[morzh]

Keep combating the AC hum.

1. The hum stops if the 75 tube is extracted.
2. The hum stops if the lower end of the volume regulator is disconnected.
3. Increasing capacitance of the cap decoupling the Chassis (10uF) to the negative reduces the hum, but does not get rid of it.
4. all GND rivets involved are checked and are fine.

I tried to complete the factory mod that was reflected in the "Changes" doc (discussed here on p.2) as it also requires disconnecting the lower end of the volume pot from C- (the midpoint between 260 and 70 Ohm resistors, connecting Negative to GND) and connecting it directly to GND.
This only made matters worse. The hum becomes just too loud.

I think I am dealing with a case of GND looping. Even though most GNDs come via rivets from the chassis locally and the rivets are fine and the chassis is low-inductance, low resistance path, somehow I think, coupled with long capacitors' leads (wires go all over the board, reaching good 5-6" in length) this will create some humming.

I think I will try to re-route the sensitive GND connection using wires to a single point where the capacitor connects to it.