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I've got an old NEWCOMB tube amp which has EL-84'S in it, I'm not sure what year it was built , but I'm sure it's the late 50's or early 60's. Now just beside the transformer and next to a 5Y3GT Tube there is an ajustable screw kinda pot that reads (HUM BALANCE) what the heck is that for ??? I'm not to familiar with amps at all.---Lenny.
 

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lenbone said:
I've got an old NEWCOMB tube amp which has EL-84'S in it, I'm not sure what year it was built , but I'm sure it's the late 50's or early 60's. Now just beside the transformer and next to a 5Y3GT Tube there is an ajustable screw kinda pot that reads (HUM BALANCE) what the heck is that for ??? I'm not to familiar with amps at all.---Lenny.
One of the tricks to reduce hum in a tube amp is to ground one side of the filament, or heater winding. That's the thingy that heats up, releases electrons and gives the tube something to work with!

It works better if each side of the heater winding sees the same amount of resistance to ground. So they put a pot, or variable resistor across the heater terminals and ground the centre wiper, that is adjusted by a knob on a shaft or in this case a shaft with a screwdriver slot. The idea is that if there are variations in the tube you might be better able to "balance" out the hum if you have a bit more resistance one side or the other. You just turn the amp on with nothing plugged into it, crank the volume with lots of bass so you can hear the hum and then adjust the pot till the hum is at a minimum.

Usually the sweet spot is broad as a barn door. Most guitar amps just wire a 100 ohm resistor from each side of the heater to ground. At maybe a nickel a resistor it's much cheaper than a pot and works just as well in practice.
 
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