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

    Well, forgot to change some text and numbers for the Dual 6DQ5 Pi-net.

    DISREGARD the set of schematics above.


    Attached Files:

  2. N2DTS

    N2DTS Ham Member QRZ Page

    I put this together to test modulating three 4X150a tubes:

    I tried various coils to play around with the Q.
    For some reason I can not get any more peak power out then two tubes will do.
    It will make the exact same peak power output as two tubes will do. (1000 watts).
    That seems odd.
  3. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    What's the impedance conversion of the pi net?

    If we knew the Ip and Vp of your Tri-4X150A's maybe we could check your Pi net because I suspect your Pi net may be the problem. I have a program that determines a lot of stuff knowing those values.

    BTW, solder braid is great for RF connections isn't it?

  4. N2DTS

    N2DTS Ham Member QRZ Page

    The plate voltage was 2000 to 2200 volts, current depends on how much I can load it up to.
    300 ma gives 200 watts out and 1 kw on peaks.

    I have a lot of coils of various sizes and turns, some copper tubing, some thick wire.
    I had a wide range of results but could not get over 1000 watts on peaks.
    I am sure some coils were in the ball park as the cap tuning had range left to adjust...

    I may try the solid state modulator and see what I get.
    Its likely a limitation of the modified dx60 modulator circuit to handle peaks.

    At one point in the past I was driving FOUR of the 4x150's at 350 watts out and over 1500 watts pep, but had issues with reverse screen current.
  5. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Here are two coil designs using different gauges of wire.

    Ip = 0.300;
    Vp = 2100;

    The 6EW7 modulator circuit I posted earlier should be able to drive the trio.


    Attached Files:

  6. KM1H

    KM1H Ham Member QRZ Page

    Reverse screen is common and all you need is to add a common resistor from screen supply to ground that will pass about 10 ma per tube. With some tetrodes it is normal. Use GM3SEK's pi net calculator for proper L and C values and preset before testing.

  7. N2DTS

    N2DTS Ham Member QRZ Page

    You guys were spot on.
    After work (and yard work) I put the solid state modulator in and I could get a bit more peak power out, maybe 100 watts.
    I was at the limit for plate tuning and the loading cap was all the way open.
    I did a quick and dirty add of a 25, then 40 pf doorknob to the plate tuning cap and stretched out the coil and I can run 300 watts carrier
    and modulate it 120% positive at about 360 to 380 ma cathode current.

    I then put the modified dx60 circuit modulator on and it will do about 110% positive modulation.
    Both sound great.

    Things are quite different with screen modulation, it seems to call for much more plate tuning capacitance.
    The somewhat low voltage on the 4x150's and the high current is different from the typical plate modulated stuff.
    To get the peak power, you have to really overload the final.

    I need to replace the plate tuning cap with something larger (more PF).
  8. N2DTS

    N2DTS Ham Member QRZ Page

    I put a smaller but higher value of capacitance in for plate tuning, then played with the coil and got things working well.
    Had a nice short qso with Borgi (K9YQQ) and will send him some extra sockets and chimneys for 4x150's I got cheap off ebay.

    300 watts is a nice power level for 80 meters.
  9. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    That's great and if the rig is putting out 300 Watts your overall efficiency is about 48%. which is about the average efficiency I get with my SG rigs.

    Here are the numbers from my software runs:

    F = 3.5 MHz:

    Pin = 630 Watts, Rplate = 3.2k, Eff = 0.476, Cptune = 139 pF, L = 14 uH, Cpload = 864 pF, LPchokemin = 1.5 mH.

    F = 7.15 MHz:

    Pin = 630 Watts, Rplate = 3.2k, Eff = 0.476, Cptune = 60 pF, L = 7.0 uH, Cpload = 423 pF, LPchokemin = 750 uH, but we keep original 1.5 mH choke.

    If any homebrewer needs a run executed just send me the actual or predicted Vp, Ip numbers and we will try to get you into the "ballpark." :)

    Last edited: Nov 9, 2018
  10. N2DTS

    N2DTS Ham Member QRZ Page

    2200 volts, 380 ma cathode current (about), 300 watts out.
    836 watts in, 300 out, 536 watts for the tubes to dissipate out of 750.

    A single 4x150 (4cx250) will do 300 watts of carrier in plate modulation, but then you need a modulator and mod iron.

    The 4X150 is interesting as the filament power is very low, the grid drive is very low, the tubes are small and work on lower voltages.

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