93 Fender Concert Amp cuts out after warming up.

[sirwesty]

I have a 93 Concert Amp (Rev A) that will begin to cut out (drop in volume) as it warms up. Sounds glorious for about 3 minutes then slowly begins to goof up. It seems to be adgitated by vibration. I've swapped out the preamp tubes w/ no change. The pwr supplies seem to be solid, but I only have a DMM to verify that. Someone had added the caps to the relays which were added on rev. B or C per the '94 schematic. (470K too) My best friend has the same amp, and it behaves the same way. what are the odds??? Sounds like a fundemental design flaw. I just really like the the sound of the amp. I've read a lot of bad things about this particular version, but if I could get it working I'd be thrilled. So, I'm just looking for some suggestions on tracking down the problem.
Thanks in advance for any help!

Westy

[Ronsonic]

Few things will make a tube amp fade out. Most common and likely is open heater connection. Occasionally there will be a break in the B+ supply that lets the filter caps bleed slowly to cause a fade.

Either way, you are looking first for bad connections. That it is affected by vibration pretty much confirms that the problem is mechanical.

I think you're looking for solders or other conx in the power supply, heater and B+.

[Amp Kat]

Patch a cable from send to return jacks and see if it stops.