Heising Reactor vs Modulation Reactor

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

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  1. WB2GCR

    WB2GCR Ham Member QRZ Page

    In the world of audio, tube audio, a version of "modified Heising" is used quite often.
    There, it's called parafeed.
    Except instead of a transformer with an RF final for a load, it's driving the primary of a transformer that
    is a step down to a speaker impedance.
    In both cases the plan is to take the DC off the transformer...
     
    AC0OB likes this.
  2. WA1QIX

    WA1QIX Ham Member QRZ Page

    An early transmitter I built back in 1974 used what I called "Push Pull Heising", which were modulator tubes connected to each end of a center-tapped inductor not designed to carry unbalanced DC, and a coupling capacitor connecting from one end of this center tapped inductor to another inductor (carrying unbalanced DC) isolating the RF amplifier from the power supply (aka, Heising reactor).

    It's essentially the old "modified Heising" scheme, but without the modulation transformer secondary.

    Worked quite well.

    The center tapped inductor could be the secondary of a high voltage power transformer or similar, and in fact I actually used several power transformers in this setup.

    The best transformers (used as center tapped inductors) were the multi-tapped power transformers - the ones with a "low tap" and a "high tap". Connect the modulator tubes to the low tap, and take the output (connect the coupling capacitor) to one end of the high tap.

    If you are building a high power tube transmitter, but don't have a good modulation transformer, this setup works extremely well !
     
  3. K4KYV

    K4KYV Subscriber QRZ Page

    United Transformer Company ads in mid 30s (west coast) Radio Handbook displayed a special mid-tapped choke designed just for this purpose. A variation is the "modulation autotransformer" that has an extension of additional turns on one side of the winding to go to the modulated PA stage. Chokes and autotransformers may be designed for use without a reactor, by including a gap in the core to reduce saturation from the unbalanced DC.
     
  4. LU1FCR

    LU1FCR Ham Member QRZ Page

    Another circuit of a "modulation autotransformer":
    100W.jpg
     
  5. K4KYV

    K4KYV Subscriber QRZ Page

    You could use a separate modulation reactor to take the unbalanced DC off B2 + B3.

    I have a similar modulation autotransformer, made by Audio Development Corporation, designed to carry the DC. It was used in the modulator section of a Wilcox Electric modulator for an aviation transmitter, with a pair of 450TLs running class AB2. I never used it as a modulation transformer, but as a reactor with a separate modulation transformer, and it worked well until I came across a real broadcast transmitter modulation reactor.
     
  6. W2VW

    W2VW Ham Member QRZ Page

    Early 90s I remember cleaning out an electronics business during their liquidation process.

    The owner escorted me to the basement where there were several unopened cases of transformers and chokes. IIRC there were a pair of Wilcox labeled chokes.

    One choke was rated 12-60 hy .5-1.7DCA. Another identical size choke was rated for smoothing service. I forget the rating.

    Also in the same case was a Thordarson plate transformer 6000, 5, 4, 3kv center tapped 110/220 primary right around 90 lbs. I use one today for plate iron for my Johnson KW.

    The part of interest was a 4 terminal? Wilcox labeled transformer which I left behind. Something had to be left behind as I could only carry so much up the steps by myself. They were about twice the weight of the chokes, same battleship color. Must have been modulation auto xformer.

    I loaned a pair of the plate transformers to another ham along with instructions to use only with a full wave centertap rectifier. He was not happy with the 4200 volts under load and reconfigured to full wave bridge. They landed up with Gus the scrap guy from Matawan. Gus was later backed over by a Honda Civic while on duty as a police officer and lived to tell about it.

    Carrying large transformers is good for you.
     
  7. K4KYV

    K4KYV Subscriber QRZ Page

    Hammy Hambone has made that mistake many times. In plate transformers the midtap is often poorly insulated from the core or from adjacent windings, since it was designed to be at ground potential.

    That same failure may occur when the filter choke is wired in the negative side between midtap and ground. The peak a.c. voltage across a filter choke may approach the DC output voltage of the power supply. That ripple voltage, filtered out by the choke, has to go somewhere.

    UTC, as I recall, added a suffix to the part # of their plate transformers to indicate that the midtap was insulated well enough for FW bridge rectifier service.
     
    W2VW likes this.
  8. AF7XT

    AF7XT Ham Member QRZ Page

    Reminds me of a Heathkit HA-10 repair or three.
     
  9. K4KYV

    K4KYV Subscriber QRZ Page

    I'd better check the schematic of my HO-10 monitor scope. Wonder if that could be the cause of the frequently reported crap-out of the power transformer. That happened to mine, and I replaced the transformer with an identical one from a parts unit I picked up at a hamfest, one that somebody had dropped and left looking like it had been in a plane crash.

    I hope you grabbed it. Not many swinging chokes are rated for that much inductance, to cover both the optimum full load inductance and the critical inductance at minimum load. At full load, the "optimum" inductance is at least twice the critical inductance at that load. That keeps the peak rectifier current to a minimum. It's especially important to use optimum inductance with MV rectifiers, since excessive peak current seriously shortens their life. The typical 5-25 Hy swinging choke is not sufficient. You need something more like 8-40 Hy for a 2000v / 500 Ma plate supply. The 5-25 Hy is OK for lower voltage higher current, like 1250v @ 500 Ma.
     
  10. W2VW

    W2VW Ham Member QRZ Page

    I rarely turned down good iron. Yes, I grabbed them : ) Still have 4 or 6. Not sure. They are mixed up with the smoothing chokes and they look the same.
     

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