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
70's and the output power went from 50W to 330W!

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
A friend has a 1950's Tannoy PA valve amplifier which works when first
turned on but the output steadily drops to zero after 20 minutes.

That's his description of the fault. I have not tested the amp yet.



Thinking about it, I'm assuming it is a problem with a capacitor.

I'm not familiar with all the failure modes of old capacitors. I know that
old electrolytics loose capacitance and become leaky when the dielectric
degrades.

Could this fault be caused by a leaky interstage capacitor which is
affecting the bias of the next valve stage?



Or could it be a valve problem?




--
Neil J. Harris