Heising Reactor vs Modulation Reactor

Discussion in 'Amplitude Modulation - AM Fans' started by N1BCG, Mar 4, 2019.

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  1. N2DTS

    N2DTS Ham Member QRZ Page

    What kind of power could you get out (RF) if you used four 6l6 tubes and a 4D32 in class A hy-zing (with choke)?
     
  2. SM0GLD

    SM0GLD Ham Member QRZ Page

    It will work as a negative limiter and I remember something was written about it on the web page.
    I am sorry, I have lost the reference to where I found the schematic. I have now spent a long time trying to recall it. No luck so far.
     
    Last edited: Mar 5, 2019
  3. SM0AOM

    SM0AOM Ham Member QRZ Page

    My very first VHF phone transmitter (1970) used exactly this scheme.

    With 10 W DC input and two EL84 in the modulator circuit, I got 100 % modulation and some margin. It sounded "good" said the otherwise critical locals.

    If I remember correctly, the 270 V HT current was divided with about 40 mA to the RF PA, and almost 60 mA to the modulator tubes.

    I am sure that it can be scaled to other tube configurations.
    Another post asked about 4 6L6 and 4D32. Using 500 V on the plates, and the 6L6s in Class A, a modulation power of around 40 - 45 W may be possible, and the 4D32 could maybe give 50 or 55 W carrier if a sufficiently "hefty" centre-tapped choke or transformer could be located.

    "Ah! Memories"

    73/
    Karl-Arne
    SM0AOM
     
    AC0OB and SM0GLD like this.
  4. SM0AOM

    SM0AOM Ham Member QRZ Page

    Inspiration for my 1970 design came from a mid-60s ARRL handbook and a similar transmitter in
    the 50s Orr-Johnson VHF Handbook.

    mod_orr.JPG

    73/
    Karl-Arne
    SM0AOM
     
  5. K4KYV

    K4KYV Subscriber QRZ Page

    Aren't the yellow and brown leads reversed? Not sure exactly where on the winding the tap connects, but that circuit wouldn't be capable of modulating 100%, since the audio voltage at the tap would be less than the audio voltage at the plate of the 6AQ5, which itself wouldn't modulate the 5763 100%. The tapped inductor as shown in the schematic would function as a step-down autotransformer. The 6AQ5 plate should go to the tap (yellow) and the line to the PA should go to the brown terminal, so that the choke acts as a step-up autotransformer.
     
  6. SM0GLD

    SM0GLD Ham Member QRZ Page

    Yes and it is a 6700 to 5000 ohm transformer.

    mod_txt.jpg

    I found it in the 1956 VHF Handbook.
     
  7. SM0AOM

    SM0AOM Ham Member QRZ Page

    The B+ or HT feed was to the centre-tap of the transformer winding in my transmitter, creating a
    1:1 impedance ratio between modulator and RF amplifier that were connected to the outer ends of the winding.

    I looked for a tapped modulation choke, but no one could be found in the junk-boxes, so on advice of my "Elmer" the
    centre-tapped circuit was adopted.

    73/
    Karl-Arne
    SM0AOM
     
  8. K4KYV

    K4KYV Subscriber QRZ Page

    The writer of the article and your mentor are too obsessed with a "correct impedance match" between modulator tube and PA. The recommendations in the tube charts are only that, recommendations. Those figures are not carved in stone, and most tubes will function just fine when working into a somewhat different load from what the tube manual recommends. The only drawback is that if the load veers too far from the nominal value, the full power output of the tube may not be realised, but this would be a problem only when the tube is running at the ragged edge of its maximum output to begin with. The difference between 5000 ohms load and 6700 ohms load on the 6AQ5 wouldn't have made a perceptible difference in the waveform or sound of the modulation other than the maximum peak modulation capability.

    Since a single-ended tube type amplifier can pull its instantaneous plate voltage only from the DC supply voltage down to about 20% of the power supply voltage, the maximum a.c. voltage swing at the plate is about 80% of the power supply voltage. To modulate the final stage 100%, as previously discussed regarding Heising modulation, the transformer must have about a 1: 1.25 step-up voltage ratio, or a 1:1.56 step-up impedance ratio, or else a series resistor/by-pass capacitor must be used to drop the DC voltage to the PA while maintaining the full audio output from the modulator. It would have been better to have left the yellow tap on the transformer un-connected and wired the PA plate to the brown connection, in parallel with the modulator plate, if using the tap to form an autotransformer resulted in too much step-up.
     
  9. SM0AOM

    SM0AOM Ham Member QRZ Page

    I think that the statute of limitations may be applied, and I was only 14 at the time...

    After some searching, the circuit that also was my inspiration surfaced in the 1969 ARRL Handbook. Here an 8-watt push-pull audio transformer was used as the coupling element between modulator and RF amplifier.

    For some reason, maybe the cost of a proper modulation transformer,
    I decided to use a single-ended class A modulator instead of a p-p class AB or B for my 10 W 144 MHz transmitter. The intent was to have 100% modulation with some margin.

    In retrospect, this may not have been too wise, as it made upgrading to higher power AM quite difficult. Instead, a separate modulator should have been built which could have been reused.

    But, at this time, SSB was the goal for weak-signal operations on 144 MHz.

    So, when my exciter and transmitting mixer were completed a year later the AM rig was retired. It was later given away when I moved in the late 70s.
    Another 144 MHz AM rig, using controlled-carrier screen modulation of the 5894 PA still survives.

    73/
    Karl-Arne
    SM0AOM
     
  10. SM0GLD

    SM0GLD Ham Member QRZ Page

    It would be interesting to try out Heising and some of the modified variants.
    I need to clear my desk and do some serious lab work.....soon.
     

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