[Joeztech]

Jeff;

Just to add a little to Steve's comment about the 80 rectifier tube, most rectifier tubes are on a separate filament winding of the transformer to keep a potential short of heater to cathode from causing a major short in the power supply. If all the tubes in a set had their filaments in a common circuit all of them would be subjected to B+ on their filaments plus their normal AC voltage. Potentially a good number of parts could be fried as a result. There are some more modern rectifier tubes that use 6.3VAC filaments, but even they are supplied with filament voltage from a separate winding of the transformer. Some of the 6.3VAC filament tube designs have a separate heater and cathode, but most of the older 5VAC filament designs have the heater as the active cathode element, so heater and cathode are one and the same in those designs. Some more modern Hi-Fi and stereo tube equipment used solid state rectifiers to create a DC voltage and filtered it with electrolytics and then applied it to the filaments of tubes in the audio preamp circuits. This was done to help eliminate 60Hz hum from the audio. Older radios such as these only use AC on the tube filaments.

Joe