[Amps] Problems with a gu-84b amp

[R.Measures]

> 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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