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Mains Transformers as Modulation Transformers

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

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  1. K4KYV

    K4KYV Premium Subscriber QRZ Page

    If the B+ is fed at the bottom end of the winding, the plate of the modulator tube at the lowest tap and the final at the highest tap, that will allow 100% modulation. Otherwise, with just a choke with no taps, the modulation will be limited to around 75-80%, unless a dropping resistor by-passed with a capacitor is inserted in the B+ line to the final. The extra higher taps will allow 100% modulation, but there will be no DC magnetic field cancellation.

    The mid-tapped choke or transformer winding can be used with a push-pull modulator and the +HV fed to the mid-tap, but that presents a 2:1 step-down turns ratio, which will not allow 100% modulation. "Modulation autotransformers" are designed to have some extra turns added to one end to present a normal modulation transformer step-down ratio. Several commercial rigs used those.

    Timtron says he has had good results by "turbo-connecting" the power transformer, so that the mid-tapped HV winding is used with the push-pull modulator, and the mains voltage primary winding is added in series at one end of the mid-tapped winding to give less step-down. Whether or not this works depends on the overall turns ratio, and whether or not the primary winding is insulated well enough to withstand the full modulated B+ voltage. This may work OK at the 100-watt class, but I think would be risky at higher power with the B+ in the thousands of volts. Plus, the amount of step-up achievable with the primary would likely not be enough for 100% modulation capability.
  2. W1BR

    W1BR Premium Subscriber QRZ Page

    Yes, I had considered needing use the primary to increase turns, but that would also add to the DC biasing since windings would no longer be equal.
  3. K4KYV

    K4KYV Premium Subscriber QRZ Page

    That's where shunt feed (blocking capacitor and modulation reactor) would help.
  4. W1BR

    W1BR Premium Subscriber QRZ Page

    Unbalanced DC bias would be minimal using the primary winding to aid one side of the CT secondary, but adding a shunt feed on the PA side of the CT would completely mess it up.
    All the DC current would be on the SE modulator half of the winding with no field cancellation on the PA half. I was just thinking that an old
    1400V plate transformer might make for good cheap mod iron, even if the ratio doesn't allow for full 100% modulation depth.

    Last edited: Jun 9, 2020
  5. N2DTS

    N2DTS Ham Member QRZ Page

    Why not just use a mod transformer?
  6. K4KYV

    K4KYV Premium Subscriber QRZ Page

    The best solution - if one is available.
    WZ5Q likes this.
  7. WZ5Q

    WZ5Q Ham Member QRZ Page

    That's why I embrace my Iron Fetish.
    If I see a piece of iron, it comes home with me whether I need it or not.
    W2VW likes this.
  8. K5UJ

    K5UJ Ham Member QRZ Page

    I don't know what this means.
  9. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Has anyone ever seen a circuit schematic of his Turbomod and the resulting waveforms it produced?

    His description of his TurboMod and what constitutes the actual circuit raises a lot of questions.

  10. K4KYV

    K4KYV Premium Subscriber QRZ Page

    The primary of the mains transformer is wired in series with the HV secondary winding. The mid-tap of the HV winding is still used as CT for the push-pull modulator tubes, and each end of the winding goes to one of the modulator tubes. The primary winding is added in series with one side of the HV winding, and the other end of the primary winding goes to the RF final. The mid-tapped winding acts like an autotransformer, and the primary in series adds additional voltage to that side of the winding, so that the effective turns ratio may be something like 1.6:1 or 1.8:1 instead of 2:1. Of course, the primary winding must be phased to that it is additive with the main mid-tapped winding and not subtractive. There may be issues with doing this, since the primary winding may not be insulated well enough to withstand the full modulated B+. You also have to consider the capacitance surrounding the primary winding, which could degrade high frequency response. But it could be worth a try. Tim says he has had good results using a power transformer in turbo mode.

    Tim has also turbo-connected a modulation transformer that had too much step-down, like the one in the Valiant. The mid-tapped primary is connected to the modulator tubes as usual, but the secondary is wired in series and properly phased with one side of the primary, thus adding the full primary voltage to the 2:1 step-down turns ratio to one-half the primary winding. This allows greater positive peak capability. Tim says this has not degraded the high frequency response in transformers he has tried it with. With a modulation transformer, there is less likelihood of inadequate insulation at the turbo winding, than when using a power transformer.

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