Convert DAC to add FM

[david winter]

The David Winter conversion will work but the question of a power source remains. Possibly using the output valve cathode bias may provide the necessary voltage but its contentious. 
Providing a mains fed transformer for the conversion and at the same time converting the poor pilot lamps to 6.3v fed from the same transformer would be the best solution.

Sam.

Powering the FM-DW module using the 6.3Vac heater circuit is the usual way to do. Of course this supposes that one side of the 6.3Vac winding be grounded. This won't work if for example the 6.3Vac is a part of an auto-transformer.
For AC/DC sets I always use a separate 12Vdc supply. You need under 100mA so those tiny Chinese PSUs will do. I also add a series resistor to drop the voltage down to 7Vdc or 8Vdc, which reduces the inherent noise of those switched PSUs.
In any case, never power the module from the cathode bias voltage.