Mains Transformers as Modulation Transformers

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

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  1. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    It inevitably causes distortion, since it makes the modulator tubes work into a non-linear load, plus it wastes hard and expensive-to-produce audio power, and I'm not sure it actually accomplishes anything.

    I once tried the original "ultramodulation" circuit that appeared in the 1956 issue of QST, using three 866-As. At first it looked good; the rf ammeter in the feedline kicked up higher with modulation, the plates of the PA tube (a 304TL) got brighter orange, and the DC plate current meter to the final kicked up slightly. I don't recall that I had a monitor scope at the time; I may have observed expanded positive peaks (actually, reduced negative peaks). BUT numerous on-air A/B comparisons yielded similar results; the signal sounded more raspy and distorted, but the audio didn't sound any louder or more piercing through the background noise.

    An analysis of the circuit explains why. The upward kick in plate current and rf feedline current, and brighter plate glow in the 304-TL were due to the upward kick in DC plate voltage, resulting from the portion of the rectified audio voltage that was not wasted in the power resistor. The main effect of the negative cycle rectification on the audio waveform itself was to add a component of even harmonic distortion. In effect, all that circuit does is to function as an inefficient form of controlled-carrier modulation with a strong even-harmonic distortion component added.

    A negative cycle recertification scheme that does work is to use a power resistor of the same DC resistance as the modulating impedance (plate voltage divided by plate current per Ohm's law). It doesn't have to be very large, maybe 10% of the modulator output power, or 5% of the DC input. Connect the cathode of the diode to the modulated B+ line, the anode of the diode to the resistor, and the other end of the resistor to ground. That way, during negative overmodulation peaks (when the PA plate is driven negative and stops conducting), the diode conducts and the resistor takes over to maintain a constant load across the secondary of the modulation transformer over the whole audio cycle, thus protecting the transformer from working into no-load (infinite impedance) during negative peaks that might exceed 100%. Since those peaks are likely to be of short duration and limited amplitude, there is no need for a large wire-wound resistor. In most cases a 20 to 50 watt resistor should be more than adequate even at the highest DC input power an AM ham transmitter might ever run.

    I don't use this circuit in any of my transmitters to-day, since the broadcast-type peak limiter pretty much eliminates over-modulation peaks, but back when I ran the 304-TL I didn't have any kind of peak limiter, so this circuit might have offered some protection to the transformer. Not having a monitor scope at the time, I went a step further and used an old globe shaped 866 (non-A) rectifier which has no shield over the rectifier filament. The blue glow was very prominent and served as an effective over-modulation indicator, since the 866 conducted when and only when the instantaneous plate voltage dropped negative to produce the blue glow. I used a small cardboard box to house the 866, spray painted the interior flat-black, and cut a window in the side of the box, making the blue glow more visible.

    I could not believe how lightly I had to drive the modulator to make the 866 flash blue, and actually operated it so that it always flashed a little on modulation peaks. I didn't have a monitor scope at the time and wasn't aware of the voice peaks that occurred at a fraction of the modulator plate current that the tube manual said the tubes were supposed to draw to produce half the audio power as the DC input power to the final. Of course I realised later on that the output power prescribed in the tube manual was for a sine-wave audio tone, and that the peak-to-average power for a voice waveform was far higher.

    Back then, most receivers were broad as the side of a barn and very few hams had panadaptors or spectrum analysers, so no-one ever complained that I was splattering even though the 866 flashed blue indicating over-modulation. I did blow a few modulation transformers back then, but that's a whole unrelated story that I have posted, in an unrelated topic.

    All that the "turbo connection" is, is to combine the two windings of the transformer to form a modulation autotransformer. Modulation autotransformers are not all that rare; they were used in several Wilcox transmitters, for example. Instead of a separate primary and secondary, the same winding is used as both. A simple mid-tapped winding would work, but it would produce a 2:1 step-down turns ratio, or 4:1 step-down impedance ratio. The autotransformer adds a section of additional turns to one side, out beyond the plate connection of the coil. That extended section of winding feeds the PA plate, producing more voltage swing than just half the plate-to-plate voltage as with a simple mid-tapped winding, thus producing less step down. The main precaution is that unless a separate modulation reactor and blocking capacitor is used, the modulator and PA must run off a common power supply when a modulation autotransformer is used.

    The turbo connection re-purposes the original secondary of the mod xfmr to serve as the turns extension to one side of the primary, converting it into an autotransformer. The turbo section must be phased correctly, to be additive and not subtractive. If the original transformer secondary has several taps, this allows the amount of additional turns for the turbo section to be chosen. Otherwise, you have what you have. The step-down may be greatly reduced, or the transformer may act as a step-up, not step-down. This allows a lot of positive peak head room, but may present too low a plate-to-plate impedance for a given pair of modulator tubes, requiring different tubes or maybe running a quad of the original types in push-pull parallel to successfully work into the lower p-p impedance. Also, since fully driving the modulator tubes may produce severe over-modulation in the negative direction, running back the audio level to 100% negative modulation may result in low modulator tube efficiency. As with any radical circuit modification, YMMV.
    N1BCG likes this.
  2. G3YRO

    G3YRO Ham Member QRZ Page

    I've never considered any of those clever modulation schemes, as most high power rigs I built were SSB . . .

