DSL 401 noise and volume fluctuations

rico

I used this amp at a rehearsal yesterday and it seemed noisier than usual and the volume was fluctuating for about 30 seconds or so - getting louder and then back down again. 

It had a service and new power valves in January. 

What could this issue be? Dodgy preamp valve?

Voxman

I have the same amp and that would be my first thought, yes probably just one duff preamp valve. I'd suggest swapping another 12ax7 in one at a time to try and find the culprit. 

ICBM

Probably either a duff preamp valve, or something to do with the master volume pot - it's a poor design which feeds the bias voltage through the pot contact, so if the contact goes bad (which it sometimes does, especially if the knob gets bumped) then not only will the signal reaching the power valves fluctuate, so will the bias voltage which can make the valves run too hot and apparently *increase* the volume. Until they then fail, anyway…

The pot needs to be bridged with 1Mohm bypass resistors to ensure the bias supply is independent of the pot contact. You can do it by adding the resistors onto the jumper wires immediately behind the pot. (I forget which ones, I'll have to look it up.)

To be fair, this and the overheating filament rectifier are among the only 'standard' faults on these amps.

rico

Thanks @Voxman and @ICBM I knew I could count on you. I'll try a good valve.