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?