    However, my early AM Transmitters just used Choke Modulation, which was very simple and sounded good, without any worry about over-modulation.

    I just had a 5763 Class C PA with a 6BW6 Modulaor Valve in Class A, with the Anode simply strapped on the HT side of the PA's RF Choke, then a small series Transformer acting as a choke.

    It ran about 15 watts input.

    Roger G3YRO
  3. SM0AOM

    SM0AOM Ham Member QRZ Page

    Operating low-power AM was quite "forgiving" with respect to
    choices of "iron". The autotransformer or "centre-tapped choke" variant
    was quite common here, as Novices (class-C) were limited to 10W DC input.

    Most were impoverished Grammar-school kids and the opportunity to beg an old push-pull output transformer from some OF:s junk-box considerably facilitated transmitter design and construction.

    With some nostalgia, I recall the times when a modulation transformer was quite expensive, but EL34s and EL84s and audio transformers were for the asking. Now it is the opposite.

    When my late father moved in the early 80s, and the storage of the remains of my "junk-box" in his shed became impossible, I had to donate two pallets packed with electron tubes and components to the University radio club auction.

    Otherwise, I "missed" the heyday of AM. I can only recall seeing two operational full-power AM rigs in operation, which must have been during the mid/late 60s. One was SM4UJ and the other was SM4YU. SSB had taken over casual operating long before my amateur radio interest. OT:s could however "tell tall tales" about the long-winded AM operators of the 50s.

    VHF was still some kind of remaining "bastion" for AM, but usually limited to low power, and the occasional controlled-carrier QQE06/40 rig. NBFM was also quite common.

    Some years ago, I listened to a recording of a 60s "old Buzzard"
    Danish VHF amateur that used an "interval signal" to raise attention before calling CQ.

    It consisted of an old alarm clock which was put in front of the microphone and its ticking was modulated onto the carrier for about an half-hour. When "CQ time" neared, some activity was heard in the background, foot-steps, the scraping of a chair and finally the CQ itself which lasted for some more minutes, and finally it was declared "listening from the low-end up".

  4. G3YRO

    G3YRO Ham Member QRZ Page

    Just looked at some old low power AM circuits . . .

    I see some single-ended circuits used a Choke with the HT fed to the centre-tap, with the anode of the modulator valve to one end and the PA valve on the other end (after its RF Choke).

    Whereas my rigs I always just had a choke with HT at the top end and both anodes connected together (again after the PA's RF Choke).

    Are there any pros and cons with either circuit?

    Roger G3YRO
  5. SM0GLD

    SM0GLD Ham Member QRZ Page

    Something like this?

    W1BR likes this.
  6. SM0AOM

    SM0AOM Ham Member QRZ Page

    Replace the 6146 with a QQE 03/12 and the 6L6s with an EL34,
    and you have the modulator and RF sections of my first transmitter (1970).

    Class-A modulation using an old push-pull output transformer as the coupling device worked admirably when working low-power.

    The "pro" was that it was somewhat easier to control the DC voltage drops in each branch, as there are separate paths for the anodes in the AF and RF circuits. This made it easier to reach 100% modulation.

    Also, an output transformer with the required performance level was often easier to find than a choke.

    Last edited: Mar 8, 2021
  7. G3YRO

    G3YRO Ham Member QRZ Page

    OK . . . but I thought that if you have a Tetrode as your PA Valve, and you modulate the Screen Grid as well as the Anode (as I always did), then it's easy to get 100% Modulation just using a Choke?

    Also, if I understand the theory correctly, Choke Mod allows you to increase the Modulation to more than 100% in the positive direction, as the Negative half-cycles can never break up the Carrier? (which is what causes splatter. (I know AM Broadcast ausio processors like Optimod use this technique).

    Roger G3YRO
  8. W1BR

    W1BR Ham Member QRZ Page

    CT arrangement allows the DC magnetic fields of both windings to cancel each other, reducing the needed core size. To get full modulation with the choke you'd need a resistor in the PA plate side to drop the PA plate DC voltage paralleled with a cap to pass the full AC modulation voltage that is available.
    Last edited: Mar 8, 2021
    K4KYV likes this.
  9. W1BR

    W1BR Ham Member QRZ Page

    I tried a very similar approach has a young teen when I first got my tech license and wanted to build a six 6 mtr hb rig. I used the HV winding on an old tv transformer, worked good enough for a young kid with no test gear to prove whether it was genius of folly. As I recall it was a 6L6 and a 2E26 final as a doubler.
    SM0GLD likes this.
  10. G3YRO

    G3YRO Ham Member QRZ Page

    Well I had no resistor in the PA anode . . . and had 100% modulation. But the Screen Grid was connected to the HT feed (via a resistor), so I was modulating the Screen as well as the Anode.

    Roger G3YRO

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