Mains Transformers as Modulation Transformers

Discussion in 'Amplitude Modulation' started by G3YRO, Jun 7, 2020.

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  1. W2BTK

    W2BTK Ham Member QRZ Page

    Here's a video I made today about using Toroidal power transformers as a mod transformer for a boat anchor. The first six minutes shows the equipment as it sits, and the rest of the video is basically me with a pencil and paper drawing a real time schematic and doing calculations on how exactly you figure out what you need. I use this setup on the air daily and it works extremely well.

    Last edited: Jun 22, 2020
    AC0OB, KC2YMO, W2NBC and 15 others like this.
  2. W4IPC

    W4IPC Premium Subscriber QRZ Page

    Nice video. always love the younger people homebrewing things. We are the future of the hobby, we soon will be the buzzards.
    W2BTK, KD4DYO and KI4LHU like this.
  3. N2DTS

    N2DTS Ham Member QRZ Page

    I used to use a slightly different setup for the 3 diode circuit, but I was running 2000 volts on the 813's and used the HV diode bricks rated for 14kv at an amp.
    I don't think they were fast enough and got reports of being wide and scrapped the setup.

    I think I would rather just use a nice 250 watt mod transformer then all those chokes plus the antek transformers, but in your case using the 4 ohm audio driver it works great. I never hear anything bad on your audio, it sounds quite nice.
    Eric (WB3CAU) used to modulate a high power rig directly with a solid state audio amp 25 or 30 years ago and sounded great doing it.

    When doing the auto transformer setup, do you need to take the DC current off the RF side?
  4. WA1EIG

    WA1EIG Ham Member QRZ Page

    Great video! Very informative and nicely presented. Learned a lot! Thanks.
    W2BTK likes this.
  5. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Three separate topics popped up in the discussion. 1. Using a toroidal transformer for modulation transformer, turbo connection. 2. The Timtron Turbo connection. 3. Using a modulation reactor and blocking capacitor to isolate PA DC off the secondary of the modulation transformer.

    You would almost certainly need the reactor and capacitor when repurposing a toroidal transformer or any other power transformer as a modulation transformer. Depending on what kind of transformer you are using, the reactor and capacitor may or may not be needed with the turbo connection.
  6. N2DTS

    N2DTS Ham Member QRZ Page

    I am going to assume you do not need to remove the dc current if using a modulation transformer in the turbo setup, but would need to use it with power transformers.

    What is the big difference between the Antek toroidal transformers and a typical power transformer?
    What the core is made of? Could they make a normal transformer core out of the same stuff?
    Did they ever make a toroidal modulation transformer designed to have DC through it?
    Could they?
  7. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    As far as I know, toroidal cores do not lend themselves to having an air gap due to construction techniques. This is what makes them prone to saturation from unbalanced DC currents.
    WB2GCR likes this.
  8. WB2GCR

    WB2GCR Ham Member QRZ Page

    IF you got distortion, etc, lower output, you were likely out of phase, and the xfmr was bucking

    Also, Tim HLR calls for using the 600ohm (500ohm? whatever) winding to improve the ratio.
    But, yes, it is not necessary to make the circuit run. But using the extra winding is an official
    Timtron Turbo Mod. (tm)
  9. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    That depends whether or not the modulation transformer was designed to carry the DC in the first place.

    Saturation is a function of the ampere-turns of the winding where the DC is flowing. In the turbo set-up, you are effectively adding more turns to the DC-carrying portion of the winding, thus increasing the ampere-turns, so at the same DC plate current to the final, it probably would be more prone to saturation than it would be in the normal mode.
    W1BR likes this.
  10. N2DTS

    N2DTS Ham Member QRZ Page

    I wonder what would happen if you cut a gap in a toroidal core.
    Most old transformers had laminations of steel, did they use different kinds of steel?
    Why laminations, why not a solid hunk of steel?
    Who thought up the lamination idea?
    How does the thickness and amount of laminations impact things?

    Most toroids are powdered metal of some sort, are some better then others for audio?
    Could you make a toroidal modulation transformer that carries dc current?
    You can make one that does not it seems as people have been using toroidal power transformers as modulation transformers with good results.
    If you were making one from scratch, how would it be different from a power transformer?
    I suppose you could make a multi match version with various taps.

    I cut into some shorted RCA mod transformers designed for 1500 volts and 300 to 400 ma (pair of 813's) and it seemed to have a LOT of very fine enameled wire, much finer then I would have thought, maybe it was in parallel somehow?
    Lots of core, with a gap, very hifi, at least on the low side of the spectrum.

    I suppose you could buy a core and wind your own mod iron, at least as well as a power transformer?

    In the old days, hams made their own parts, capacitors, resistors, coils, etc.

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