Older Peavey Blues Classic 50 Watt 1 X 15 Volume Fadeouts (Capacitor?)

[rocknrollchef]

Hello:

I love my old tweed Peavey Blues Classic w' one 15" speaker. I changed ALL the tubes recently, it's never had any traumatic falls etc. But lately, completely at random (onstage!), the volume just inexplicably drops out to "barely there." Sometimes it will go literally for hours just fine, and then... Is this a bad capacitor perhaps? Can a capacitor be tested? And where is the cheapest/fastest replacement if I'm doing that? THANKS FOR YOUR INPUT.

[Enzo]

Plug a spare cord from FX send to FX return and se if that helps.

No, it doesn't sound like a cap.

[rocknrollchef]

Yeah we tried the FX send already - no go

But really appreciate you responding so fast!

[txstrat]

What do you usually do to make it sound right again? Does wiggling the tubes help in any way? Are the input jacks alright? Does it happen on concrete as well as on a wooden stage (the wooden stage would make it quiver while somebody stomps with the rhythm).
What I'm saying is, maybe some part is moving inside (maybe one of the new tubes). Or maybe just a faulty tube.

b.t.w Enzo I'm glad you're back

[rocknrollchef]

There's nothing I can do to make it sound right - it's completely random. And it has nothing to do with vibration, it's dropped out without any of that around. The input jacks have been checked, and the tubes are pretty new. FRUSTRATING!

[txstrat]

Yeah I can imagine that it is frustrating. But be aware even NEW tubes can be faulty. I happened to have the same in my main amp. A brand new tube was cutting out just like that. Changed it and everything was alright - I found out by wiggling it and the sound came back.
I'd try to locate the problem. Change all tubes one by one.

[Enzo]

What do you mean you tried the FX send already? You mean you connected the send to the return? Or you mean you did something with just the send?

If the patch didn't help (cure for a bad return jack), then connect the send to some other amp. That way when it happens you will hear either that both the internal speaker AND the other amp both diminish, OR you will hear the internal speaker diminish, but the sound from the other amp remains strong. SInce the FX send is more of less the preamp out, this will tell you if the preamp is OK or is where the problem sits.

Furthermore, plug a signal into the FX return so you are listening to ONLY the power amp. (This bypasses the preamp) Plug a MP3 player in or somethig so you can listen while busy elsewhere. Now, does the power amp come and go like the problem? Or not?

The point of all this is to isolate the problem to a particular section of the amp.