Terry,
The 12AD6 was designed for car radio use and was meant to operate from 12 volts DC for both heater and plate. The heater has a fairly wide "safe" voltage range since it was designed for the automobile electric system and I am sure it could be operated from AC also. But all you need for B+ is 12 volts DC.
Radio and TV News had an article "12 volts Heater and Plate" that detailed the operation of these then new tubes. They were used in the hybrid auto radios that used tubes in the RF and IF stages and transistors for audio.
Here is a good website on the low plate voltage tubes that might be helpful to you:
http://www.junkbox.com/electronics/lowvoltagetubes.shtml I built an 80 meter receiver using low potential tubes many years ago when I was a novice but it is long since gone.
Rodger WQ9E