Having repaired many guitar amplifiers in my time, I have seen probably every make out there up until the late 90's.
I remember fitting a new O/P transformer to a Marshall jcm 800 That went into violent oscillation at around 25kHz. The speakers response
at that frequency was practically zero, all that was heard was a swishing sound with a slight hum, and the poor old output valve anodes had a nice fireside glow. This was because the output leads had been mis-identified due to the sealing varnish being so thick.
On reconnecting it properly, everything was fine and although the EL34's were working, I fitted a new pair for the customer F.O.C.

 

Aye! I've known 'HIWATT' Amps to 'take off' in the low HF region (i.e. fundamental at 4MHz - 6MHz)

Best regards
HP

hey guess what. the plate voltage is 900mv now. overnight. I dont know what happened. Whys is to bad?

 

Having repaired many guitar amplifiers in my time, I have seen probably every make out there up until the late 90's.
I remember fitting a new O/P transformer to a Marshall jcm 800 That went into violent oscillation at around 25kHz. The speakers response
at that frequency was practically zero, all that was heard was a swishing sound with a slight hum, and the poor old output valve anodes had a nice fireside glow. This was because the output leads had been mis-identified due to the sealing varnish being so thick.
On reconnecting it properly, everything was fine and although the EL34's were working, I fitted a new pair for the customer F.O.C.

check my post. plate voltage 1 volt now bascially

 

Time for those filter caps you wanted to delay replacing.

They made the choice for you

 

It's time to start thinking about disconnecting some wires and zero in on the problem.

 

Hello,

This is continued in the following thread:
No sound tube audio circuit.

Bertus