Peavey Classic 50 - cracking and sizzling

[wagdog]

Greetings all,

Working on an abused peavey early 90's classic 50 I picked up as a project. It was stored in a basement after several years of not working.

schematic: http://www.blueguitar.org/new/schem/peavey/c50schem.pdf

I resuscitated it with a set of tubes I had laying around and replacing the leaking PS filter caps. I also replaced the cathode bypass caps on the pre-amp, plate load resistors on the pre-amp and the dropping resistors in the B+ rail. Also replaced the electrolytics in the bias supply, c42, 43 and 49.

Amp sounds great, but it has a strange problem; After sitting overnight, if I apply power with the master dimed and the pre-amp vol down to a reasonable level (how I usually play), after the tubes warm up and it springs to life, it will crackle/sizzle and pop - fairly loudly. Adjusting the pre-amp vol (this is on the clean channel, btw) has no effect, but lowering the master vol to 0, then bringing all the back up will cure the problem.

I did clean all the pots thoroughly, but I'm thinking that perhaps years of sitting in a basement took its toll on the master vol pot?

Any other thoughts on where to look?

Thanks in advance.

[The Dude]

After being stored in a damp basement for several years, it's likely there could be some corrosion on the socket pins. Have you tried cleaning and retensioning the tube socket pins? The sound you describe sounds like something is arcing. You could also have a noisy tube. Does the noise change at all as the amp warms up?

[wagdog]

Sorry, should have mentioned that I did clean all of the tube sockets and re-tensioned them. The tubes feel very snug going into the sockets.

It doesn't go away until I bring the master vol down to 0 and then back up. It's worth noting that is what I fiddled with to get to stop - it may just be that it stopped making that noise when I did that by coincidence.

[Jazz P Bass]

Run a patch cord into the FX send & return jacks.
See if that helps.

[wagdog]

Run a patch cord into the FX send & return jacks.
See if that helps.

Thank you for the reply. This model does not have an effects loop - it's a 2x12. I did have a classic 50 head and 4x10 cab a while back, and it did have an effect loop. It looks like these older 2x12's didn't.

I have traced the problem down to a bad 12ax7 in the PI. It's interesting that turning down the master vol (before the PI) stopped the issue. I sort of found the problem by accident, as the PI tube started oscillating last night. Replacing the PI tube with a known good tube and all is good.