Simple way of getting on A.M.

Discussion in 'Amplitude Modulation' started by G3YRO, Mar 5, 2021.

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  1. G3YRO

    G3YRO Ham Member QRZ Page

    40 Years ago I was given the PA section of an old rack-mounted Transmitter (a Tiger 300). (the VFO exciter section was a separate rack-mounted unit, as was the Modulator, neither of which I had)

    So I put a 5763 crystal-controlled oscillator in it, which was enough to fully drive the PA to about 150 watts input on 80m.

    I had a 100 watt output transistor audio amplifier with a Mic input . . . so to modulate it I found an old Mains transformer, put the mains input winding in series with the PA Valve HT Supply, and fed the output of the Audio amp into the 6V heater winding.

    It fully modulated the PA . . . and sounded really good !

    Just thought I'd point this out, for anyone wanting to find a simple way of getting on AM with high-level modulation.

    Incidentally, back in those days everyone I knew that built AM rigs just used old Mains Transformers as the Modulation Transformer (as proper ones were pretty rare or expensive) . . . although they were obviously designed for 50Hz, they had a pretty flat frequency response. (I also built a couple of Valve Hi-fi amps, just using Mains transformers for the output, with the Speakers connected to the Heater winding . . . they sounded really nice too!)

    Roger G3YRO
    K4YNZ and KA9JLM like this.
  2. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    The problem with that method is that the DC flowing through the output winding tends to magnetically saturate the core, effectively reducing the inductance to a very low value, since a mains transformer normally has no physical gap in the core. A practical fix for the problem other than going to great trouble to dismantle the transformer and re-stack the laminations, would be to use an additional inductor, most likely one designed to be a smoothing reactor in a power supply, as a modulation reactor to carry the DC to the final, and then couple the a.f. output from the mains transformer to the PA through a blocking capacitor. This is somewhat reminiscent to the old Heising "constant-current" scheme dating back to the earliest attempts to plate modulate, hence the oft-heard misnomer "modified Heising", which is more correctly described as shunt-feeding the modulation transformer.
    N1BCG and W2BTK like this.
  3. G3YRO

    G3YRO Ham Member QRZ Page

    Why does it work OK then? (using Mains Transformers both as a Modulation Transformer and as a Valve Audio Output Transformer?)

    Roger G3YRO
  4. SM0AOM

    SM0AOM Ham Member QRZ Page

    If you have sufficient margins in core area it will work,
    as the flux density in the core is inversely proportional to frequency with everything else kept constant.

    When the operating frequency is increased from 50 to 300 Hz, the peak flux is reduced by a factor 6, and the remaining head-room can be filled by DC magnetisation in a modulation transformer.

    The use of a mains transformer in a push-pull AF amplifier feeding speakers is even easier, as the net DC magnetisation becomes close to zero.

    From what I recall from the 70s, it was quite safe to use a 50VA mains transformer as a 30 W modulation transformer.

    Continuous sine-wave excitation at maximum power level may stress the margins however.

  5. G3YRO

    G3YRO Ham Member QRZ Page

    I guess it very much depends on the actual transformer used then . . .

    Back in the day I lived in Portsmouth, which was a big Naval base back then. So a lot of very high-quality Parmeko oil-filled transformers made their way out of the Dockyard and were sold cheap at our Radio Club junk sales !


    Roger G3YRO
    AC0OB likes this.
  6. W2BTK

    W2BTK Ham Member QRZ Page

    Try it with toroidal mains transformers.
    AG5CK likes this.
  7. K4YNZ

    K4YNZ Ham Member QRZ Page

    My Dad's generation used old Juke box amplifiers. He never mentioned how they were hoked up. Dad said AM was easy.
    Roger, Mains transformers for output? How did it sound? Valves? Speakers? I saw a circuit that used an auto ignition coil for an output transformer.
    Oh I'm a retired Rolls -Royce Chauffeur/mechanic...from Birmingham Alabama. LHD and all. LOL
  8. AG5CK

    AG5CK Premium Subscriber QRZ Page

    Antek toroidal transformers work very well and aren't too expensive.

