[Boatanchors] Physics of plate current dip

[Ian Wilson]

Tuning a grounded-cathode power amplifier with a pi network goes as follows:
 1. set loading to minimum (maximum capacitance on loading capacitor)
 2. tune plate capacitor for resonance as indicated by plate current dip
 3. increase loading (reduce loading capacitance)
 4. go to 2 until there is no increase in output power

I have done a lot of reading on tube output stages and have searched for
information online about the mechanism of the plate current dip but with very
little success.

Thinking about what happens as the plate circuit is tuned through resonance,
the thing that is special about the resonant condition is that the plate voltage
and current are 180 degrees out of phase only at this point (in other words, the
load presented to the tube is purely resistive at resonance). However, I can't
quite make the connection between this condition and the DC plate current.

Grateful for any enlightenment.

73, ian K3IMW