> From a friend.
>I've had some kind of problem with my gu-84b amp.
>When I turned the amp on it went through its 3 minute warm up period. I
>switched the amp to operate without keying the radio and the plate
>current went to 2.5 amps and one of the 30 amp fuses blew on 240 volt
>mains input. After replacing the fuse, I'm able to tap on the top of the
>tube and see that the bias voltage drops and the plate current raises as
>I tap. This sounds like a tube problem to me. What do you think?
>
>This is a very new amp and has never been driven with more than 50 watts.
>Are we seeing some kind of parasitic problem here?
It's hard to say unless one does one's homework. I would start by
measuring the resistance of the suppressor-R. If it's more than 50%
high, I would be suspicious. High-potting the tube at 2x the rated DCV
is also useful. Also, measuring the grid-cathode potential @2uA of
leakage gives a clue.
>...
good luck, Jimmy
>
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