Ian,
I have just been through this exercise. The long answer is that the plate
load impedance is the peak RF voltage divided by the peak fundamental RF
plate current. These values are determined from the constant current
characteristics of the tube. The Eimac book "Care and Feeding of Power Grid
Tubes" explains the procedure.
The short answer is that the plate load impedance is the DC plate voltage
divided by 1.8 times the single tone DC current as read by your plate meter.
This value assumes class AB or B operation and is close enough for working
purposes unless you have some unusual operating conditions.
David C. Hallam
KC2JD
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