Resolving Hum Issues In Vacuum Tube Amplifiers

Transcribed from: "Transistor Audio and Radio Circuits" 2nd edition, Mullard
"It should be remembered that the input sensitivity of an amplifier is typically 3mV at 1KHz, on the magnetic pick-up position, and the output voltage is of the order of 20V, implying a voltage gain approaching 10^4 (40 dB) It is therefore essential to keep the output separated and screened from the input. The magnetic field from the mains transformer may cause hum, so the transformer should be remote as possible from the input.
Earthing
Currents of several amps magnitude circulate in the power supply and output stages. It is important that no wiring carrying these currents is included in the input circuit, otherwise hum or instability, due to the small but significant resistance of the wires, will result.
The earthing arrangement for stereo amplifiers is considerably more complex than for mono, since the single power supply and common earth for the two signal inputs make it more difficult to avoid earth loops. "