Mains Transformers as Modulation Transformers

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

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  1. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    We tried the Turbomod on a DX-100 according to KD5OEI's schematic (which was developed from Tim's textual discussion) but the modulation percentage actually dropped and the audio was distorted. A sine wave was converted into a trapezoidal wave.

    So did we miss something? Here is the schematic WITH transformer wiring color codes.


    Pheel

     

    Attached Files:

  2. K5UJ

    K5UJ Ham Member QRZ Page

    This must be some scheme that departs from a p.p.class B audio stage and transformer primary secondary to B+.
     
  3. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    It converts the conventional modulation transformer with separate primary to secondary windings, to a modulation auto-transformer, with one single winding, with taps for B+, each modulator plate, and the modulated B+ to the PA plate. Some modulation transformers are made that way at the factory, for example, one that I have that came from a Wilcox modulator that used a pair of 450-TLs, to plate modulate an aviation band base station transmitter.

    BTW, I used that autotransformer in conjunction with another modulation transformer, to serve as my first modulation reactor, until I acquired a real modulation reactor that came from a BC transmitter.

    You get exactly the same turns ratios with the auto-former as with a conventional transformer, except there are no separate taps on the transformer for modulator B+ and PA B+. These transformers are designed for a common power supply, although separate supplies work OK when shunt-feed (modulation reactor and blocking capacitor) is used. The autotransformer is probably a little more efficient than the conventional transformer when it is actually wound with one single tapped winding.


    That looks to me like the circuit as Tim described it. I'm not sure about the 600-ohm tap, maybe that portion of the winding was not designed to carry the full PA plate current. If the modulation percentage dropped, the turbo winding could be out of phase with the main winding, so t hat it subtracts, instead of adds voltage. One way to check it would be to grab a handful of clip leads and check the turns ratio with a voltmeter and 120 v.a.c. Connect the 120 volts to TMPA and TMPB (through an incandescent light bulb to serve as a series current limiting resistor for safety). Now connect the black to the blue wire, per the schematic. Measure the voltage between the red and green wire. This will tell you what the turns ratio is in Turbo mode. It basically converts the conventional modulation transformer into a modulation auto-transformer. It works only if each segment of winding in the transformer is wired to be additive: red to TMPB, plus each section of TMSB.

    I'm not familiar enough with the DX-100 transformer to know what the turns ratios of the windings are, but (just guessing) the above test should yield something like a 1.2:1 or even 1:1 ratio, much less step-down than with the original connection. If there isn't enough step-down or if it even steps up, try connecting the Mod B+ to the strap between sections of TMSB instead of to the green wire, assuming that tap is accessible. If the configuration as shown in the diagram results in more step-down than the original, try connecting the black wire to the opposite 807 plate (brown) instead of blue. I'd check the whole transformer to see what voltage appears between each set of taps with 120v applied to the 807 plate connections (brown and blue wires). Wire it up so that every additional segment of winding connected in series to the 807 plate connection at the transformer, is additive with respect to CT (the red tap).
     
  4. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    We phased it according to scope results.

    Here is Tim's DX-100 text regarding the “Turbo-Mod

    …The Turbo-Mod reconnects the modulation transformer as an auto-transformer. Rather than having separate primary and secondary windings, you take the 500 Ohm tap on the secondary and connect it to one of the modulator plates, and the bottom end of the secondary that went to B+ is disconnected. This provides a much lower impedance for the modulator tubes to look into, thus providing a better match. They will deliver more power with less distortion…”

    I would agree it should but not in this case.

    It shouldn't matter because the above quote was from Tim's DX-100 Turbomod writeup. We did all of that and went back to the original configuration.

    Pheel
     
  5. KE0ZU

    KE0ZU Ham Member QRZ Page

    I've used filament transformers and this 50 Watt/channel stereo power amps modulating ARC-5 transmitters. I later configured one channel to 100 Watts out, and used the remainder of the other channel as a 30dB pre-amp.

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    Last edited: Jun 15, 2020
    AC0OB likes this.
  6. WB2GCR

    WB2GCR Ham Member QRZ Page

    That's clearly incorrect, as a winding hanging with nothing on one end is not connected.

    Afaik, the "Turbomod" is to set the thing up as an autotransformer... from what I can glean.

    Think I will ask WA1HLR to draw the dang thing out on a piece of paper... take a picture and send it.

