Peavey Ultra 120 Plus Head PRoblemo

[JohnnyBoy]

I recently bought this head used. It was in pretty good condition, but when I play it for longer than about... 30 minutes, there's this LOUD hum. Sounds like I'd say a 60Hz hum. There a ground lift switch in the back, and it doesn't do anything at all. there's a good sound coming out of this head, not crackling or anything out of the ordinary. When I put it on stand by, and then put it back on, it'll go away, but a couple of minutes later the loud hum is back. No matter at what volume anything is, it is the same.

[PRNDL]

Most likely the electrolytic caps need replacing, although it could be bad output tubes. A tech would begin by measuring voltages and bias currents and go from there.

[Enzo]

My first four guesses would be tubes, but caps is possible.

[JohnnyBoy]

changed them...

I replaced the power tubes. The hum is sometimes still there after a while of playing, but this time, when I turn the master down the hum goes down as well. The guy from the store told me to resolder the capacitors if it keeps happening. This sucks.... Any other suggestions?

[PRNDL]

Did you measure the bias and match the tubes?

[JohnnyBoy]

yep

[Enzo]

If the remaining hum goes away when you turn down the master, then the power tube stage is fine. If there was a bias or balance problem, it would hum at any volume setting, even zero.

It might be a cap, but at this piont, I'd be looking for a loose ground connection. Look inside and make sure all the hardware is tight, especially the mounting bolts on the power transformer. MAke sure all the nuts are tight on the jacks and ALL the control bushings.