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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

    Yeah. this thing is hooked up wrong. stand by.....
  2. K4KYV

    K4KYV Subscriber QRZ Page

    Mine are all rebuilt. The ones made by B&W used cellulose acetate (I think) for the plastic strips that hold the wires together and in place. The stuff deteriorates after a certain amount of time and begins to shrink and eventually it disintegrates into a brown powder. As a result, the coils tend to deform, sometimes putting enough stress on the ceramic bar to cause it to break. As the plastic deteriorates, it loses its dielectric characteristics. I literally had one to catch fire and break into flames while I was transmitting. Hard clear polystyrene would be excellent for re-building the strips, but I don't think anyone makes that anymore (you used to be able to order it from mail-order radio parts stores like Allied and Burstein-Applebee back in the 60s), so I use Plexiglass sheet stock. Not as good a dielectric, but it's mechanically and chemically stable and since there is so little of it actually in the coil, I don't think it makes any difference. Coils made by Bud and National used some other kind of plastic, maybe polystyrene, and they don't seem to deteriorate with age.

    Also, the stock HDVL coils tend to have too many turns; they were calculated for the maximum likely plate voltage to plate current ratio, which requires more inductance and less capacitance for a given Q. I needed fewer turns in mine. I never did try to calculate the number of turns mathematically; I set the variable capacitor for the approximate correct capacitance, as best as I could determine, following the coil charts in the Frank C. Jones Radio Handbook and 1930s-40s-50s ARRL handbooks, and removed turns, one turn at a time, until I could get resonance with that correct amount of capacitance.

    Some of mine are more homebrew than commercially made; I salvaged coil stock from various junk coils picked up at hamfests and used it to put together my own coils with the proper number of turns, wire size and spacing. My 160m coil is homebrew from scratch. I bought the correct size wire from a motor rewinder and wound the coil on a glass jar the proper diameter for the coil, then glued the plexiglass strips in place. The glass jar was wrapped with wax paper before winding the coil, so the glue wouldn't bond the plastic strips to the jar. Once the glue hardened, I used a small hammer to break the glass jar, and when the pieces of glass fell out, a beautiful air wound coil was left. I then glued another plexiglass strip on the underneath side under each outside strip, so that the wire is sandwiched between the two strips, and the space in between the turns of wire and the inside and outside strips is filled with the glue, forming a solid insulating plastic bar to hold the turns in place.
    W2BTK and WZ5Q like this.
  3. N2DTS

    N2DTS Ham Member QRZ Page

    I loved the push pull stuff.
    I built a pair of 812a's modulated by 811a's, a very smooth working transmitter.
    I used vacuum variable neutralizing caps out of paging transmitters designed for 70 MHz, 3 to 30 pf at 15kv.


    Never should have sold it, the Harley I got sucked!
    AG5CK and W2BTK like this.
  4. W2BTK

    W2BTK Ham Member QRZ Page

    Okay so I found a multitude of problems.

    So many I will only list the majors:

    Grids of 810s were tied to the same condenser, the output connections were wrong.

    Had a 10 ohm short to ground on the grids through a choke in the MB-150 that needed to be isolated and tied to Bias - instead

    One of the shafts of the tuning condenser was not grounded (the clip was dangling)

    Did not have a link for the 10Mhz and up side of the MB-150 (that's what the two wraps I was trying to figure out were for)

    God dammit if this wasn't educational. I seriously feel like I learned a lot today.

    I believe this rig may very well work now. Still a whole lot of buttoning up before it sees any HV.
    Last edited: Aug 30, 2019
    AG5CK likes this.
  5. W2BTK

    W2BTK Ham Member QRZ Page

    Here's what I ended up with.

    Attached Files:

  6. W8KHK

    W8KHK Ham Member QRZ Page

    It looks like your bias source is connected to the link side of the MB-150. If your circuit diagram is accurate, there is no bias connection to the 810 grids. With no plate voltage, you will have grid drive, but no bias, so do not enable plate voltage until you verify the bias is reaching the grid pins.

    I like Push Pull finals too!
  7. W2BTK

    W2BTK Ham Member QRZ Page

    Funny you say that, I thought the ratio was off. We used the 40M coil to get resonance on 75M. Probably a combination of my transmitter still being weird and what you're talking about. Great info
  8. W2BTK

    W2BTK Ham Member QRZ Page

    OH CRAP.. No, it's not. I drew it incorrectly!!!!!!!!! Thank you for pointing that out!

    Edit*** We actually tested it with no bias at all... drew more current but nothing took off, tubes blushed evenly... pretty good, eh?

    Attached Files:

    Last edited: Aug 30, 2019
  9. K5UJ

    K5UJ Ham Member QRZ Page

    Look for a Cn that matches one of the ones you are using. will make neutralizing easier I think. Get a permanent magnet and use it to check your RF hardware to eliminate anything that's magnetic. Any nuts, bolts, washers in the RF paths should not be steel. EFJ used nickel or nickel plated steel which is okay; modern zinc plated steel is not. copper, brass, aluminum, silver all okay. Get yourself a Roper Whitney Jr. no. 5 hand punch and tin snips. Makes cutting and punching copper strips for use replacing braid a piece of cake. Plus any other ground strap inside and outside is much easier to work.
    W2BTK likes this.
  10. K4KYV

    K4KYV Subscriber QRZ Page

    What he said.

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