Plate output capacitance of grounded grid amp?

Discussion in 'Homebrew and Kit Projects' started by K0IZ, Jun 6, 2021.

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  1. K0IZ

    K0IZ Ham Member QRZ Page

    I'm designing a grounded grid amp for 20M using two 813's. Tube spec shows output capacitance of 14pf per tube. So in my tank calculations I used 28pf plus 5 stray cap plus 5 minimum on my 23pf tuning variable. Total of about 38. With Q of 12 and load resistance of 3050 ohms (2200V, 400 ma), tank values are 44pf, 3.1uh, 262pf.

    However in "Radio Transmitters" (Gray, 1961), says output capacitance in grounded grid is about 1/2. I would suppose this is because, as per W8JI, input and output circuits of GG are in series. Since input capacitance on 813's (2) is about 32pf and output is 28pf, that would be about 1/2.

    There have been tons of 813 GG amps built, but other than that one book I haven't seen anything about the output capacitance. Comments?
    KN4LGM likes this.
  2. W9WQA

    W9WQA Ham Member QRZ Page

    im no expert but ive built a few 813 amps by trial n error. i dk how to max effieciency
    but i never ran out of range with a tuning cap. also didnt do well at 20 and up.
    ill be watching.
    KN4LGM likes this.
  3. W1VT

    W1VT Ham Member QRZ Page

    Page 56 Gray & Graham Radio Transmitters
    "The output capacity is is approximately one-half that of the typical capacity neutralized grounded-cathode amplifier."
  4. K0IZ

    K0IZ Ham Member QRZ Page

    Yes, the Gray & Graham document is what caught my attention. Didn't recall seeing anything similar anywhere else. Lots of 813 GG amps over the years but they all talk about the 2x14pf output capacitance. When I get my circuit mocked up, I guess I'll find out.
  5. W9WQA

    W9WQA Ham Member QRZ Page

    and regardless of calculations, you will end up turning a knob with a cap on the other end !!
    SWL37632 likes this.
  6. SWL37632

    SWL37632 QRZ Member


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