[Mondial]

I think you are better able to compensate for mismatch in output tube characteristics with two individual filament windings. If the two 6A3's were perfectly matched, then a single winding with a hum balance pot would be fine. The pot just serves to find the exact electrical center point of the AC winding for balance.

But if the tubes characteristics differ slightly, you may be able to get a better hum cancellation with individual windings. Effectively each has its own center tapped filament supply, one being variable to achieve exact balance and hum cancellation.

Too bad Philco didn't use 2A3's instead of 6A3's. The lower the filament voltage the less hum induced in the output, but then they would need a separate 2.5V winding.