Hum balance pots

[Stedes]

just built my second ever kit (a Stereomour) recently.

Having had to adjust hum balance pots at the end of the build for the first time I was wondering if someone could explain what they are, what they do and why they're needed in the Stereomour but not in some other kits (like the Smash I built before that).

Many thanks.

[Paul Birkeland]

In a directly heated triode, the source of free electrons and the source of heat to free them are one and the same - the filament.  If you were to present signal to this filament while the tube was operating, you could modulate the departure of electrons from the filament, thus imparting that signal to the plate of the tube.  The hum pot allows for the cancellation of this signal, which is present as the 2.5V of AC voltage at 60Hz that heats the tube in the first place.

In the Smash, the filament is heated with regulated DC, which has very little residual noise present, so no hum pot is needed.

[Stedes]

Thanks for that.