Could this, if inverted, cause hum ?
Yes I think so.
How to identify the wires with multmeter ?
Yes.
The PT B+ CT should read, resistance, about 1/2 way in between the 2 B+ red wire leads.
The bias tap should have a little more resistance from 1 B+ red wire then from the other B+ wire. Not as centered resistance wise as the B+ CT.
The heater CT should read in resistance about 1/2 way in between the 2 green heater wires. I think you need to take all the tubes out to measure them or you'll be measuring the tube filaments too and that will through off the reading.
If it were me I would fix/trim those 3 PT wires 1st, then listen for hum. The longer they are the more noise they can push/transmit into the air that can/will get picked up by other circuitry in the amp. That's why you shorten them.
If there's still hum then I would use shielded wire on the input jacks.
Sometimes there's 3 or 4 things causing an amp to hum, as you track them down 1 at a time each fix will get rid of some hum, the next fix will get rid of some more hum.
You have that very long B+ CT wire looping around AND the very long OT secondary speaker wires running around the end of the preamp end of the circuit board and preamp tubes.
These both could be major hum sources.
You could run the OT secondary wires outside the chassis over to the speaker jacks then drill a hole there, use a rubber grommet in the chassis, then feed those 2 wires into the hole. I'd use some type of cloth insulator tubing or shrink wrap around them, on the outside of the chassis, to give them extra protection.
This will keep them away from the circuit board and preamp tubes and the chassis will act as shielding.
