There are several possibilities of hum in this model. One of these is leakage from the “B” supply through the insulating material of the wire-wound resistor to ground. Only the very first production [run] had any of this trouble, which was corrected by replacing the resistor with the later type, which was darker in appearance.
Any wire insulation defects, particularly where the wires pass over sharp metal corners, can cause a hum, due to the grounding conditions which is established.
Grounded resistors can produce a hum, which condition can also be produced by a defective type 75 tube. [A filament to cathode short or leakage is common.]
From the Philco Serviceman – October, 1933.