Mains Transformers as Modulation Transformers

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

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  1. W1BR

    W1BR Ham Member QRZ Page

    You essentially have a 1:1 transformer ratio. The plate voltage swing on the audio plate is lacking. modulation.htm
    Since modulator and PA anodes are in parallel for audio signals, the anode voltage swing in the modulator and PA track each other. This means the 6146 PA has voltage swing limitations similar to the modulator tube. Because of the lower voltage limit established at point C and point A during positive grid voltage swings, the 6146 anode can't possibly reach zero voltage on negative modulation envelope peaks. The solution is the addition of R106 and C110. Resistor R101 causes the anode of the 6146 to operate at reduced voltage. C110 provides a low-impedance path for audio variations. Both are necessary to achieve 100% negative modulation
  2. G3YRO

    G3YRO Ham Member QRZ Page

    No, but that is just modulating the Anode (Plate) . . .

    If you modulate the Screen Grid too (like I did in my rigs), you DO get 100% Modulation.

    From the same article: "The exception to this is with a tetrode tube. Because the tetrode is combination screen and anode modulated, we do not need to double anode voltage for 100% positive peaks."

    Roger G3YRO
  3. W1BR

    W1BR Ham Member QRZ Page

    Article said peak negative modulation. Note the plate dropping resistor added to the 6146. It has nothing to do with reaching 100% peak positive. 6146 is a beam power tetrode. The RC combination was needed in the Globe despite also having modulation applied to the screen.
    Last edited: Mar 10, 2021
  4. G3YRO

    G3YRO Ham Member QRZ Page

    Well I achieved 100% modulation on a 5763 PA Valve just using a choke, by modulating the Anode & Screen Grid . . . no RC combination on the PA Valve.

    And as any of the thousands of people that used the Codar AT5 will tell you, it also achieved 100% modulation, with a similar circuit to my rig (although they used a centre-tapped choke)


    Roger G3YRO
  5. W1BR

    W1BR Ham Member QRZ Page

  6. WD5JKO

    WD5JKO Ham Member QRZ Page

    That schematic on post #65 is what I did with my Gonset G50 Transceiver. That schematic was drawn by me.

    The stock G50 with choke modulation would only get about 50% modulation before severe distortion set in, and anything under 500hz was distorted.

    I attach a modulated pattern after the modification.


    Attached Files:

  7. SM0GLD

    SM0GLD Ham Member QRZ Page

    Will you please explain the purpose of the diode and how the pot should be adjusted.
  8. W1BR

    W1BR Ham Member QRZ Page

    Who, that is not choke modulation! That is transformer with a 2:1 turns ratio!!!
  9. SM0AOM

    SM0AOM Ham Member QRZ Page

    The AT-5 is able to reach 100 % modulation by the use of a quite clever "trick". An autotransformer is improvised by the use of the primaries in a small (15 VA) mains transformer with 110 and 240 V primary taps.

    This gives a slightly asymmetric transformation, close to 1:1.2
    and this makes for a modulation voltage swing of about 240 V peak for a modulator plate swing of 200 V, which is about what a class-A audio stage running "flat-out" may provide.

    The small core area and low winding inductance gives a
    quite marked "pre-emphasis" or "treble lift" which should have contributed to its good readability signal.

    W1BR likes this.
  10. G4COE

    G4COE Ham Member QRZ Page

    I had a Codar AT5.... and built many AM's using a center tapped mains tranny for modulator, we are only transmnitting speech and not music.

    It'd put out a useful signal on 40Mtrs in the 80 Mtr mode and tuning the PA for the second dip, then one day came I took the 6BW6 pa bottle out in went a EF80 - qrp here I come!

    I solved a problem with a portable TV for a army chap who was based in Germany and had a 120V expensive portable TV, it tuned all the way from about 40Mhz up to some 800 and odd MHz, you could select various demodulation formats, connected it to our 240V system and he said there was a almighty bang!!!!! I repaired it and fitted a transformer that had a twin 120+120V primary. across the mains it went with the telly connected to the center tap, it ran for years until the tube lost it's blue gun.... and I got my 'tranny' back, a bit dusty mind.


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