Using toroidal power transformers as a modulation transformer

Discussion in 'Amplitude Modulation - AM Fans' started by W2BTK, Jan 11, 2019.

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  1. N2DTS

    N2DTS Ham Member QRZ Page

    I picture guys with strings of chokes, toroids and oil filled caps covering half the basement floor with 2000 volts on them.
    I tried one antec toroid transformer (the big one) then two in all sorts of combinations and they all sounded nasty or worse, with the DC through them.
    Gave up on that idea, they are a mess to mount, heavy and if you add some big chokes and an oil filled, its a huge amount of space taken up all with high voltage on them.

    I would rather use a good mod transformer designed for DC (like the RCA ones) or do screen modulation and have none of that mess.

    But there ARE those that think the more iron, the more glory. Piles of glory!



     
  2. KB3OUK

    KB3OUK Ham Member QRZ Page

    The rule I've always used was .0045×plate Z for the choke, and 10000/plate Z for the coupling cap, I've also heard 10 henrys for every 1000 ohms, there's so many variations to know which is right. Like i said, I'm using as low as a 10 henry choke on 6000-7000 ohms (the actual inductance could be a little higher at 150 ma since the choke is rated for 10 henries at 600 ma) with just a little loss in bass compared to the transformer by itself, but in this case it helped by flattening out the audio, the transformer alone was a little too bassy below 500 hz or so. I suspect that the inductance of the iron itself might come into play, since the secondary is somewhat in parallel to the choke, if the inductances in parallel work out to still be high enough for decent bass response. With the choke in the transmitter now, i actually noticed an increase in highs, which makes the audio response look fairly flat out to about 4k, where before everything seemed to fall off past 1000 hz. Of course lately with this transmitter I've been doing a lot of things that conventional wisdom say shouldn't work but they do.
     
  3. WZ5Q

    WZ5Q Ham Member QRZ Page

    AMEN!!!!! :D
    You can never have enough Iron! ;):cool:
     
  4. WA4JK

    WA4JK Ham Member QRZ Page

    I have a 47h 625ma mod reactor on it now and it’s a pretty steep drop at 300 and 6000hz from the bc-610 mod iron, but that was where it is designed to work for our military and rightfully so.
     
  5. K5UJ

    K5UJ Ham Member QRZ Page

    try a cap of value 1 or 2 mfd from the cold end of the mod. trans. secondary to ground. get an oil cap around 4 KV. change coupling caps in ur audio stages to .1 or .05 mfd.
     
  6. WA4JK

    WA4JK Ham Member QRZ Page

    It’s as responsive as Borgi could get it. It’s just the design of the mod iron.
     
  7. K4KYV

    K4KYV Subscriber QRZ Page

    If it's a BC-610E or other early model with the open frame iron, it would be easy to disassemble the transformer, remove the laminations and re-stack them like the laminations in a power transformer, alternating the E - I orientation at each adjacent set of laminations, thus eliminating the core gap. That would greatly improve the low frequency response. Of course, the cross-laminated transformer would not work with any DC passing through the secondary, so you would have to use a reactor with it.
     
  8. N2DTS

    N2DTS Ham Member QRZ Page

    I gave the ones I had to Bruce (WA3JVJ) and he reports very good results from them.
     
  9. WA4JK

    WA4JK Ham Member QRZ Page

    Short of rebuilding the mod ironas Don has said I am planning on building a new mod transformer using the toroidal cores and keeping the BC610 mod iron intact.
     
    W2BTK likes this.

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