[Arran]

http://www.nostalgiaair.org/Resources/772/M0021772.htm

Well, I can't seem to help the hum.
The schematic at the linkmis very close, practically the same, and no matter what I do the AC hum persists.
The hum stops when I short the grid of the 12AT6 (pin1) to GND.
IF it is disconnected with the 10M resistor to GND it hums, if connected and the volume pot is in the low position, the hum gose away, in high position the hum is pronounced.  The reception is fine.

Oh, if the output tube's grid is disconnected the hum stops. So it likely comes from the 12AT6.

I replaced 10M resistor with 100K one, the hum went down quite a bit with the reception still being fine.

But all in all, I am not sure why the hum is there. I tried a cap from AC lines to chassis, or across the lines, or parallelling capacitor across the electrolytic.

If it is a result of AC lines routing....it is where I think it was originally.

Heck with it, maybe it was designed to hum.

Mike;
  Maybe it has some heater to cathode leakage in either the 50C5 or the 12AT6 tubes? Try some tube substitution.