[Amps] Problems with a gu-84b amp

[Jimmy Jones]

 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?

Tnx in advance
Jimmy
n5ifi