'72 SF MV Twin Reverb "bright switches" cause pop and hum

[booj]

When the volume and treble controls are at minimum on one channel, the other channel works normally. No problems. If the vol. and treble are at "normal" settings on the unused channel, and the bright switch is activated on the used channel, an insideous hum is evoked at 60cps with suspicious higher frequency noises mixed in, like tuning a radio reciever, along with a really loud "pop" noise.
Also, if the unused channel is set normally, the used channel will emit a strong "thump" when turned up in volume at the highest volume, close to 10.
All this with no signal applied, and nothing plugged into the amp.
I'm suspecting internal oscillation from the haywire approach to making internal connections in this beast.
Anybody had similar problems?

[pdf64]

If the vol. and treble are at "normal" settings on the unused channel, and the bright switch is activated on the used channel, an insideous hum is evoked at 60cps with suspicious higher frequency noises mixed in, like tuning a radio reciever, along with a really loud "pop" noise.

Maybe the shorting contacts on the jack socket are dirty. Does plugging in a guitar with its volume turned down improve the noise?
Could be a bad tube V1 or V2, or a failing 100k plate resistor.

Also, if the unused channel is set normally, the used channel will emit a strong "thump" when turned up in volume at the highest volume, close to 10.

It could be that the circuit board has developed some degree of conductivity, that's a known failing with fender boards - has it been stored in a damp location?
I'm thinking of some dc getting to the grid.
But also the haywire lead dress can result in ultrasonic oscillation that causes dc bias shifts, ie thump. Does the treble control affect this.
Pete.