What is the effect on plate current of the correct setting of a vacuum-tube RF power amplifier’s TUNE control?

When tuning a vacuum-tube RF power amplifier, the TUNE control adjusts the resonant frequency of the output tank circuit. At resonance, the impedance of the parallel LC tank circuit reaches its maximum. Because the impedance is at its highest, the current flowing through the plate circuit is at its lowest. Therefore, you look for a "pronounced dip" in the plate current meter to indicate that the amplifier is properly tuned to the operating frequency. Other options like a "pronounced peak" apply to output power or grid current, not plate current. A slow oscillation or no change would indicate a malfunction or failure to tune.