Peavey Valveking volume drop in lead channel

[Tilman]

Hi,

I am trying to fix this amplifier. The clean channel works OK but the lead channel drops in Volume and comes back from time to time. The drop is kind of soft - like a heater problem or maybe a JFET shunt switch or something.
I have replaced ALL Tubes, a switching relays and cleaned ALL Loop Jacks.
The problem is still there, although only sometimes and only at very loud settings.

The problem with the dropping out lead channel on this amp seems to be quite common. May people report it - but I did not find a solution or hint on the net so far.

Maybe you guys know where to look next ?

Thanks!
Tilman

[Tilman]

Just found the problem. Very strange. A new tube JJ12AX7 has one dead heater (hitting the tube restores the heater function).
That tube did run a view hours max. ( I installed a new one when the Peavey was repaired the first time).
This leads me to another question : The heater voltage at V1+V2 is 5,5V only. Can this low voltage destroy the heater somehow earlier than expected ??
Tilman