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toroidal power transformers as modulation transformers.

Discussion in 'Amplitude Modulation' started by N2DTS, Jul 1, 2016.

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  1. N2DTS

    N2DTS Ham Member QRZ Page

    I was working someone who was using a power transformer as a modulation transformer and sounded great doing so, so I ordered a pair of 120vac to 800 vac transformers.
    Each transformer has two primary's and two secondary's.
    120
    120
    800
    800
    That is 8 pairs of wires. White and yellow each for the 120 volt windings and red and black for the 800 volt windings.
    I tried every combo to use on a pair of 811,s modulating a pair of 4x150's at 1200 volts, and can get maybe 20% of highly distorted audio out.
    4000 ohm on the RF side, about 12K ohms for a pair of 811 tubes at 1200 volts.

    What is wrong with this setup?
    Someone is using this iron and its working for them....
     
    NISHAYENG likes this.
  2. K4KYV

    K4KYV Premium Subscriber QRZ Page

    Are you sure you had the polarity of all the windings phased properly, and that the insulation on none of the windings was shorting out, since they likely weren't all designed for high voltage?
     
  3. K5UJ

    K5UJ Ham Member QRZ Page

    One of the guys doing this is a ham in Indianapolis, Borgi, K9YQQ. I don't know a great deal about it, but I vaguely recall you do something like buy an 800 v. transformer and a 600 v., something like that, with the needed VA or KVA and voltage rating and you parallel connect the 120 v. coils so you have a 800/600 : 1 turns ratio transformer. I think he mounts them up on insulators and he uses a mod. reactor. I think each toroid has a pair of coils so you series the 800 v. coils and use the center junction as your modulator B+ connection. you connect the 600 v. coils in series also and put one end to ground via a 1 or 2 mfd oil cap of adequate v. rating and the other end gets the final B+ via the mod. reactor. You can select other voltages for a more favorable turns ratio. Another thing that's being done is to use a single transformer to somehow modulate a tube final with a solid state audio source but I don't know much about that at all. I think Dave W2VW knows something about this so he may correct/expand on what I've written. Don't hold me to this but I think you have to use a reactor. I hope I'm wrong because the reactor is tough to find.
     
  4. W2VW

    W2VW XML Subscriber QRZ Page

    Check the hookup using a filament xfmr. See if you get expected voltage out of other end.

    That setup works best at 1:1 impedance ratio.

    I've used the Antek transformers all the way up to 3kv fully modulated for a while without failure. That's way over their specification. The "secondary" side is grounded on one end and fed thru .95 uF DC blocking cap.

    Rob's info is correct. I believe Borgi did not use the reactor. One should really be used.

    A pair of 810s would work FB with 1:1 turns ratio. I did it with 813s all the way down to 8K plate load impedance. They were working pretty hard but held up for a long time.
     
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    Why would it take a pair of 811's to drive a pair of 4x150's ?
     
  6. W2VW

    W2VW XML Subscriber QRZ Page

    High level modulation. Audio amplifier essentially in series with RF power amplifier B+.
     
  7. N2DTS

    N2DTS Ham Member QRZ Page

    Yes, the Antek transformers.
    I did a bit more playing around and could get it so it did not sound bad, but I got a lot of 811 plate current swing and not a lot of audio on the carrier.
    Maybe 40%.

    A pair of 4x150,s have 500 watts of plate dissipation, and can run around 1600 volts at 300 mills or more, about 480 watts input, 400 watts out.
    I like to have as much audio power to modulate a rig as carrier output, in this case, a pair of 811a's is only good for about 300 watts.

    I normally screen modulate the 4x150's at 2000 volts, with 200 watts carrier output, sounds good but I would like to use the 811 modulator and run the rig at 300 watts with less stress on the tubes.

    I have two of the Antek transformers, can you use two separate ones in series parallel?
    I will have to test that...
     
  8. W2VW

    W2VW XML Subscriber QRZ Page

    A modulator that hauls enough current to play into a load impedance similar to the modulation impedance of the RF final will satisfy the power delivery issue. 1:1 does not appear in classic amateur literature. Selected quality commercial MW AM broadcast transmitters used 1:1 with common supply but not often.

    811s will not keep up with yer final.

    You can parallel and series all that is necessary keeping in mind leakage reactances are the enemy.

    If you heard me on the air in the last year it was with Antek toroids as modulation iron. They work extremely well as Stu AB2EZ originally found out and chronicled.

    Would like to have an Antek to Antek QSO in the near future..........We can go in circles.
     
  9. N2DTS

    N2DTS Ham Member QRZ Page

    I put both in series/parallel and can get 100% modulation, low end is very weak, but my power supply sags badly.
    I had changed it to a voltage doubler to get up to 2000 volts under a steady load, but with a big class B modulator it drops 300 or more volts under mod peaks, never going to work that way so I need to put it back to full wave center tapped choke input.
     
  10. W2VW

    W2VW XML Subscriber QRZ Page

    ru using B+ shunt feed to the final?
     

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