Curiosity comes knocking: Plate Modulation VS. Screen Modulation

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

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  1. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    With an Isg = 3X20 mA and Icg = 3X15 mA, thats 105 mA you need to subtract from your plate current.

    Plate Input Power to Finals = 2200 voltsX(380ma - 105ma) = 2200 voltsX275 mA = 605 Watts.

    Rerunning the equations:

    F = 3.5 MHz:

    Pin = 605 Watts, Rplate = 3.7k, Eff = 0.49, Cptune = 121 pF, L = 16 uH, Cpload = 760 pF, LPchokemin = 2.5 mH.


  2. N2DTS

    N2DTS Ham Member QRZ Page

    Gave the new setup a good workout lately, many old buzzard transmissions at 300 watts with no issues.
    I really like the results.
  3. N2DTS

    N2DTS Ham Member QRZ Page

    Borgi (K9YQQ) built one using a pair of 4X150's and used the WA1QIX dx60 modulator circuit.
    He says it works great at 175 watts out.
    I think he built it for 20 meters and up, but he may get bit by the screen modulated bug and start using it on all bands.

    I had my 40 meter rig on last night and it was making 250 watts carrier output and 130% positive modulation.
    Not sure why the 40 meter TX works better then the 80 meter one...

    Fun stuff!
  4. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

  5. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    I get the same output or better with a single 813 Final screen modulated by a single 572B. And the system can be "upscaled" to modulate even higher powered tetrode's without changing modulators.

  6. N2DTS

    N2DTS Ham Member QRZ Page

    Yes, many tubes can be used I suppose, I would like to try some sweep tubes as well.
    But its fun to use a 6EW7 to do the job.
    That is a lot of power out of a single 813 though, that tube has a 125 watts of plate dissipation..
  7. N2DTS

    N2DTS Ham Member QRZ Page

    Been having fun with the WA1QIX circuit into three 4x150a's.

    That is the 2nd half of the modulator, some changes made it even better:
    There are only 3 Zener diodes in the string on the cathode of the cathode follower, that was just an experiment.
    The driver stage (2nd 6SN7 stage) has a 330 k plate resistor to get more swing, and a 1 meg resistor from the grid to ground, not the cathode.

    I will take a picture of the earlier stages with the mic preamp and tone controls.

    Three 4X150a's,
    10 to 15ma grid drive and -120 volts grid leak bias (adjustable fixed bias set at 100 volts),
    700 volts going into the modulator,
    Resting carrier screen voltage 200,
    Resting screen current 6 ma,
    2200 volts on the plates,
    360 to 380 ma plate current,
    320 to 350 watts carrier output,
    130% positive modulation at 95% negative.


    No transformers in the audio path, takes a D104 and has negative modulation limiting built in, the D104 takes care of the high end roll off.
  8. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    You won't need feedback if you bias V1A into the linear region of it characteristic curves.

    This design has been my most linear screen grid low-to-medium-power modulator to-date and it doesn't have zener switching transients present on the cathode rail of the CF:


    Attached Files:

    Last edited: Dec 12, 2018
  9. KM1H

    KM1H Ham Member QRZ Page

    That 125W is super conservative and closer to 225-250W but just for the graphite anode version. The sheet metal plate version is easy to melt down.
    I could get up to 1500W out of a quad on SSB at 3200V and still look decent on the SA. I later rebuilt that with 803's and it loafed at 1500W on 40M and lower; the 803 is another seriously underrated tube that was purpose built for SG AM for the USN.

  10. N2DTS

    N2DTS Ham Member QRZ Page

    I will have to give that a try!


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