Bassman 70 - both channels hum

[booj]

60 cycle. Very similar to the noise from a guitar cord not plugged into an instrument. Max hum is at "5" on each channel. When turned all the way down, hum disapears. When turned all the way up hum is almost goes away completely. The input jacks are shorting properly. Power supply is ok.
If the Master is turned down, and either channel turned all the way up, the amp is usable if the Master is used for the volume control.
The leads from the 68k input pairs go to unshielded wires which go under the fiberboard to the tube socket. Could it be as simple as using shielded wire from the resistors to the tube sockets?
Anybody with a similar experience?

[loudthud]

I'm not a big fan of shielded wire...but I'd suspect the wires from the volume controls back to the tubes. But first check the hum balance pot and the two 4.7ohm "fuses".

[Enzo]

Since the hum is worst in the middle and goes away all the way down AND all the way up, I start to suspect the volume pot itself. Is the pot body grounded - the round cover part?

[booj]

Loudthud, Enzo, thankyou for your kindness in replying. I'm on the same page your on. I'll get back when I have it on the bench again. Another amps in the way right now.
Loudthud, what are the 4.7 ohm resistors? I don't have/can't find a schematic. It has a hum balance pot. Are they connected to the end lugs of that?

[pdf64]

Schematic
http://www.schematicheaven.com/fende..._bassman70.pdf
The parts loudthud is refering to are just above the 'caution' notice at the bottom area of the schematic. These give a ground reference to the heaters, if they lose that reference they will pick up stray electromagnetic signals and in turn pass them on to the grid circuits.
If any of those parts have failed, use the '2 x 100ohm resistor' method of ground referencing the heaters (see numerous other fender schematics from above website). Peter.

[booj]

Thanks Peter! I had decided to ry the two 100 ohm resistors before getting on here, and it worked. Thanks for the scat.
Even though the hum control did adjust either side of the filaments to close to zero at the extreme settings, and was able to adjust to about two volts, after that the voltage would just read full filament voltage. The pot is probably open in the middle.

[Enzo]

Loudthud and I are on very different pages, actually. He refers to the heater hum balance pot, which certainly could be the problem.

When you say on either channel the hum is worst when the pots are at mid setting, I assumed you meant the volume pot for the channel. SO my idea to check if the cover is grounded on those pots is a totally separate idea from the hum balance pot.

However, whichever pot it was that hums worst at the middle is the one to check.


And that Fender schematic can't be right. One of the 4.7 ohm is in series with the pilot lamp, and the other is in series with the hum pot. Why would they want the hum pot off center by 4.7 ohms? I could believe a 4.7 ohm in series with both sides of the light.

[booj]

Loudthud and I are on very different pages, actually. He refers to the heater hum balance pot, which certainly could be the problem.

What I meant by being on the same page is that you guys were both accepting my diagnosis that it wasn't the power supply. I guess that could be a different page in itself. Anyway, thanks for your suggestions!