Valve amplifier loosing 'gain' over 20 minutes

Dried out electrolytics usually improve as they warm up, leaky capacitors usually worsen with temperature which among other things can cause biasing faults - glowing tube plates is a dead giveaway!

At this point, that's a bit like saying " My friend says his car engine loses power after 20 minutes. Could it be a bad spark plug ? "

Well yes, it could, but there's also a score of other things it could be as well. Same applies with this amp. Until you have looked at it yourself, and have some real observations of the symptoms to tell us about, any opinions that we might give are pure speculation, and likely to be misleading and way off-beam.

Give us some more info and checks to work with, and we can probably help more.

Arfa

The resistors have probably gone high in value, easily checked with a meter.

The "paper" capacitors will probably be leaky, change them anyway. You can get polypropylene ones which "look right".

Be gentle with the electrolytic- if you hear any hissing sounds run!

I remember replacing the output valves in a Marshall Major back in the

There are several specialised newsgroups and websites. (I remember reading a wonderful account of complete rebuild of Quad amp, it wasn't until it was finished that the bloke realised it was a 100V line job!.)

Obligatory safety warning:- I often have to tell the game-boy generation at work to keep their fingers out of old equipment with >>50Vinside.

In message , Rudge writes

gassy tubes also heat up and draw grid current over a period of time like that...

I had an old Eico scope that did that due to a gassy tube...

check the plate voltage of all the tubes as it warms up...

if you have a gassy tube, the plate voltage will DROP as it heats up..

of course it could be 100 other things as well..

Mark