sounds great but the volume is low. I hope the issue comes also from that C11.
I have been thoroughly enjoying your videos. I started looking at your videos to see if I could diagnose the problem I am having with my Blues Junior LTD. Low volume and a bunch of distortion. Can't seem to get to the bottom of it however. I am currently in north africa and don't have a tech to call on so I am on my own to diagnose the problem. I would be glad to pay you for a remote diagnostic session.
Hi Happy to help of course but it's not easy doing these things at a distance. If you go the very end of any of my videos you'll see a splash screen with a contact email on it. Contact me via that.
I always hate the repairs in which everything checks fine and therefore there is no reason why the amp isn't working, but it's not. In those cases, it always turns out to be that I overlooked the obvious and the problem is something dead simple that I should have seen right away. Oh well, that's how it goes sometimes. You are correct that diagnosing is having a hypothesis and then checking for evidence. Sometimes the original hypothesis is dead wrong and there is no shame in that as long as you used a methodical and step by step diagnosing process to find facts. Eventually the problem will reveal itself.
I Can imagine the frustration.. end user, had a jcm 900 that first blew a speaker.. (Amp never run hard) took to two local techs and two authorized Marshall service centers (in the U.S.) each shop got two tries at it... and even after a new transformer, the problem was never resolved.. ran thru tubes right and left.. ugh.. finally traded it off at a loss.. would have happily paid full service cost had it been fixed.. spent much, much more for it never being fixed.. when it was new it was a lovely sounding amp.. when i traded it away it was a heart ache..
@@stuartukguitarampguy5830 just found your channel a week or two ago.. love your content and your detail.. First Rate! wish I knew what really had been the problem with that JCM900.. though a bit to much gain for my taste. traded it and some $$/££ for a fender deluxe reissue.. which thankfully I adore played clean along with well worn AC30...
At 21:20, the ribbon cables and one end of most of the resistors and other components have red marker on them; was that from the factory? Or another tech, checking off the things he or she already measured/ tested/eliminated as being the source of the malfunction?
So would there be a safe way of creating the same effect the faulty C11 was creating without harming the amplifier longterm? This might be a cool mod to do for a higher gain, lower volume, saggier Pro Jr. Maybe a different value for C11?
Thank you so much for sharing this! I have a Fender Pro Junior iii that also has low output and am not getting a reading from C11. But the 1st pre-amp tube - the one all the way to the right (V1 on schematic) - it does not seem to be lighting up even when I change it out. Do you have any idea why this would be? Would the C11 cause this too or is that indicative of a different issue? Like a heater wire or dead filament? Thank you!
Great instructions ! I have the same amp,,,,,,,,and seems like the same Low Output problem - What was the name of the component that fixed it ??? Thank you
That was fun! Thanks for showing us your work. Before you hooked up the scope I was thinking it might be the speaker. What do they call that cap/resistor across the anodes,...a something network? I can't remember. On another note, I've heard some people complain that 100 ohms is enough on modern EL84 screens. Have you ever had problems with that? Thanks again!
I've heard it called the conjuntive filter. It is in the RCA tube manual, but the actual name escapes me at the moment. If I come across it, I will let you know. Cheers
You didn't test the speaker first, or substitute another speaker temporarily to see if the fault remains with the amp or not! BTW, I've seen a Silvertone amp with similar R/C circuit across the transformer primary; the capacitor was a tunable ceramic and mica type! Presumably there to eliminate oscillations?
