SF Bassman 100 hum problem

[ValveBytes]

This bassman has a fairly loud hum once at 5 on the master volume increasing up to 8 and then decreasing slightly by 10.

bias circuit has been checked out caps ok, filter caps replaced and most of the grounds associated with the PI and master volume have been checked, if you remove the PI the hum goes away (low level hum still in background but this is probably just induced in to the OT TF) and another clue if you turn the Output Tubes matching pot all the way to one side this reduces the hum and toward the other side you get an increase also lowering the bias reduces the hum?!

any ideas would be great the amp is stock apart from a 10K bias adjust pot I added in series with a 5K6 resistor, the output stage is idling at 18.5 static with about 455VDC on the plates.

[ValveBytes]

two thoughts since posting maybe I don't need to bias them at 40mA thereby reducing the and or swapping the primaries on the OT hmm

[Jazz P Bass]

SF Bassman

Make sure ALL of your input jacks are grounding out with no plug inserted. Any jack that is not grounding will make the output hum.
The hum balance should be turned until you achieve the lowest possible hum.

[Gtr_tech]

Make sure the hum balance rheostat is good. Tube failures like to smoke those. Sounds like it may be open on one end.