HB 813 amp VHF resonance in plate circuit

Discussion in 'Homebrew and Kit Projects' started by K0IZ, Jul 13, 2021.

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  1. K0IZ

    K0IZ Ham Member QRZ Page

    I'm nearing completion on a pair of 813's for 20 meters, grounded grid. Plate Pi-net is tuning properly as verify with my trusty Millen GDO. Decided to check for other resonances and found a plate circuit resonance about 125 Mhz. This is in addition to the desired 20M resonance.

    In troubleshooting, I have removed the tubes and disconnected the plate choke (and parasitic supressors). So now only have the plate coil, tuning variable, and loading variable. Still have resonance at about 125Mhz. Frequency slightly changes with changes in either variable. GDO coil is fairly closely coupled to the plate coil (closer than I need for the primary 20M resonance).

    Plate coil is #8 copper wire, 1 3/8 dia, 14 turns in 2 1/2". Tuning variable is 23pf wide-spaced, loading is 206pf plus 150pf doorknob in parallel. I have tried without the doorknob and still about 125 Mhz. I connected wiper of both variables directly together, no change.

    If I hadn't checked, I would be dumb and happy, but now I worry as to why I have this resonance, and what will happen when I fire up the amp.
     
  2. K0IZ

    K0IZ Ham Member QRZ Page

    UPDATE: I pulled out the plate coil. #8 copper wire, 1 3/8"dia, 14 turns. See photo. GDO shows a self resonance (ends not connected) of 140 Mhz. I assume combination of the 3.17 uh inductance and distributed capacitance. So is this a problem or what? P1040845.JPG
     
  3. W7FOX

    W7FOX Ham Member QRZ Page

    IZ,
    I suspect all tank coils have a self resonant frequency. In my own 813 amp that covers 80 through 10 meters, I have absolute stability without use of parasitic suppressors in the plate leads. I have never looked for resonance in the tank though, but I suppose its there. What I did check for was series resonances in the plate choke, those will really cause trouble for you. I have added or subtracted a few turns from the choke to move these resonances to an unused frequency. Kudos for building your own equipment, its the best part of the fun.
     
  4. K0IZ

    K0IZ Ham Member QRZ Page

    Thanks, Chris, for comments. I did check the plate choke and then checked the rest of the circuit. I decided you are totally correct, all plate coils have self resonance, but nobody bothers to check. I think I'm good to go, and anyway doubt the 813's have much gain at 125 Mhz. I did notice that most 813 amps did not have suppressors.

    This amp goes with my "Covid" project, a tube 20M SSB transceiver with single 6146 in final.

    JoHn.
     
  5. W9WQA

    W9WQA Ham Member QRZ Page

    my dumb luck, never tested anything. just ran it and tinkered with micas till it tuned.
    im not too good on 20 and up, yet...160,80,40 hot !!

    using just 2 813. tested 3...
     

    Attached Files:

  6. K0IZ

    K0IZ Ham Member QRZ Page

    Tom, love those transmitting micas. Solid state!
     
    W9WQA likes this.

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