good-un Yes probably to eliminate parasitic high freq, oscillation. Stuart, I love Your channel, and You give excellent advice, and analysis in these videos. When i test amps, I always attach the output to a speaker with a 'scope across the speaker, and use a Variac to bring the supply voltage up step by step, starting at about half voltage. Most tube amps work fine under these conditions with about 1/4 output, so there is a much lower risk of burning things, further. On viewing the video of this amp, I suspected something in the output section or phase inverter. So then it is easy to look at the waveforms at both places and see where the clipping occurs. You don't have to make theories, or hypotheses, with a little more testing, and observation, that tells You exactly where the problem is. Divide and conquer is the name of the game. You can always insert a signal at the phase inverter, and see if the thing works perfectly before that point or after! You tend to always mention HT 1st which is great... serious problems from the power supply should be eliminated 1st,... for Your American viewers, HT means what we call B+ or the initial High Voltage Supply. (High Tension) If I had seen this schematic, I would have immediately removed C 10 and C 11 as they are unnecessary, and elements of poor design. As Tom Colangelo chief designer at Mark Levenson Amps explained to me long ago, an amplifier with two 90 degree phase shifts in a negative feedback loop, always run the risk of creating oscillation, at the given frequency where 180 degrees total of phase shift occur, (as 180 degrees is already produced by the amp). This is in fact exactly what a phase shift sine oscillator is! Actually stray capacitance can easily introduce another 90 with the parasitic capacitance in parallel with the signal. These caps are intended to suppress hi frequency oscillation, but instead can easily CAUSE it. Good design, seen in for instance, the Fender AB 763 circuits, or the 5f6 bassman, do not use such caps. It is always better to remove the parasitic capacitance in preamp or tone circuits or introduced by bad grounding schemes in the output, than adding more BS, to a clean circuit hoping to eliminate High Freq Osc. If You are monitoring the output with a 'scope You can immediately see when hi freq oscillation occurs, and then go about snubbing it out with one cap, and a resistor in series to ground in a part of the circuit without a feedback loop! Any amp can be safely returned to a common sense design that more closely resembles an AB 763, which is pretty close to all the designs out there anyway! I am guessing that C 11 shorted because it is a small cheap ceramic cap and was probably seeing parasitic oscillation, which ran to almost full output.
Brill. Thank you for sharing very valuable knowledge that takes as you stated 50 years to obtain. and pieces of info can be passed on in 20 min or so.
I'd like to just say that I appreciate the videos that you make especially on amp repairs Mr Stuart. You're channel is one of amp repair channels that watch. Kudos to you and goodluck on your channel!
Thanks Mark!
When I taught troubleshooting techniques, I emphatically stressed the importance of history. Always assume someone else has seen the same problem. In his book, "Guitar Amps and Their Problems 2.0" Michael Fratus describes this exact failure. His book contains over 2500 amp repair cases. Every amp tech should have a copy. It just may prevent a sleepless night!
I shall get a copy!
Nice...........IS that book still in print ? And, who sells it ? Thank you............
Well done!
This video is more realistic in the sense that you take us along through the diagnostic process without a lot of heavy editing. This way those of us who do maintenance on our own amps can actually come away with something to remember for future reference. It reminds me of my high school days in electronics lab.
Thanks that's exactly what I am trying to achieve. So easy to edit it to make you look like a miracle worker!
Good job! I really enjoy watching you diagnosing and repairing amps.
Thanks Zack!
I have watched many channels, many techs online and have a real love of amps and the old-school valve tech. You sir are becoming one of my favourites! Your honesty and explanation of the full diagnostic process are proving to be not only enjoyable but really educational. Thank you. Please keep them coming.
Thanks! I really appreciate those comments.
@skmann22, which other channels do you find interesting? I watch Uncle Doug, D-Lab, and just happily found Stuart here.
@@stevelark9840 Yes Uncle Doug of course, he got me started with the idea of videoing my repairs. The Guitarologist is also pretty good.
Also Psinoc Audio are great and very knowledgeable. There's also a woman (cab;t recall her name) who is a bit of a beginner but is doing good work out there her channel is Fazio Electric. I also enjoy Fran Blanche of 'FranLab'.
Thanks very much!
The only good thing about a truly baffling problem that you eventually solve, at the cost of great time and effort and brain cells is that you will never, ever forget what the fix was!
I've forgotten already! :)
I watch all your videos and I don't even repair amps... I think I'm the one that needs to get out more! Keep them coming, Stuart!
Ha ha! Yes I watch all sorts of stuff on CZcams too. It's amazing what people get up to!
You are right - struggling through the problem is painful for you, but highly beneficial for your viewers. Thank you!
Your verbal tour... or dialogue on your diagnostics.... mirrors my own Internal logic circuit perfectly.
Good job .
I get a kick of the difficult ones also. Nicely done.
I'm a bit relentless or compulsive on those sort of problems.
(My coffee mug also says 'Grumpy Ol' Man'...very strange. So funny.)