    _-_-

    EDIT: actually the BLACK wire would be taken off that point and moved to the YELLOW
    wire at the bottom of the 500ohm winding... OR that Yellow wire would go to the other 807 plate depending on the phase relationship, choosing the correct one, and the BLACK one not used. I am confident this is correct.
     
  7. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    That's my understanding as well from what I could tell from the text.


    Yes, so which it?

    Asking Tim to clarify as to whether the schematic is correct or needing correction would be a good step forward since a good graphic or schematic would really help. The text of the description of the autotransformer leaves out a lot of details, and most us us are visually driven.

    The primary reason I published Pat's schematic was to ask the question, "Has anyone had any success with implementing this autotransformer and is this schematic correct?"

    We tried the Turbomod on a DX-100 according to KD5OEI's schematic (which was developed from Tim's textual discussion) but the modulation percentage actually dropped and the audio was distorted. A sine wave was converted into a trapezoidal wave.

    So did we miss something? Here is the schematic WITH transformer wiring color codes.

    I want to try this on my Apache and make waveform measurements before and after to see if it could improve the modulation characteristics.

    Pheel
     
    Last edited: Jun 20, 2020
  8. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    The modulation autotransformer is basically a mid-tapped reactor with the power supply B+ connected to the mid-tap, and the plates of the two modulator tubes connected to the ends of the winding. The modulated B+ lead to the RF power amplifier is connected to one end of the winding. The half of the winding that is connected to the PA simultaneously doubles as one half the primary and as the secondary of the modulation transformer. By definition, this configuration inherently yields a 2:1 turns ratio step-down, and thus a 4:1 impedance step down. With that much step-down, it would be impossible to modulate the final more than about 80%. Another thing that is essential is that the leakage reactance of the inductor be low, so that the two halves of the mid-tapped winding are close-coupled magnetically.

    To overcome the excessive step-down, in a modulation autotransformer, some extra turns are added to one end of the mid-tapped winding, for the PA tap. This extension to that half of the winding reduces the step-down to the same ratio as what a regular modulation transformer would have.

    What the turbo connection does, is to use the mid-tapped HV secondary winding of a power transformer, or the normal primary winding of a regular modulation transformer, as the mid-tapped primary that connects to the modulator tubes, and to re-purpose what was supposed to be the primary winding of the power transformer, OR the secondary winding of a modulation transformer that had too much step-down ratio, to serve as the above-described "extension" to one end of the mid-tapped primary to feed the PA. Of course, this extension must be phased in polarity to be additive to the half-winding on the primary that is doubling as secondary.
     
  9. WB2GCR

    WB2GCR Ham Member QRZ Page

    The Heath transformers, iirc have a "600 ohm" winding... Tim is using that winding.

    Pheel... which one? the one that works out to be in phase? Just run like 10vac on the
    plate leads, and swap the added in windings in both directions, see which one doesn't buck??
    Compare to the output of the secondary in stock form.
     
  10. N2EY

    N2EY XML Subscriber QRZ Page

    But...what was the "high end"? 5 kHz? 10 kHz? 15 kHz?

    Also, note this: In your case, the original primary was almost certainly 250 volts, not the 120 volts found on this side of the pond, so the turns ratio would be more suitable.

    ----

    Couple of other considerations:

    - A common source of mains transformers used to be old TV sets. A hefty TV power transformer could easily be good for a couple of hundred watts continuous duty, so 50 or 100 watts of audio would be loafing. This may also be why core saturation wasn't such a big issue.

    - Running two power transformers back-to-back could solve the turns-ratio problem.

    - I have heard of folks who took apart a power transformer and restacked the laminations with an air gap to deal with the core saturation issue.

    And a question:

    Has anyone tried using a microwave oven transformer as a modulation reactor? Remove the heater winding and the magnetic shunts, wire the primary and secondary in series-aiding, and you may have some serious henries. All of the MOT's I've seen did not have interleaved laminations for some reason, so core saturation might not be an issue.

    ----

    Way back during WW2, there was an ad in QST that suggested using power transformers as audio output transformers for PA and home entertainment use. HV secondary for the center tapped primary, heater/filament windings for the speaker, tape up and isolate the primary. In a time of priorities and unobtanium parts, could do the job.

    I think it was a National Radio ad.
     

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