Home-brewing a Plate modulated Push-Pull 810 deck

Discussion in 'Amplitude Modulation' started by W2BTK, Aug 29, 2019.

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  1. W2BTK

    W2BTK Ham Member QRZ Page

    Hello AM community.

    So I want to start a thread about this deck that I'm building. It's a pair of 810s in push pull config being modulated by a solid state crown XLi-800 audio power amp. Me and my friend Dave got it lit up on tuesday, it was conducting and didn't oscillate. I didn't have the grid input ready, so we weren't able to feed it any drive. I am using a national MB-150 tank assembly for the grid input tuning. The only info I could find about it being used backwards for a grid input was in this article. http://n4trb.com/AmateurRadio/GE_HamNews/issues/GE Ham News Vol 14 No 5.pdf

    It seems they modified the MB-150 and added a second link. The pictures suck. I am going to try what I *think* they did based on the wording. Has anyone done this? Any pro-tips?

    Attached Files:

  2. W2BTK

    W2BTK Ham Member QRZ Page

    I'll include some pictures of the deck as it sits so far.

    I'm not gonna be very organized about this thread. My main concern is finding somebody who has used an MB-150 in a similar fashion to that GE article, who could maybe show me a better picture of the modified MB-150!!!!

    Attached Files:

    W1BR, W1TRY and AG5CK like this.
  3. W2BTK

    W2BTK Ham Member QRZ Page

    Oh and hello to all the lovely moderators!!!!!
  4. W2BTK

    W2BTK Ham Member QRZ Page

    Oh yeah. Sorry for the mass posting. Like I said, organization is out the window today. Welcome to the mosh pit.

    I wanted to show you guys this capacitor... It's a dual-gang vaccum variable. Never seen one before this build, but I found it and it works. Definately never seen a ham radio transmitter with one in it. Anyways. More eye candy. Check it out. And tell me about your MB-150.

    Attached Files:

    W1TRY and K6RBK like this.
  5. N2DTS

    N2DTS Ham Member QRZ Page

    Those tuners use the series and parallel setup to cover a wide range of frequencies.
    I would just try it backwards without the extra link first.


    I have the 40 watt version, never used it, its been sitting on the shelf for 40 years....
  6. W2BTK

    W2BTK Ham Member QRZ Page

    It didn't work, 100W of drive vanished into thin air. (nothing blew up either....)
  7. W7TFO

    W7TFO Premium Subscriber QRZ Page

    Very nice job overall. The use of braid carrying RF leads to complications, better to use thin copper ribbon.

  8. W2BTK

    W2BTK Ham Member QRZ Page

    Thanks! It's not done. I was actually replacing those things as I read your post. Haha.

    Attached Files:

  9. K4KYV

    K4KYV Subscriber QRZ Page

    Have you measured to see if each section of the dual vacuum variable is the same capacitance? Or is this stamped somewhere on the unit. Never saw or heard of one of those before.

    My final uses a pair of HF-300s in push-pull in the final. Plug-in coils and link coupling. I have also used 810s and 8000s. Mine uses split stator air variables for both grid and plate tuning. I found that 810s require about 1/3 more rf driving power for class-C plate modulation service than do type 8000s or type HF-300s. I drive mine with a single VT-4C/211 (eat your hearts out, audiophools). The driver stage runs close to 100 watts DC input to get full grid drive. When I first built it back in circa 1970, it was tricky taming the parasitics.

    I still use a pair of 810s in class-B for the modulator, driven by a quad of 2A3s in push-pull/parallel. (once again, eat your hearts out, audiophools)

    Click on the photos to enlarge.
    HF300 final.jpg HF-300 final.JPG
    WZ5Q likes this.
  10. W2BTK

    W2BTK Ham Member QRZ Page

    Yeah the values are close. I may not have it hooked up right.. I am going to try adding the second link showed in that article and feed it how they are feeding the kilowatt parallel 810 amp in the article I posted, except both my leads will go to the grids.

    Hey that's really cool... There's your balanced output! I talked to you on the air once and marveled at the fact that you don't have any unbalanced transmission line in your setup. There it is to look at! Very cool.

    Edit: also very nice HDVL tank... I couldn't find any that nice. Going to be rebuilding the few I found

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