Ground like hum on 1968 Bassman

[1971SuperLead]

Hello!
I have a 1968 Fender Bassman that hums even with the volumes on zero.
It hums the same even with the phase inverter tube removed.
It's a low hum that's steady and sounds like a grounding issue.
It does have a new 3 wire AC cord installed.
The stand by switch and death cap are still in place and wired up!
Moving the polarity switch makes no difference.
It has all new Sprague caps in the power section.
I checked the polarity on them.
I checked for loose wires, solder joints, etc.
I checked to make sure the heater wires on the power tube sockets go from pin 7 to pin 7.
When switching on from standby the hum is momentarily very loud and then goes much lower.
The Mica caps that go to V2 are microphonic. Strange?
I switched power tubes from a 70 Bassman that has absolutely no hum and the hum did not change.
Adjusting the bias raised and lowered the hum, but when the hum was gone the amp was all farty sounding, not a good distortion at all. lol
Interesting is that using a Variac, the hum increases with changes in the voltage. As soon as I stop adjusting the voltage the hum dies back down. Something inside is hating voltage changes.
Holding my guitar 12 inches or closer to the amp head significant increases the hum.
It's a great sounding head besides the hum.
There is evidence of a power cap having failed under the cover. There is also evidence of a fire under the inside power tube socket. The green heater wires are scorched and new resisters seem to be in place for both tube sockets.
The bias adjustment seems to be of the silverface variety as I lose power to the tubes when turned down.
It just sounds like there's a grounding issue somewhere in the power section.
Any ideas anyone? I'm a novice at this but I do have a multimeter, Variac, bais probs and 20 years experience in the aerospace field.

[52 Bill]

Do you know what circuit this amp is, AC568?

[Leo_Gnardo]

You have covered most of the usual suspects, good! How about the bias filter cap(s). Don't forget to freshen up the filters there. A failed bias filter is the culprit in too many good old Fenders. And a failing one will induce a hum that's otherwise hard to track down.

Removing the phase inverter tube is a good analysis technique. No signal from the preamp will get to the power amp so you know the problems you hear will be related to power amp, power supply, bias supply.

Holding guitar near amp - you have to expect it will pick up hum from the power transformer. Your pickups sense a varying magnetic field. It need not be the amps' own transformer. Another amp or any other item that throws a field will do it. One of my customers asked why his guitar went tick tick tick but only when his right hand was in playing posiition. It was his quartz-timed watch. Every pulse that moved the second hand, tick tick tick. Good metronome as long as you want 120 bps.

There's no need to retain the "death cap" now your chassis is grounded via a proper AC cable. Shouldn't make much difference in hum or anything else. Sometimes I forget to remove one and nothing awful happens. Unless that cap is shorted. One day in an Ampeg V4 I left a bad "death" cap in, and when I moved the polarity switch, the cap went zooming across the chassis leaving a trail of sparks and a spiral of aluminum foil in its wake. Oh boy Fourth of July! Excitement I could have done without.

[The Dude]

Not sure exactly which chassis we're talking about here, but since you said the heater wires were scorched, perhaps do you have burnt 100 ohm resistors near the lamp (virtual ground)?

[Enzo]

Hums with no PI tube? My first thought then is imbalance. Check for B+ on ALL the pin 4 screen pins of the power tubes. And check for ripple on the bias supply. That will hum up your power tubes.