Hi everyone, I have a valve guitar amp ( 5E5A schematic) that I have been using since 2005, it has recently had all its electrolytic caps replaced, it has a strange oscillation when I turn up either volume knob, it's really faint, untill I plug a guitar lead in then it's a loud noise, its a fast oscillation sounds like a cat purring, was wondering if anyone has come across this before?
There is no hum, it is literally like a purring cat. Like.... A fast series of pulses .... Maybe motorboating? The only motorboating I have ever heard was quiet slow,. I don't have an oscilloscope all the caps in the amp are new tried a different speaker, not that, tried different tubes, not that. Did a resistance test on the output transformer, seems normal. Cleaned all valve sockets. No difference. The sound of the amp doesn't sound as clean as it used to? Seems to breakup at lower volume levels?? Could the output transformer be failing? Thanks for the reply by the way!
Yes it does sound a bit like motorboating which is a common fault in old valve/tube radios due to HF instability which ( if you had an oscilloscope ) would be easy to trace --no its not a failing output transformer but more likely a faulty small value capacitor .
Using a analogue multimeter start at the input on the AC range and progressively check each stage , an analogue meter can quickly show the meter needle rise and fall much easier than a digital meter.
Of course the frequency response on the AC range should have a wide bandwidth--not just mains frequency.
Using a analogue multimeter start at the input on the AC range and progressively check each stage , an analogue meter can quickly show the meter needle rise and fall much easier than a digital meter.
Of course the frequency response on the AC range should have a wide bandwidth--not just mains frequency.
Hi Jan, no I didn't change any values, I rather like the amp the way it is, hardly any hiss (even with carbon composition resistors) and no hum! This is the first time anything like this has happened, been gigging with this amp for years!��
Any loose connections of the new electrolytic capacitors, particularly the ones that filter the supply?
Hi MarcelvdG, no that was something I already checked, actually I was wondering if one of the filter caps was a bit duff I'll check everything again tomorrow, I obviously missed something. Thanks
Feedback is always around, whether or not we call it by name. An amplifier passes signal forward and backward, intentionally *and* unintentionally. Even at the grossly simplified "schematic" level of modeling, it's possible to see the feedback path through the ubiquitous common B+ supply - plus - decoupling - low pass filter. More is more better, and perfect is impossible.
All good fortune,
Chris
All good fortune,
Chris
You might have gotten bit by a fake capacitor - with less microfarads there than what is marked on the case. Might have a problem handling the voltage for extended periods of time, too.
The term "motorboating" is generally used to describe low frequency oscillations and has no direct relation with HF instability.
Morgan Jones describes "motorboating" as:
"This is an oscillation at about 1 or 2 Hz, and is invariably caused by unintentional feedback via the power supply, due to the rising impedance of filter capacitors at low frequencies. In effect, the entire amplifier becomes a relaxation oscillator."
(page 467, Valve Amplifiers, 4th edition, 2012)
Motorbating an HF instability can ofcourse happen at the same time but that doesn't change what motorboating in itself is.
He says its occurring at a higher frequency not at--1 to 2 Hz --changing small value capacitors in valve radios usually cured it, never touched the power supply .
Well, than unlike you wrote in post #4, the problem in the amplifier of TS by definition is not motorboating.
One sneaky possibility is called "squeeging", bursts of supersonic oscillation that increase to the point where the oscillating stage(s) is driven towards cutoff, so then relaxes, normal DC conditions are restored, so then resumes oscillating, repeating at an audible rate. Copies of vintage guitar amps, usually without grid-stops or any layout provision against parasitics, are vulnerable.
All good fortune,
Chris
All good fortune,
Chris
It's a guitar rather than a HiFi amp. I wouldn't be surprised if those motorboated at a bit higher frequency, enough to imitate a purring cat, as the RC time constants could be smaller.
In any case, it's apparently not a design issue as it used to work, so something must have gone wrong when the electrolytic capacitors were replaced - or at least that would be my guess.
In any case, it's apparently not a design issue as it used to work, so something must have gone wrong when the electrolytic capacitors were replaced - or at least that would be my guess.
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And you haven't read the OP,s first post and his third post--"purring like a kitten "
You are also simplifying the term "motorboating " - what you are saying if it isn't in the frequency range = 1 or 2 Hz it isn't motorboating --not as far as I am concerned I have listened to some higher frequencies above that ( a bit quicker ) .
Your too tightly defining motorboating I am dealing with actual practical repair not some term that is so tightly defined by you that you don't seem to be regularly dealing with practical repairs that I have .
You are reacting like I formulated the definition of motorboating, which ofcourse is not the case.
To not derail this thread any further, I will stay out of it.
To not derail this thread any further, I will stay out of it.
Sounds like a scope job.
Just follow stages through until discrepancy occurs.
Guitar amps dont usually have feedback so are easier to debug.
Its possibly a faulty coupling capacitor if the amp is of old age.
Just follow stages through until discrepancy occurs.
Guitar amps dont usually have feedback so are easier to debug.
Its possibly a faulty coupling capacitor if the amp is of old age.
Guitar amps (notably Fenders) are easily made unstable, if the preamp-stage electrolytic supply decouplers are out-of-specification.
this means the value of capacitance, of course, but also the ESR (series resistance).
So I think Marcel is correct, there is likely be a question over the exact part used to replace one or more of the caps.
Choosing caps that are smaller than the size of a Nichicon VR (same µF and voltage) may mean higher ESR, and that can be enough to destabilize it.
http://products.nichicon.co.jp/en/pdf/xja043/e-vr.pdf
this means the value of capacitance, of course, but also the ESR (series resistance).
So I think Marcel is correct, there is likely be a question over the exact part used to replace one or more of the caps.
Choosing caps that are smaller than the size of a Nichicon VR (same µF and voltage) may mean higher ESR, and that can be enough to destabilize it.
http://products.nichicon.co.jp/en/pdf/xja043/e-vr.pdf