Hi Brian: Consider what the efficiency is with no drive. The tube is drawing current and has voltage therefore it has input power but no output. Efficiency is 0%.
As input drive is applied the efficiency increases but so does the plate current. The trick is to reach the 25% power point (or 50 Voltage point if monitoring on a scope) with out hitting the spot where the dissipation is 65 watts / 811A. John, WA5BXO
