[AMPS] How to determine the input Z of a Big tube?

[Tom E. Miller]

I have been a subscriber to the reflector for some time but this is my first post.  I have a question for the experts.

How does one determine the input impedance of a tube say a 3CX1200A7 for instance.  I have the Eimac data sheet which lists a lot of things but related to the input it says "Peak driving power 102 watts at 2500 volts on the plate"  It lists 255 ma of grid current at this voltage.  How do you figure it out.  You need to know this to design the input matching network.  DO you use the constant current characteristic chart or what.  I know for some tubes, they give the input z.  I believe it depends on the plate voltage and a lot of other things.  I am NOT an expert but maybe someone can explain it to me without flaming me.

Thanks in advance.

Tom Miller
WB8ICT
Tom E. Miller

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