They must clone us somewhere. Hah.
From the Sea Islands of S.C. USA.
CHEERS !
Yes we are a breed!
Totally agree , and admire your honesty regarding tricky fault finding . This is true to life and helps set your channel apart .
Many thanks.
Stuart, your channel is magnificent! Thank you so much for your time and tutelage.
Thanks! I'm pleased you are enjoying it.
📌Love watching and listening to Mr Stuart at work, really informative covering many bases, keep up the good work sir🤘😉
Cheers Daz much appreciated!
Hello my friend. All of us have had jobs that just kick our butts. Excellent video and fantastic video.
Cheers Monty!
Thoroughly enjoyable 45 minute journey. Thank you!
You need to get out more! Seriously though, thanks for the thumbs up.
Hi Stuart. Do it your way. My profession was in fr feild. This is great fun for me. Keep going my friend.
Hello Stuart, thank you for showing this fault Diagnostics, alot of people may not understand how much effort it takes, I'm sure your other tech mates can help when something is not so obvious such as the PCB on Marshall DSL tsl amps that got me on my first fault finding mission. As a Electronics Technical Officer in Communications PBN and every other transmission device in telecommunciations that's 15 years experience before you progress to upper levels of building Networks World Wide. Cheers I enjoy your channel and respect the knowledge base and be so careful when dealing with Valve amps.
Hi I'm pleased you are enjoying them! Yes I'm quite careful. 400V DC is no fun - but was talking to a friend yestrday who regulaarly worked on 33Kv (!) equipment. Yowza!
Great Video, As it clearly shows that we sometimes do mistakes. I'm an amp tech and juste like you I had an issue on C11 on a Pro Junior too. Like you I spent hours finding the cause. Instead of your case, the amp was working fine until it behaves like a compresser-sustainer when pushed. The capacitor had no sign of wear or burn, and it blocked DC when testing, but the remaining 90% capacitance clearly showed that the lost 10% had turn into leak at high volume.
Thanks Frank. NAsty problem you had there!
That was a good catch. Good for you, and an excellent repair. Thanks for the video. 😀
Thanks Ken!
Well done. Enjoyed the video and your sense of Humor.
Glad you liked it!
Wonderful. Subscribed. Great video. Greetings & blessings from the SE USA.
Glad you enjoyed it thanks for subscribing.
great job mate..so good to see your wizardry...one correction about all musos being great people ??? how many drummers do you know ha ha ha. not joking there a breed of there own..whats nice to know im a producer and when i go to bed I'm going maybe redo that bridge or remove it etc..your a perfectionist that means and dedicated..love it
Cheers Pat
I can tell you do this for the love of the challenge. We all develop our own strategies over time. And sometimes, hard to avoid confirmation bias. Being able to back up and go in another direction is what brings home the win. Aint it the truth, it is never as simple as a lot of videos represent it. Still watching here. :) Nothing like a good struggle to entertain us.
Yes I like to keep the struggle in the video if possible. All too easy to make yourself look like a brilliant fault finder. What you say about confirmation bias is SO true.I have stories....
Thanks. it was very educational, it helps to understand. I have yerasov GTA-15(with variable R29 resitor Mod to adjust bias)
Thank you from Kokomo, Indiana. USA!
Glad you enjoyed it Mike.
Good job diagnosing the amp!
Thanks Michael!
Colored magic markers or Sharpies are handy for marking the male spade lugs on the board; it can be removed with alcohol, but I generally mark the thin end or edge of the connector so that the Sharpie colored marker can't be seen when you plug the female connectors on again. Of course, taking a picture with your smartphone is always a good idea.
Good tip thanks.
Top fix, so interesting..Wouldn't think such a small capacitor could cause so much of a problem..! Thanks for sharing..Ed..Herts..uk..😀
Thanks for the feedback Ed.
Hi Mr Stuart! Love the channel, new subscriber here. I am a professional luthier/guitar tech and have owned and operated a guitar repair shop in Texas for the last 15 years… and as you mentioned, due to such a shortage of good amp techs around and the one I had been using retiring, I’ve decided to take my knowledge and experience with guitar/pedal electronics and expand it to servicing amplifiers as I get requests for it all the time.
