Ian, G3SEK wrote:
"A related topic would be the effect of tank circuit Q on bandwidth of
HF amplifiers;"

Class A amplifiers are little used as HF finals, so in practical
amplifiers current is only part-time.

Impedance of a parallel resonant circuit is high. Circuit impedance
rises with inductance. Q rises with capacitance.

A Class C plate tank introduces a load on tube or transistor. It should
waste only a small percentage of the power generated. It should have
enough Q to linearize the output of the amplifier.

Terman says it is easy to show that the Class C tank circuit efficiency
is: 1 - Qloaded/Qunloaded.

Loaded Q is the ratio of the circulating volt-amperes to the transmitted
watts. If Q is too high, bandwidth is too narrow. If Q is too low,
harmonics are high.

As Q is ordinarily high, the tank circuit impedance is higher than the
load on the amplifier.

Impedance on the Class C amplifier has little effect on the tube or
transistor loading. Output impedance presented by the transmitter to the
load is determined in many cases by the percentage of the time the
amplifier is switched-off.

Best regards, Richard Harrison, KB5WZI