Valve amplifier loosing 'gain' over 20 minutes
Neil J. Harris wrote:
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
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
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