Curiosity comes knocking: Plate Modulation VS. Screen Modulation

Discussion in 'Amplitude Modulation' started by N6YW, Jul 6, 2016.

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  1. N2DTS

    N2DTS Ham Member QRZ Page

    Can I get 200 volts out to the screens if I put 700 or so in?
    The 6EW7 does fine for two 4x150's, 700 volts in, 200 out.
    The original WA1QIX circuit I used had about 600 volts going in I think, for the dx60.

  2. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    I don't know why you would need 200V quescient on the grids of the 4X150 as that seems too high. What you need to do is to find a quescient voltage that gives the nominal power output and then have available sufficient audio voltage to make enough peak to peak audio voltage to arrive at 100% modulation. That's where your speech amp and the first stage of the 6EW7 comes in.

    The voltages shown on the schematic were for typical SG rigs. For a DX-60 you only need about 60V quescient, for the Knight T-150 you only need 35V quescient.

    To your answer yes, I have run 700V on the plate of the 6EW7 of my Turbo-SGX rig. However I prefer to run no more than 600Vp on the second stage of the 6EW7. I also use a relay to reduce standby power dissipation since I see no need to keep the Speech amp and Modulator active during standby.

    If you find you need a higher quescient voltage than the RAMpower pot. can give you, change Rg2 to 3.3 Megohms. I have found the second stage of the 6EW7 to have a fairly high DC current gain for a very low mu tube.

    Here is a higher voltage version that you may find useful:

    When the audio voltage peaks to 95V (from the 190Vp-p source) the peak SG voltage will be 185 volts, or about 75% of the max Vsg of the 4X150.

    If you find you need a tad more audio, then place a 22 uF cap across Rk1.

    If you start paralleling tubes and need higher currents and voltages the 572B CF stage is the way to go.



    Attached Files:

    Last edited: Dec 13, 2018
  3. N2DTS

    N2DTS Ham Member QRZ Page

    On my current transmitters, it takes 200 volts to the screens to give the correct power output and modulation levels.
    That gives me 225 watts out of two tubes, 300 to 350 watts carrier for three tubes.

    When I run the 4x150's in AB1 as modulators, they have 360 volts on the screens....
    My guess is the screen modulator does about 350 volts to 50 volts for 100% modulation, or a 300 volt swing from 200 volts.

    On all my transmitters the plate voltage is off of both the cathode follower and its driver as its the screen voltage for the transmitter.
    Its a keyed variac supply with adjustable current limiting and adjustable dropping resistor.
    The resistor is dialed off for screen modulation.

    Been running 700 volts into the 6EW7 modulator (40 meter rig) for years without issue.
    It does not like going much higher though...

    The 6EW7 will break down when pushing three tubes, so I tried a triode connected 6L6.
    Seems to work fine.

    The modulators are standalone so I like trying things.
    I can extend the cables to the desk (B+, screen output and ground) and try things live.
    With 3 connections and 120 vac power I can swap out the modulators quickly.
    The solid state one is even easier, no AC power needed.

    This stuff is really fun to play with!

  4. N2DTS

    N2DTS Ham Member QRZ Page

    The first part of the modulator:
  5. N2DTS

    N2DTS Ham Member QRZ Page



    This is what I am using lately, mostly with the D104 input.
    Seems to sound good, drives three 4X150's at 300 to 350 watts carrier and over 100% modulation.
    The negative limiter hits about 90 to 95%.
    This circuit is made with a 6SL7, a 6SN7 and the 6L6.

    Anyone see anything wrong with the design?
    I made some changes to get more grid swing to the 6L6.
  6. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Do you get Electric Radio?

    If so, see my modulator design in #350 using a 572B as it will handle 2500 Volts Vp at 500 mA peak Ip and drive just
    about any tetrode in existence.

  7. N2DTS

    N2DTS Ham Member QRZ Page

    I no longer get electric radio, but I think you may have sent the design to me before and I printed it out.

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