[Mondial]

As was mentioned, the hum balance pot nulls the 60 cycle component in the output caused by the AC on the directly heated filament of the output tubes. When properly adjusted, the in phase hum components of each output tube cancel in the push pull output circuit, which only responds to signals 180 degrees out of phase.

I guess to save money, Philco did not use separate windings for the output tubes alone, but combined the heater load of the other tubes, putting half of the total filaments on each output tube filament winding. Since the rest of the tubes have indirectly heated cathodes, they are not affected by the phase of the heater AC set by the balance pot.To the remaining tubes on the winding with the balance pot, it looks just as if they were fed by a center tapped filament winding.