I’ve got what I believe could be an issue with my Fender Silverface Bassman 70 that brought me to this video:
I believe it’s got a low output “problem”. I use quotes because it’s not necessarily a problem as is and what I mean by that is, that amp still sounds good it seems to me and I am actually liking that I can turn it up and it’s not killing anyone with volume, but I’m running a single 12” speaker rated probably around 80 watts and with the volume up it still seems way quieter than a 70 watt tube amp should be. My question then is, if there’s a problem in the amp causing low volume/output but it still sounds good (no weird distortion or anything) and I actually like that it’s not so loud especially for recording, could that be risking any future abnormal fault or failure of anything in the amp if I continue to run it that way? I would hate to ruin the tranny. Thanks for all the information you share, it’s much appreciated!
Hi. Great news on your getting into the amp repair business. Watch as many CZcams videos as you can as you can learn a lot that way.
On you amp that's a tricky one. If you're going to start repairing amps you'll need to get yourself soe way of measuring the actual power the amp is giving. I do that with a dummy 8ohm load and I hook the scope across that. You can then work out the power (if you don't know how ti do that jut ask me). You're going toi need to start getting some test equipment together so a scope and a signal generatot are essential I'm afraid.
Thanks very much for sharing your work.
It's the only vidéo i found that shows this issue and hiw to fix it.
I have recently bought a hot rod from 1996, it sounds great but the volume is low. I hope the issue comes also from that C11.
Let's hope so!
Was it the C11 after all? Fairly certain I have the same problem and C11 getting no reading.
Anticipate high bias on all Blues and Pro Jr amps and address bias resisters early on.😊
Agreed. There's also a quick mod you can do to ake sure it's on the 240V tap not the 230V. This brng down all the voltages by about 5%.
Another great informative video, thanks
Many thanks glad you enjoyed it!
@@stuartukguitarampguy5830 No problem, it’s building up my understanding of how tube amps work 🤙
I have been thoroughly enjoying your videos. I started looking at your videos to see if I could diagnose the problem I am having with my Blues Junior LTD. Low volume and a bunch of distortion. Can't seem to get to the bottom of it however. I am currently in north africa and don't have a tech to call on so I am on my own to diagnose the problem. I would be glad to pay you for a remote diagnostic session.
Hi Happy to help of course but it's not easy doing these things at a distance. If you go the very end of any of my videos you'll see a splash screen with a contact email on it. Contact me via that.
I always hate the repairs in which everything checks fine and therefore there is no reason why the amp isn't working, but it's not. In those cases, it always turns out to be that I overlooked the obvious and the problem is something dead simple that I should have seen right away. Oh well, that's how it goes sometimes.
You are correct that diagnosing is having a hypothesis and then checking for evidence. Sometimes the original hypothesis is dead wrong and there is no shame in that as long as you used a methodical and step by step diagnosing process to find facts. Eventually the problem will reveal itself.
You're a genius !
Aww, shucks!
I Can imagine the frustration.. end user, had a jcm 900 that first blew a speaker.. (Amp never run hard) took to two local techs and two authorized Marshall service centers (in the U.S.) each shop got two tries at it... and even after a new transformer, the problem was never resolved.. ran thru tubes right and left.. ugh.. finally traded it off at a loss.. would have happily paid full service cost had it been fixed.. spent much, much more for it never being fixed.. when it was new it was a lovely sounding amp.. when i traded it away it was a heart ache..
Hi David I'm dying to know what was actually wrong with it!
@@stuartukguitarampguy5830 just found your channel a week or two ago.. love your content and your detail.. First Rate! wish I knew what really had been the problem with that JCM900.. though a bit to much gain for my taste. traded it and some $$/££ for a fender deluxe reissue.. which thankfully I adore played clean along with well worn AC30...
At 21:20, the ribbon cables and one end of most of the resistors and other components have red marker on them; was that from the factory? Or another tech, checking off the things he or she already measured/ tested/eliminated as being the source of the malfunction?
So would there be a safe way of creating the same effect the faulty C11 was creating without harming the amplifier longterm? This might be a cool mod to do for a higher gain, lower volume, saggier Pro Jr. Maybe a different value for C11?
