[Boatanchors] Dipping the plate current at resonance

[mac]

A class C amplifier conducts current only during a short interval  
during the positive peak of the incoming RF wave cycle.  The  
concomitant current pulses excite the plate tank circuit to produce an  
oscillating voltage at the same frequency as the incoming grid  
signal.  When the plate tank circuit is adjusted to resonate at the  
exact frequency of the incoming signal at the grid, the maximum  
excursion of the instantaneous voltage on the plate in the negative  
direction will coincide with the maximum positive excursion of the  
voltage on the grid and the total potential across the tube will be at  
it's lowest point which in turn limits the current the tube can draw.   
This is why the DC plate current is seen to decrease or "dip" at and  
near resonance. (More specifically, seen to "dip" as one tunes across  
resonance.)  At exact resonance the dip is greatest because: a) the  
impedance of the tank circuit is greatest and hence the P-P plate  
voltage excursions are the largest, and b) the peak positive voltage  
on the grid and the peak negative excursion of the voltage on the  
plate line up exactly thus giving the lowest potential across the tube  
and hence the lowest current figure.  As we tune the plate tank off  
resonance the DC plate current rises for two reasons.  First, the  
impedance of the tank circuit decreases causing the P-P voltage  
excursions to decrease, resulting in a higher potential across the  
tube hence higher current. Also, when the plate tank is tuned off  
resonance, the impedance of the circuit is no longer purely resistive  
and the now introduced reactive component (capacitive or inductive  
depending on which side of resonance we are now on) introduces a phase  
shift in the tank circuit voltages, the peak voltage excursions no  
longer line up exactly and so the potential across the tube is  
increased even more.

As the loaded Q of the plate tank circuit decreases (as a result of  
loading up one's antenna, for ex.)  the amplitude o the P-P voltage  
excursions in the plate circuit will decrease further causing the  
plate current at the bottom of the dip to be a higher value in  
accordance with the mechanism described above.

Dennis D.  W7QHO
Glendale, CA