"Rudge" wrote in message
...
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?


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