Channel 2 Fading Out

[MarshallPlexi]

I have a two channel amp that I'm working on and after about 2 minutes of play, Channel 2 fades out and after a bit, Channel 1 sounds like it is about to fade out.

My first suspicion is that there is a connection that is faulty and that the heat from the tubes is causing it to reveal itself only after the amp has been played for a bit. Possibly a bad connection on the heater wiring?

Anything else I should be looking at? I'll be able to spend this weekend working on it that's why I ask the questions now so I'll have ammo in hand to attack the problem.

If it makes any difference, the preamp tubes are DC heated.

Thanks!

[pdf64]

Smooth fade outs are usually due to a bad heater connection. If you can check all the tube base voltages when it's in the failed condition (big if), it should be obvious where the problem area is.

[MarshallPlexi]

That's going to be my first test. If it does fail I'll just reflow the solder joints there and see if that helps things.

Thanks for confirming my suspicions!