    W2BTK has a good YouTube video with the math needed for transformer selection.

    I use the same audio chain with my Anan SDR and a Johnson Valiant. People have a hard time figuring out what rig I'm on.
  9. W2VW

    W2VW Ham Member QRZ Page

    Mains xfmrs can work OK. OK is a relative term.

    Oversizing the transformer can go a long way.

    If one doesn't detest youtube videos, one can refer to this:

    Start at 6;31 for the technical part.

    Thanks to Stu AB2EZ for developing the Antek application.
    W2BTK and AG5CK like this.
  10. W4KJG

    W4KJG Subscriber QRZ Page

    Oh, I hate it when somebody posts ideas like this.:D It will just lead to a few more sleepless nights of playing until dawn. I'm just about to complete the full integration and installation of my new weather proof automatic remote controlled antenna matching network with multiple antenna selection. It is built around a $35 N7DCC AT-100 kit.

    So ...

    Years ago, probably late-60s/early-70s, I built an amplifier and power supply on a decent sized Bud aluminum chassis. It was designed as a peice of test equipment for my shack. I used it with various low power transmitter/exciter circuits. I initially used it to test a low level DSB exciter. I've used it with my old Heathkit SB-10 SSB adapter. It provides adequate power for the SB-10. It has been used with quite a few Ten Tec boards and transceivers including my very venerable old Ten Tec 505 transceiver, and various other 505s and 509 transceivers. I last used it about two years ago at our beach home with my Ten Tec 505 for CW and digital on 40 and 20 meters. So I believe it still works.

    The amplifier uses a 6CL6-6146 combination based on the Heathkit DX-60 design. I remember that I modified the circuit so that the 6CL6 always runs class A. I further modified it so that the 6146 can be switched to operate Class A, AB1, AB2, or Class C. The power supply has has a high voltrage regulator, probably an OB2 for the 6146 screen voltage. The 6CL6 amplifier has tracking input and output tunable tank circuits using a single double section old broadcast receiver 10-365 pF variable capacitor. It is designed to tune from about 1.5 to 4 MHz, 3.5 to 8 MHz, and 7 to about 15 MHz. The 6146 Pi network output can be switched work over about the same range.

    I think the plate power supply for the 6146 runs about 700-750 volts. I'm guessing the 6146 puts out about 50-60 watts CW. I think it also requires at least a watt, maybe more, at 50 Ohms to drive the 6CL6 and 6146 to full output when running Class C.

    I think I could drive it to full CW output with a small amplifier connected to one of my DDS VFOs. I have two DDS VFOs with equalized amplifier outputs at +10 dBm +/- about 3 dB out over about 1 to 30 MHz.

    I have a monster stereo audio amplifier that my son deafened himself with when he installed it in his car while in college. I'm guessing the outputs were designed for 8 Ohm speakers. He used my Astron 7 amp 12 (13.8) VDC power supply to shake the walls and floors of our house when he would bring his stuff into his cave in our basement. I'm guessing the amplifier runs at least 50 watts out on each channel. I've also got his great little switch-selected input equalizer/pre-amp that runs on 12 VDC.

    If I wanted to use this audio amp to plate modulate the 6146 amplifier for AM, what would I need as a modulation transformer?

    Could I use what I would consider a filament or car battery charger transformer?

    I've got a collection of old 120/6, 120/12, 120/24, and one 120/48 VAC trasformer. I've also got an old military surplus sealed transformer that is about 4 x 4 x 4 inches (10 x 10 x 10 cm) that has taps for 12, 24, and 48 volts. All are of these transformers are capable of several amps at their rated secondary outputs using 120 VAC inputs.

    I'd love to put the old amplifier back on the air -- particularly using AM on 160 and 75 meters.

    Thanks for putting up with my crazy questions,

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