The hum which appears only when a station is tuned in is called Modulation Hum and is the result of RF being brought into the set via the house wiring. Any non-linearity in the set, and any valve, diode, transistor and even dry joint will be non-linear, that this mains-borne signal meets will cause modulation of the RF by the 50Hz mains so that there are now two RF signals in the set - one clean, from the aerial and the other modulated by 50Hz. When these two signals meet, there will be a resultant 50Hz "hum" produced. The caps placed across the mains input of sets are there to help prevent the RF from getting into the set, in other words, filtering the mains input. A typical cure would be to add X2 rated caps across the mains at the first possible point, probably the mains switch. Ceramic caps across the smoothing electrolytics could also help.
Hum which is present all the time will be due to inadequate HT smoothing, pickup by high impedance circuits (grids etc.) or heater-cathode leakage in valves. HT hum can be 50Hz (half wave) or 100Hz (full wave) and if the hum is 50Hz in a set with full wave rectification, then it is not due to HT smoothing.
I hope this helps a bit.
Hum which is present all the time will be due to inadequate HT smoothing, pickup by high impedance circuits (grids etc.) or heater-cathode leakage in valves. HT hum can be 50Hz (half wave) or 100Hz (full wave) and if the hum is 50Hz in a set with full wave rectification, then it is not due to HT smoothing.
I hope this helps a bit.
