Simple way of getting on A.M.

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

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  1. G0GSR

    G0GSR Ham Member QRZ Page

    And nothing to restrict the bandwidth..... Bliss :)

    My first AM rig was the same.
    A few low value interstage coupling caps limited the bass a bit but the bandwidth was what it was.

    Re. Mod reactors.
    I was always amazed at how big these needed to be. Physically much larger than its accompanying mod transformer, which itslef was larger than the HV transformer.
    I seem to remember 30Henrys used in our broadcast TX's before ironwork went out of fashion.

    Last edited: Mar 6, 2021
  2. SM0AOM

    SM0AOM Ham Member QRZ Page

    Using the design rules outlined in Langford-Smith chapter 5,
    it may be estimated that the critical parameter for a modulation transformer used between a low-impedance AF output stage and a high-impedance modulated RF stage is allowed flux density, which in turn depends on number of turns, operating frequency and core area.

    A transformer with a high output voltage has more turns, which eases the core area requirement. Generally, the maximum peak linear flux must not be exceeded by the sum of the AC and DC magnetising forces.

    As a dimensioning example, the case of a 6146 RF stage operating at 50 W DC input at 600 V plate voltage, or 80 mA
    current, results in a required peak secondary voltage of 600 V.

    If no DC component is present, the peak AC magnetisation of a 100 VA EI-138 core, or 15 cm2, using a winding for 240 V at 300 Hz would be 1/4 of the design value of 12000 Gauss (1.2 Tesla), and the winding would have approximately 950 turns.

    However, to ensure linear operation, the DC magnetisation must be considered. Using a DC value of 80 mA, the DC magnetisation force for 950 turns is approximately
    950*0.08 = 75 AT.

    The average reluctance of the magnetic circuit without air-gap for an EI-138 core can be approximated to 250 so the DC part of the flux would be around 0.5 T, which needs to be added to the peak flux of 0.3 T.

    The sum must be less than about 1 T in order to keep out of saturation and severe non-linearity.

    So, a 100 VA mains transformer with a 300 V primary/secondary and a turns ratio of the square root of 6000/8 = 25 or 28 which means using a 9 V winding on a 240 V transformer when feeding it from an amplifier intended for an 8 ohm load.

    This is however quite uncritical.

    Last edited: Mar 6, 2021
    W4KJG likes this.
  3. G3YRO

    G3YRO Ham Member QRZ Page

    Well for this Modulator lash up I used the kind of transformer you'd find in a 100 watt Amplifier . . .

    The Mains winding was 240V (put in series with the PA Anode feed) and the Audio amp was fed into the 6.3V Heater winding. (this was while I was away from home at University, so didn't have any of my 'proper' gear to get on the air)

    In terms of using Mains Transformers in audio amps, I have done everything from using them in single-ended Class A amps (with ECL86s), to big push-pull amps using a pair of EL34s or KT88s (I needed a twin-winding primary for this, but many British transformers have this) . . . and the speaker connected to the Heater winding.

    I never did a proper frequency response test . . . but the audio always sounded pretty good ! (and I'm quite critical)

    Roger G3YRO
    Last edited: Mar 6, 2021
    AC0OB likes this.
  4. SM0AOM

    SM0AOM Ham Member QRZ Page

    I was not very much into AM, but built some modulators in the 20-30 W class using p-p 807s and "repurposed" mains transformers.

    One modulator, using p-p parallel EL34s converted from a public address amplifier, used a proper modulation transformer salvaged from a fire-damaged Army 100 W transmitter.
    The 813 rig that it was intended for did however never materialise, due to SSB.

    This was in the early 70s, long before I had the foggiest knowledge of magnetics...

    For some reason, I paid some attention to "Electromagnetic Field Theory B" classes while at University where the finer details of magnetics design were taught. During a session, it was "hinted at" that one or two exam problems were likely to be about DC biased iron-core components.

    I worked several problems that had a very clear connection to designing a modulation transformer, and learned about the importance of the air-gap.

    Judging from speaking with engineers 30 or 40 years my junior, it seems like the art of designing "modulation iron" is about as a lost art as designing steam locomotives...

    My worn copy of "Taschenbuch der Hochfrequenztechnik" does however contain a chapter on high-level AM, somewhat in the manner that my contemporary "Dubbel Taschenbuch der Maschinenbau" contains detail design advice for locomotives.

  5. W2BTK

    W2BTK Ham Member QRZ Page

    Lol. Yeah. These young kids dont do anything except twiddle with their SDRs and FT8. Its ruining ham radio!
  6. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Yes when you are in college or just starting out on AM you build your AM gear using what you have. It doesn't have to a broadcast quality audio chain; at least you got on AM.

    As you gain experience with your electronics you can always make improvements as your financial situation improves, but at least you made the effort to join the AMers.

    W1BR and W2VW like this.
  7. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Steve Johnston, WD8DAS, has purchased the AF4K Crystals business and will be returning it to service as soon as possible.

    N3RYB likes this.
  8. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    I still wonder what happened to all the ham-band FT-243s. At hamfests I see abundant quantities of non-ham surplus xtals for sale, usually in the 8 MHz range or below the 80m band, but very few amateur frequencies. Since so few hams these days operate xtal control, I can't imagine that a lot of hams would have a reason to hoard them, as audiophools did to generate a scarcity of triode tubes and audio transformers.
  9. W3SLK

    W3SLK Ham Member QRZ Page

    I look for crystals all the time since my MK-214D is rock bound. (Roger G3YRO would be interested in this rig!). I do have a T-368 PTO and associated circuits I guess I could use if I built a proper power supply for it.
  10. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    I am certainly rockbound on my HT-40 and mostly on my Allied Knight T-150 and am always looking for 80-20 meter crystals.

    Occasionally I am at a hamfest and I see cigar boxes, open them up and there are FT-243 crystals in there, usually for frequencies on the lower (CW) part of each band.


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