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drive an 813 tube with mosfet rf amplifier at mw band

Discussion in 'General Technical Questions and Answers' started by TZITZIKAS, May 31, 2009.

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  2. N4CD

    N4CD Ham Member QRZ Page

    AM transmitter

    I assume you want to operate in 'AM'.

    Your mosfet stage is a 'linear amplifier'. It will amplify an AM signal.

    The 813 stage you show is an untuned amplifier. First, you need to rethink everything.

    You will need either a linear amplifier with an 813 tube - not that easy to do


    you will need a modulator for the 813 stage (not easy to do) and run the 813 as a AM modulated output stage.

    The circuit diagram you show for the 813 has no 'tuning'. You will need tuning in the amplifier stage , both on the input and output.

    Of course, the 813 will take a HV supply of significant size. You don't get 400W output without 600W plus input. if you run it linear, then you need to have a 1000W supply, and I doubt whether a 813 will run 1KW input - You'd need a pair of them, plus likely 30-40w of drive.
  3. W2VW

    W2VW Ham Member QRZ Page

    I was gonna wait until somebody asked exactly what flavor of R.F. is supposed to be amplificated. Or the usual what's your callsign let me see your papers....

    If linear service is desired:
    Cathode driven. Leave the screen and grid supplies in place. 1 X 813 good for 250 watts P.E.P. out all day long with a little air on it.
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    The 813 amplifier schematic not only has no RF tuning but also has no grid bias circuit. It appears they're trying to run an 813 with no bias supply, this isn't going to make for a linear amplifier.

    The input won't be 50 Ohms, and the output won't be anywhere near 50 Ohms. The "untuned" output for the schematic shown has a plate load impedance of >3000 Ohms. You can't connect that to any sort of transmission line and antenna system.

    The only "m.f." amateur band is 160m, obviously. The 813 should work fine at that frequency but only if the amplifier is built properly. You'll need both an input and output tuning network, and a grid bias supply.

  5. W2VW

    W2VW Ham Member QRZ Page

    It pays to look at the whole schematic:D Whoops.

    It would work cathode driven without a negative supply but not with a whole 400 volts on the screen. Maybe 50 volts on the screen. More drive would be needed.
  6. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    ::The picture I have only shows a choke and 10K to ground connected to the grid. You see something else? Same picture only shows filament power requirement and no RF bypassing of the filament, which makes an incomplete circuit for RF. I don't see how this could possibly work.
  7. W2VW

    W2VW Ham Member QRZ Page

    I think the schematic was simplified by showing only DC connections and ignoring R.F. Maybe from some place where that's how it is presented to students.

    You are correct Steve. I see the same thing. There is no grid bias circuit. That circuit would not work even if completed for R.F.

    That's probably part of the reason the question appears here.

    We still don't know what it is intended to amplify either.

    Could just be a mental exercise from a veiled poster.

    Maybe it's really Cotroversial Burt or The Dawg. Could Be Glenn Baxter too.
  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    ::I doubt it's the "Dawg," because he identifies here on the Zed with his real call and creates havoc the best he can.:p Actually, he moved to CA and is now a local to me, I think in the Pasadena area.

    Haven't heard Baxter in a while, and don't miss him.

    Back to the amplifier circuit, I don't think I'd use an 813 today for anything except maybe a reading lamp. It's a tube that's far older than I am, and just a silly choice for almost anything unless you happen to have an old amp already using them.

    Today I'd pick a 3-500Z and get it over with. Still not a bad night light, but a far more useful tube.:)
  9. AC0OB

    AC0OB Subscriber QRZ Page

    I have to agree with previous posters, even at 300 m BC band, without any tuned circuits or grid bias, I don't see how any useful RF power can be derived.

    Here is the spec sheet for an 813

    and it show that even for clas B RF, the grid should be biased to at least -60 volts.

    The solid-state portion of the schematic appears to show three class A or AB stages with the last in pushpull, and may go from BC to 160 m in broadband. One would still need an AM or SSB modulator to drive this circuit and with proper input matching.

    But you had better have the proper licencing to go on the air or feed it into a dummy load for experimentation.

  10. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    ::It's worse than that. The schematic doesn't even show any RF bypassing on the filament, which means the transmitter would be looking into "nothing," and no amplification could occur. This is a very incomplete circuit, kind of a Thevanized analysis for a test, but not a real working amplifier.
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