3x4-125 rf deck

Discussion in 'Amplitude Modulation - AM Fans' started by N2DTS, Sep 4, 2017.

ad: L-HROutlet
ad: l-rl
ad: L-MFJ
ad: Subscribe
ad: l-assoc
  1. N2DTS

    N2DTS Ham Member QRZ Page

    Stuck the 4-125 rf deck in place of the 3x4D32 rf deck and tuned it up.
    [​IMG] [​IMG]

    It tuned up nicely, tried it at 1500 volts on the 811's and the 4-125's and could get 300 watts out.
    No fans, so its quiet, but it wants higher voltages.

    Three is overkill for plate modulation but only does about 150 watts with screen modulation.
    It would go well with a pair (or four) 100th modulators at 2000 volts.

    Fun with radio.
    KD2ACO and W2VW like this.
  2. WQ5Q

    WQ5Q Ham Member QRZ Page

    Nice rig...I see the "grid tune" control...what are you using for a grid input circuit? And what drives it?
  3. N2DTS

    N2DTS Ham Member QRZ Page

    Typical coil and cap with a link input is the grid setup.
    This is not it but it looks like this:

    Grid coils wound on pvc pipe.

    The exciter is a homebrew vfo and driver (6146):

    Really simple exciter as its on frequency and the vfo always runs (to feed the digital display and to make it stable), one band (40 meters), no band switch or multipliers, etc.
    I did not need the driver tube (empty socket) as the buffer drives the 6146 ok.
  4. K4KYV

    K4KYV Subscriber QRZ Page

    PVC pipe is a poor material, not really recommended for use as a coil form, since it is lossy at RF. Probably not too different from winding coils on wooden dowels, as they did back in the 1920s. Better to use air-wound inductors. If you can't find suitable air-wound coil stock like B&W Miniductor or Air-dux, it's not all that difficult to make your own. Plexiglass strips and glue to hold the wires in place works, or better still, if you can find them, solid polystyrene strips. Other methods are described in detail in pre-WWII Handbooks. Maybe a little more work involved, but you end up with a lower loss coil.

    My converted Gates BC1-T used a solid phenolic coil form for the PA grid coil. I replaced it with an air-wound coil I found in the junkbox, and that brought the grid drive up about 15%.

    Click on the photo to enlarge.

    Grid coil.JPG
  5. N2DTS

    N2DTS Ham Member QRZ Page

    That rf deck only works on 40 meters, it could operate on 80 with a different exciter.
    I have no problem with drive.

    I have tried making coils like B+W used to and I just do not have that skill.
  6. N2DTS

    N2DTS Ham Member QRZ Page

    I liked how the 4-125 deck worked and looked, so I thought it might be nice to pair it up with a higher voltage modulator tube.
    I have a bunch of 100TH tubes, and have the quad 100th modulator assembly, and may just make a deck that holds a pair, four is overkill for this application.
    Run the 4-125's and 100th tubes at 2000 volts.
    Since I have the rf deck, the 100th control deck (filament, bias, metering) I just need to make a new deck to hold two 100TH tubes behind glass.
    An easy fun project.
    I love 100th tubes.

    Something that glows almost white hot in normal use is very interesting to use!
    W2VW, KD2ACO, WQ5Q and 1 other person like this.
  7. WZ5Q

    WZ5Q Ham Member QRZ Page

    KD2ACO likes this.
  8. K4KYV

    K4KYV Subscriber QRZ Page

    Plus, four tubes would be optimised for a more usable plate-to-plate impedance. Hard to find good 100THs that haven't gassed up on the shelf, though.
    Last edited: Sep 5, 2017
  9. N2DTS

    N2DTS Ham Member QRZ Page

    I am thinking I will just install the entire PPP 100th mod deck, I can install only two tubes.
    If it works well, I can make a tube deck that only holds two tubes.

    The mod transformer is a Thordarson 11tm77 300 watt multi match.
    I have it set up for parallel primary and secondary.
    I can run the three 4-125's at 2000 volts and 200 to 300 ma, 400 to 600 watts input.
    From memory, a pair of 100th tubes will do about 320 watts audio at 2000 volts.
    Should work ok.

    I could also retire the 3X 4-400 screen modulated deck and plug the 4-125 rf and 100th mod deck in and run it on 80 or 40 meters.
    That might be a better idea.
  10. K4KYV

    K4KYV Subscriber QRZ Page

    So you're not going to use that beautiful four-100TH modulation deck in the rig after building it? Or is that a photo from someone else's project?

Share This Page