Need A Modulation Transformer For That 500 Watt Final?

Discussion in 'Amplitude Modulation' started by W2VW, Jun 4, 2020.

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  1. K5UJ

    K5UJ Ham Member QRZ Page

    I'm 95% certain that it's FW only. No plans to try FWB and find out. Too many stories of hams doing that and burn out. FW is good enough. UTC made a CG series of plate iron. The model numbers ending in W could be run as either FW or FWB (at reduced current). My 3-400 rig plate trans. is a CG303W (or 301W, can't remember which) and I run it with four 3B28s because that's how the power supply was originally built. Specs say 500 ma. FW and 350 ma. FWB. Been running it at 350 ma. Ip for years & no probs.
     
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  2. N2DTS

    N2DTS Ham Member QRZ Page

    The BBQ grill sounds like a good idea, no way the XYL would put up with baking anything in the house!
    I also love the T368 mod iron, oil filled so no worry about it degrading with age.
    No air, no moisture, plus it was designed for 2500 volts.
    It works with the 2x 4-125 X 4-125 setup just fine, and also at 1200 volts on the 3X 4d32 rig.
     
  3. W2VW

    W2VW XML Subscriber QRZ Page

    T368 mod iron is tough but it has a crummy ratio for use with a common supply.

    If the modulators are run on a higher voltage than the final amp it would be able to make more useful positive peaks.

    Of course it was designed to be war proof and positive peaks were way down the list of design goals.
     
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  4. K4KYV

    K4KYV Subscriber QRZ Page

    The same is true for the BC-610 transformer. In addition, many older broadcast transformers have a 1.6:1 to 1.7:1 step-down turns ratio, since the FCC once limited AM broadcast positive peaks to 100%. When they changed the rules to allow up to 120% positive, mod iron began to appear with something like 1:3:1 to 1.4:1 turns ratio. One solution when using a common power supply and a transformer with excessive step-down is to insert an adequate power resistor in series with the bottom end of the modulation reactor to drop the DC plate voltage to the final but deliver the full audio swing. It wouldn't hurt to by-pass the resistor with a capacitor; if the transformer is used without a reactor, then the by-pass capacitor is mandatory. This is similar to the R-C network in series with the B+ line to the final in a rig using Heising modulation. That idea was first published in QST back around 1929 in an article explaining how to achieve 100% modulation, which back then had been a rarity with ham radio transmitters.
     
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  5. K5UJ

    K5UJ Ham Member QRZ Page

    I always assumed finding any mod. iron for a 610 other than the stock transformer would be difficult because of the high p. to p. impedance of the 100THs. Ditto for the 30K1. Is that true? But there's nothing to prevent a 610 owner from trying a mod. reactor of the needed inductance to get the current off the transformer secondary and eliminate talk back. I've never owned a BC610 but I've heard the early production transformers did a lot of talk back.
     
  6. N2DTS

    N2DTS Ham Member QRZ Page

    I run a common supply on both transmitters and get 100% modulation in both directions on both transmitters per the REA mod monitor.
    No one is going to hear the difference between 100% positive and 120%, are they?
    100% might sound cleaner in some receivers.
     
  7. K4KYV

    K4KYV Subscriber QRZ Page

    A lot of the infamous talkback in the BC-610 is not due to the modulation transformer, but to the overload relay whose coil is wired between CT and ground of the plate transformer. The power supply DC passes though that coil to ground (and pulls in the relay if the current is excessive), and it carries enough audio component from the modulated DC to make it talk back. The fix is to by-pass the relay coil with about a 100 mfd electrolytic capacitor.
     
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  8. K5UJ

    K5UJ Ham Member QRZ Page

    Probably less distortion with envelope detectors.
    In my opinion, it's all about high average modulation achieved by processing. I'll take that over 120%. But, a stock classic ham or mil. rig is best off with a D104 and nothing else. Again my opinion. I occasionally hear new AM ops doing 200% positive. That borders on DSB with a pilot carrier. Completely unnecessary, and unlistenable with a vintage AM receiver.
     
  9. W2VW

    W2VW XML Subscriber QRZ Page

    Lots of people can hear the difference between 100 and 120% positive modulation.

    Most simple diode detectors start distorting the recovered audio envelope above 30%.

    Have you calibrated your audio monitor for verification of the numbers it shows?
     
  10. N2DTS

    N2DTS Ham Member QRZ Page

    From wwhat I have been told, the REA mod monitor needs no calibration, its all done in software.

    I could never tell 100% positive from 120%, I hear people who sound loud at under 100% and people who sound low with over 120% positive.


     

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