Noisy Mesa Engineering Express 5:25

[Swantastic]

Hi, folks. New to the forum. It comes highly recommended. I have a question about my Mesa amp. It is a Mesa Engineering Express 5:25. I really like it, but it emits a constant hum that I’d like to be rid of. It is much more dominant on channel 1 and increases as the volume is turned up. It is affected by the tone knobs and is most pronounced when increasing the treble with very little change when increasing the bass. It seems almost reversed on channel 2 where the bass and midrange increase the hum more. This amp has a rear switch that changes it to a lower wattage class A. The hum is present no matter which way I set it. I did have the tubes replaced and am wondering if they weren’t matched or if the bias wasn’t set. Thoughts? On a side note, I live rurally in north Missouri and there are no nearby repair shops.

[The Dude]

Here's the schematic:

http://bee.mif.pg.gda.pl/ciasteczkow...Schematics.pdf

It appears the unit does not have a variable bias control and bias is adjusted with a resistor change (See the resistor marked "EXPRT"). Since you say the volume controls effect the hum, I suspect it's not a bias or amp problem anyway. It sounds to me like more of a preamp or grounding issue. You could pull V4 (PI tube) and see if the amp still hums. If not, that would eliminate the output tubes circuit.