|steve conner||Re: 12.6V filaments?|
"The 6V6 output tubes are wired in series going from one side of the heater tap to the centre tap to the other side of the heater tap."
In that case I would say they are not in series. After all, if you pull one tube, the other will still receive 6.3V. It's more correct to say that one tube is running off each 6.3V leg of the 12.6V centre tapped heater winding.
This was presumably done to equalize current draw. But you should be aware that running heaters off 12.6V may give more hum than off 6.3V. The reason is (maybe) that the heater is designed so the hum field cancels if the two ends of the heater are at equal and opposite voltages. If you run it off 12.6V centre tapped then one end of each heater will be at ground and the other at 6.3V and the hum cancelling will be defeated.