811A's take -4.5 volts bias @ 1500 volts and give about 340 watts audio
level. They will take 2000 volts OK since the am rating is 1250 volts ( 2500
volts peak). 9 volts is the standard for 2000 volts I think.The Viking 500
uses 811A at 2000 volt and OK by the tube manufacturer at that time. Since
the 572B is a 2750 volt tube and will give way over 500 watts audio I would
just use them since the transformer is adjustable and going to changed
anyway. That way the voltages can all be the same.Mod and final. 73, Mike
