What is the correct adjustment for the LOAD or COUPLING control of a vacuum tube RF power amplifier?
Why A tube amplifier's pi-network has two adjustments: PLATE (TUNE) sets resonance, and LOAD (COUPLING) sets how tightly the tube is coupled to the 50 ohm load. Increasing loading draws more power from the tube, raising both output and plate current, so you advance it until you reach the power you want while keeping plate current within the tube's rated maximum. Running beyond that current rating overheats the tube and shortens its life, and under-loading wastes power as plate dissipation.
Watch out Minimum plate current describes the dip you look for with the TUNE control at resonance, not the loading adjustment, and SWR on the antenna is a function of the antenna system, fixed by an antenna tuner rather than by the amplifier's loading control.
TUNE for the dip, LOAD for the power - and never exceed max plate current.