MC-30 hum adjust pot: what's it for?

[efhjr]

I have never figured out what the HUM. ADJ. pots on my MC-30s are supposed to do. When I adjust them, I hear no change in the sound whatsoever.

Should I keep them turned all the way down? Half way? All the way up?

 

[Rob]

In most tube amplifier circuits a hum adjust pot (100 ohms if 6.3 volt filaments) is usually connected across the AC filament supply with the wiper to chassis ground. It allows one to balance out (neutralize) the capacitively coupled 60 Hz hum that can get onto the cathode of the low level stage tubes from their filaments. Sometimes the wiper will go to a positive bias supply tap of +30 to +50 volts to keep electrons from jumping off the heater to the cathode.

If you cannot notice a difference in setting then it doesn't matter where you leave them set. Sometimes it takes a very quiet room with high efficiency loudspeakers to hear the difference, or you might do a tube change one day and suddenly find you need to try this adjustment control.

If it makes you feel better, set them dead center for now.