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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    TZITZIKAS Banned

  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 Ham Member 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.

    TZITZIKAS Banned

    first thanks all of you. the stage with 813 tube its working now and it is the final stage of an mw band transmitter at 1431khz in Thessaloniki - Greece.This is an amateur student radio station which is transmitting from university of thessaloniki. the 813 tube is driven by an 807 tube at 430volt dc. the 813's anode voltage is 1400volt dc. between constant capacitor at the output of 813 and antenna, there is a match circuit which you can see at the following link. also you can see the previous stages.
    the 813 now is plate modulated. the final audio amplifier is an 2x811 tubes amplifier at 1100volt dc. G1 current is 12mA and G1 voltage is -120volt. Is not necessary to have a negative power supply for G1.
    i wan't to remove the other stages, the modulator and the final audio amplifier and to drive the 813 stage with mosfet amplifier like this i have posted above. the modulated stage will be the first stage: the 50mw pll. its is possible?

    thank u and sorry for my English.
  12. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    This is all beginning to make sense now.
  13. W5HTW

    W5HTW Ham Member QRZ Page

    It is starting to make sense. We didn't have enough information at the beginning. I think, as I understand this, we have a plate modulated 813 output stage, drive by an 807 driver. He wants to remove all stages prior to the 813 final, leaving it unchanged, and drive that final with the mosfet exciter. I see this as completely workable, no serious problems at all. The mosfet exciter should have sufficient output to drive the final without difficulty.

    I would very simply remove the preceding stages, and connect my mosfet output at the grid coupling capacitor of the 813 final, and go for it. Doesn't seem very complicated at all.

  14. W2VW

    W2VW Ham Member QRZ Page

    I'd still want to see a safety bias supply in case drive is lost.

    Voltage step-up to the grid would make things easier. A 1:9 toroid would do it.
  15. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    To me, it still doesn't make sense.

    The 813 PA as drafted in the schematic cannot work since there's no RF bypass on the filament; the tube's an open circuit for RF.

    The "no bias required" statement also doesn't work for a linear amplifier. It might work okay for a Class C modulated stage, as the original amplifier has been described. But if you want to modulate at the low level and then amplify, the 813 must be changed from Class C to Class AB2, and that will require a bias supply.

    It will also need a tuned input circuit, since the tank of the original 807 driver won't be there.
  16. AC0OB

    AC0OB Ham Member QRZ Page

    813 Tube stage

    Since you allready have a solid-state linear stage (or at least a schematic for one), I would abandon the tube circuits alltogether and add another pushpull linear MOSFET stage for higher power with a Vds of 50 volts using say NTE 2396's with about 2.5 Volts on the gate for AB1 bias. It appears you would have enough drive for a final higher output stage.

    Give us some system requirements if you can such as :

    What is your expected power output, antenna types/impedances, etc.
    Last edited: Jun 1, 2009
  17. KL7AJ

    KL7AJ Ham Member QRZ Page

    The 813 makes a good linear amplfier, but not in grounded grid. In grid-driven mode, the 2-15 watts will drive a pair of them comfortably to the full gallon input. Check out ARRL Handbooks from the 1960s.

  18. W2VW

    W2VW Ham Member QRZ Page

    Sure it does. Tie all the grids together and it acts exactly like a large 811.
  19. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    ::It sure won't "drive" with 15-20W that way! Maybe with 100W. Then, he'd need to add filament chokes and a tuned input network capable of handling that, plus some sort of cathode bias to put the tube into the linear region. Not sure this is "less work!":p
  20. AB1GA

    AB1GA Ham Member QRZ Page

    Good luck, but it's best to take care when describing your station.

    Using the term "amateur" when referring to a radio station around here usually means a station in the Amateur Radio Service, which is not permitted to use 1431 kHz in Greece, the US, or anywhere else I know of.

    I assume you are referring to a college broadcast station, which makes more sense. However, in your balun thread you called it a "pirate" station, which implies illegal operation! I'm sure many of us would regret helping you to get fined or imprisoned! :eek::eek::eek:
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