Another Marshall Hum Problem - help!

[bobe]

It's a 1980 Marshall 2104 (combo, valves, 2x12). Seems to have developed a bit of background hum over the last few months - time to diagnose and do something about it. Hums when no guitar is plugged in, increases as the Master Volume is increased. also have 3 new 12AX7's, didn't help, amp has never had caps replaced. Any ideas?

[Robert M. Martinelli]

What about the power tubes?

class AB1 output stages are hum-canceling by design, provided everything is OK and the tubes are reasonably close; since tubes do not wear at the same rate, this "loss of symmetry" could be the reason for the increased hum.

You stated the amp is 29 years old.....well, maybe the filter caps are still good, maybe not ( and that would cause some hum too ), but in any case I would not wait until they decide to spew their guts out

Hope this helps

Best regards

Bob