[Mondial]

OK, but both windings of the original Philco transformer are 6.3 V. There is no 12.6 V winding. The center tapped winding is 3.15 V each side of the tap. Otherwise, with a 12.6 V winding you would be applying 12 V to the 6 V heaters.

That one winding is only center tap grounded for hum reduction, feeding the low level audio tubes and FM detector. Grounding one side of an untapped winding for the heater supply, and feeding all the heaters is OK, but you may have a bit more hum.