Hey, amp repair legends!

timmysoft

I'm sure some of the amp legends on here will have a fix for this!

The high gain input on my peavey ultra causes a really high pitch feedback style squeal, the low gain is fine (which is the one I always use), changed out all my preamp valves but it's still giving me the squeal. It only does this on the gain channels with the gains wound up a bit

Any ideas?

ICBM

Does it still do it if you turn the guitar volume(s) down to zero?

timmysoft

No, it doesn't seem to.

timmysoft

Tried different leads and guitar too!

ICBM

See if you can isolate it by damping one preamp valve at a time (just grasp it with your hand should be enough) while it's squealing. Even if you've changed the valves, all valves are microphonic to some extent.

If it's not a valve it's probably a capacitor - they can sometimes go microphonic as well.

timmysoft

And would the capacitor be localised to that one input?

I've tried the holding each valve trick and that didn't give any kind of result for me.

ICBM

timmysoft said:

And would the capacitor be localised to that one input?

I've tried the holding each valve trick and that didn't give any kind of result for me.

If the schematic I have is for the same version as your amp, there are two caps in the input circuit. Because of the way the high/low switching is arranged, it's possible that if either of these caps is microphonic, it would be damped out enough when using the low input to stop the feedback.

It's pretty much got to be something like this because there are only three resistors and these two caps in the entire input circuit; everything after this point is common to both inputs.

timmysoft

Cheers ICBM! I knew you'd be clued up I an old peavey. Are the caps something I could change myself? I have 20 years experience soldering on subsea remote vehicles so ive probably got the skills, just not the knowledge

ICBM

Probably fairly easy, although I think you'll have to take the PCB out. It's probably 20 years since I was inside an Ultra though, and I can't remember exactly. The caps you would need to change are (logically enough!) C1 and C2 and should be very near the input jacks.