Thank you so much for sharing this! I have a Fender Pro Junior iii that also has low output and am not getting a reading from C11. But the 1st pre-amp tube - the one all the way to the right (V1 on schematic) - it does not seem to be lighting up even when I change it out. Do you have any idea why this would be? Would the C11 cause this too or is that indicative of a different issue? Like a heater wire or dead filament? Thank you!
Hi James I can;t reall help I'm afraid. It's beeen 3 years since I did this! I hope you manage to get it sorted.
Great instructions ! I have the same amp,,,,,,,,and seems like the same Low Output problem - What was the name of the component that fixed it ??? Thank you
Hi it was C11 which is 2200pF at 1KV or greater. Good luck
Watching this made me realise that I should buy a hand-wired, point to point amp!
Yes they are always best but it's all down to money of course. PCBs are used to save a LOT of cost and hence make the amp cheaper to buy.
That was fun! Thanks for showing us your work. Before you hooked up the scope I was thinking it might be the speaker.
What do they call that cap/resistor across the anodes,...a something network? I can't remember.
On another note, I've heard some people complain that 100 ohms is enough on modern EL84 screens. Have you ever had problems with that? Thanks again!
Hi Rick Sorry but the network you mentioned doesn;t ring any bells. I've not had any issues with EL84 screens, but other may have?
I've heard it called the conjuntive filter. It is in the RCA tube manual, but the actual name escapes me at the moment. If I come across it, I will let you know. Cheers
@@kengevers8738 Right, its called a Zobel network.
Thanks Rick. 😊
"the volume is lower and it distorts alot sooner" and the problem is??? hahaha
You didn't test the speaker first, or substitute another speaker temporarily to see if the fault remains with the amp or not! BTW, I've seen a Silvertone amp with similar R/C circuit across the transformer primary; the capacitor was a tunable ceramic and mica type! Presumably there to eliminate oscillations?
good-un Yes probably to eliminate parasitic high freq, oscillation.
Stuart, I love Your channel, and You give excellent advice, and analysis in these videos.
When i test amps, I always attach the output to a speaker with a 'scope across the speaker, and use a Variac to bring the supply voltage up step by step, starting at about half voltage. Most tube amps work fine under these conditions with about 1/4 output, so there is a much lower risk of burning things, further. On viewing the video of this amp, I suspected something in the output section or phase inverter. So then it is easy to look at the waveforms at both places and see where the clipping occurs. You don't have to make theories, or hypotheses, with a little more testing, and observation, that tells You exactly where the problem is. Divide and conquer is the name of the game. You can always insert a signal at the phase inverter, and see if the thing works perfectly before that point or after! You tend to always mention HT 1st which is great... serious problems from the power supply should be eliminated 1st,... for Your American viewers, HT means what we call B+ or the initial High Voltage Supply. (High Tension)
If I had seen this schematic, I would have immediately removed C 10 and C 11 as they are unnecessary, and elements of poor design. As Tom Colangelo chief designer at Mark Levenson Amps explained to me long ago, an amplifier with two 90 degree phase shifts in a negative feedback loop, always run the risk of creating oscillation, at the given frequency where 180 degrees total of phase shift occur, (as 180 degrees is already produced by the amp). This is in fact exactly what a phase shift sine oscillator is! Actually stray capacitance can easily introduce another 90 with the parasitic capacitance in parallel with the signal. These caps are intended to suppress hi frequency oscillation, but instead can easily CAUSE it. Good design, seen in for instance, the Fender AB 763 circuits, or the 5f6 bassman, do not use such caps. It is always better to remove the parasitic capacitance in preamp or tone circuits or introduced by bad grounding schemes in the output, than adding more BS, to a clean circuit hoping to eliminate High Freq Osc. If You are monitoring the output with a 'scope You can immediately see when hi freq oscillation occurs, and then go about snubbing it out with one cap, and a resistor in series to ground in a part of the circuit without a feedback loop! Any amp can be safely returned to a common sense design that more closely resembles an AB 763, which is pretty close to all the designs out there anyway! I am guessing that C 11 shorted because it is a small cheap ceramic cap and was probably seeing parasitic oscillation, which ran to almost full output.