73 Pro reverb output balance control

My amp has a adjustment pot near the output tubes.I know it is not a bias adjustment. I believe it's called a output balance control.Can anyone tell me what this control does?

It is a balancing pot for the bias voltage. IT doesn't adjust the bias voltage but balances it between the two sides of the OT primary....between the two power tubes. IF the power tubes are unmatched to some certain amount or if that bias votlage is unbalanced far enough, there will be a hum. That control can eliminate that hum by balancing the voltage between the tubes. You can run unmatched tubes in a circuit like this...just by balancing the hum out. Play with it...turn it until you hear a hum. IF it doesn't go to a hum in one direction, then turn it in the other direction. Most of us will run the amp with the control in the quietest position there is on the 'dial' there.
Some folks change this balancing circiut out for a true bias voltage adjustment circuit. Some leave this balancing circuit and add a bias voltage adjustment pot so as to have the best of both worlds....selection of the amount of bias voltage and the ability to balance mismatched tubes..or to very slightly unbalance mateched tubes for harmonic 'richness'.....

Does this amp have a 'Hum' pot on the right side of hte rear panel?? If so, that is an adjsutment for the heater filaments. This pot should always be run to achieve the quietest operation...nothing to do with bias.....it balances the heater filaments which are set-up in a humbucking circuit.

Wally,thank you very much for all the great information.