I've had some experience with two 1 KW AM broadcast rigs, the Sparta
701 and Gates BC1G. Both meter the final PA plate current by putting
the amp meter in series with the CT of the final PA filament supply
transformer secondary and ground, using appropriate meter shunts of
course. The Sparta runs 4-500As; the Gates is a triode rig, 833As.
Usually your grid current meter on the PA is placed between the grid
leak resistor and the filament supply CT.
These are class C grid driven transmitters, very different from a
grounded grid, cathode driven class AB amplifier which, because it is
cathode driven does not require a neutralizing circuit. I believe
Bill Orr developed the grounded grid cathode driven linear amplifier
when he was with Eimac, and covered the design in a 1969 Ham Radio
Magazine article using the zero bias 3-500Z. I imagine he worked
out the metering.
With such an amplifier, there's a lot going on around the cathode: It
has to be lit up, biased, metered, and driven with RF. It takes a
lot more drive, but manufacturers liked it because the cathode Z was
easy to match, or with tube exciters they didn't even bother, and no
Cn circuit was needed. Band switching was easy.
Merry Xmas
Rob
K5UJ
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