What does an amp with dying filter caps sound like? Diagnose my amp!

[Will]

Well last time Martin had my amp he said it would need doing, so that is my first guess, but he said the hum /buzz isn't noticable until its loud - however, recently it has spurts of buzzes that last about a second, and have no pattern at all.
I have just tried swapping all the preamp valves, so it isn't that atleast

[hamfist]

Could well be a dodgy ground connection somewhere. Although, could possibly be a faulty Electrolytic (as you suspect). These intermittent faults are very annoying !
  If the electrolytics are old (ie. over 15-20 years old) then I would replace them anyway. If that solves your problem then all well and good. If not, then it was a job that needed doing anyway.
 
BTW, Does the amp have an FX loop. If so, you could play for a while through a pedal or MFX unit into the FX loop return. If you get amp symptoms that way then you have identified the problem as a power amp or power supply issue. If the problems disappear, then it would identify it as a pre-amp problem.

[Will]

Its a '78 2203 :P I think I will give Martin a ring and probably send it up to him

[HTH AMPS]

Its a '78 2203 :P I think I will give Martin a ring and probably send it up to him

Fantastic amp - the late 70s JMP 2203s are awesome amps, I've repaired and modded LOTS of those amps.

Spurts of buzzes don't sound like a filter cap problem - likely to be preamp valves, a dry solder joint or a noisy plate resistor.

[gwEm]

nevertheless, i would also like to know what an amp with dying caps sounds like...?

hope you get your jmp fixed will!

[martinw]

nevertheless, i would also like to know what an amp with dying caps sounds like...?

Hummmm....... :)

A continuous mains frequency hum is what you'll hear, and what I picked up on Will's amp. In this case, it wasn't the worst I'd ever heard, but indicative that the caps would need changing at some point. Above a certain gain setting it was intrusive, but IIRC Will wasn't using the amp up there.

This symptom, being intermittent, is probably along the lines of what HTH said.

[HTH AMPS]

further to what Martin said, I'd also add that filter caps on the way out may cause fluctuations in the amp's volume as well - I've seen this in a 1959 model JCM800 where the volume would fade to nothing, then ramp back up once you stopped playing.  New filter caps sorted this out.

Old filter caps also make the amp lose punch - you'll notice how much more balls the amp will have with new caps.

This is essential maintenance imo - players shrug this kind of work off, but if your amp is 10+ years old you should really have a tech look at the caps.  Some caps are ok for longer than this, but prevention is the way forward - you DO NOT want a filter cap exploding inside your amp, the mess is causes is NASTY!!!