Using an SB-200 as an example - The HV power supply will supply ~2000 v under load. The pair of 572b tubes work at ~.5 ma of plate current in tandem. That provides you with 1000 w input power. Add to the the wattage from the exciter - let's say 60 - upi get 1060 watts of input. At 65% efficiency you will get ~689 watts output.While on the subject I just want to see if I got it right. When they say input they mean how much juice it pulls from the power supply, and input times efficiency equals output. Is this correct?