What is the effect on plate current of the correct setting of a vacuum-tube RF power amplifier's TUNE control?
Why In a vacuum-tube amplifier the TUNE control resonates the plate tank circuit. At resonance the tank presents its highest impedance to the tube, so the DC plate current drops to a minimum while RF output peaks. Operators therefore 'dip the plate' with TUNE, then advance the LOAD control for the desired output and re-dip.
Watch out A pronounced peak describes what the grid drive or RF output meter does at resonance, and what the LOAD control tends to do to plate current as coupling is increased; plate current itself dips at resonance.
TUNE dips the plate, LOAD brings it back up. 'Dip for resonance, load for power.'