Marshall JCM 2000 TSL100 weird cap

[CJ]

working on this amp, had a screen resistor heating up, 120 vac across it when power testing, wtf?

easy to find, just follow the smoke signals.

turns out there was a 22 pf cap from plate to screen that went belly up,

this cap is only on one of the tube sockets, so they must have had some stability issues with just that one power tube,

never seen a cap on just one tube,

if you ever work on one of these, check that cap, C46, bottom right,

 

[aomahana]

I have come across that fault.
Hum at the output.
R74 1K grid resistor measured low in circuit , but was OK when removed.
I replaced C46 with a 6000V HV ceramic.

No doubt you are aware of the problem with this model of the bias rising because of a fault with the pcb design.
Over time the small gap between the bias and HV tracks become carbonised and leaky.
This causes the bias to rise as the amp warm up.
I never found a permanent solution for this, other than replacing the board.

 

[CJ]

i thought i had been through this before,

https://groupdiy.com/threads/marshall-jcm-2000-triple-super-lead.61779/

https://www.hullerum.de/Marshall/TSL122repair.html