[AMPS] Feedthrough power - a question

[John Fielding]

An interesting comment was passed this week on the reflector about the feedthrough power
from the driver having to be added to the calculated output power to arrive at the total
power - or something along those lines - when running grounded grid amplifiers.

I have often checked  an anode cavity for resonance by applying a few watts drive to the
cathode and monitoring the output with a spectrum analyser or milliwatt power meter. 
(This is with no heater or anode supplies).

However these amps where for 1296 MHz using 7289's where the anode to cathode capacitance
is neglible, so it is easy to see why the input needs to be a few watts to get 10mW or so
at the output.

So to my question - in a typical hf amplifer, using say a pair of 3-500Z's, how does one
calculate how much of the output power is due to feedthrough?  One can use various ways
of calculating the efficiency, but without knowing how much of the measured power is
feedthrough how can one calculate efficiency correctly?

I suppose that when the tube has anode potential applied the capacitances remain fairly
close to the no volts condition - am I right or not?  How does having the heaters powered
up affect the inter-electrode capacitances?

An interesting puzzle.  Anyone care to show me were I am going wrong in my reasoning.

John	ZS5JF

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