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Yet another L4B rescue

Discussion in 'Amateur Radio Amplifiers' started by W9JSW, Apr 13, 2018.

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  1. N8FVJ

    N8FVJ Ham Member QRZ Page

    Largest Antek transformer is 1500 va. Is not this a little 'light' for 2500KW input? On the other hand Antek states 20% more power for 60 Hz. That would be 1800 va?
     
  2. W9JSW

    W9JSW Ham Member QRZ Page

    I am also just running one 3-500.
     
  3. N8FVJ

    N8FVJ Ham Member QRZ Page

    Oh, way more than enough power for one 3-500Z.
     
  4. W9JSW

    W9JSW Ham Member QRZ Page

    I would certainly like to find a reasonable priced higher power transformer. I will mention that this transformer I used is 800VA. It sags from 3600V no load to 2800V loaded. I could have used the larger 950V 1500VA transformer and double the secondaries. And it was almost twice the price and much larger. So probably good enough.

    What led me down this path is the results from a one tube amp modification here -

    http://wb4hfn.com/DRAKE/DrakeArticles/L4B-AIO/L4B-AIO-TOC.htm

    His results with one tube are more than satisfactory.

    The reason I have not messed with it further is that I am also building a 2x813 x 2x813 plate modulated transmitter. That will be my go-to transmitter. This project was more of a "will it work" effort. I had bought the RF Deck with 2 tubes and planned to part it out using the tank on my 813 rig. Figured I could steal parts and still be money ahead for a cost of $200.

    One of the tubes turned out to be gassy. I looked at it for over a year and decided the tank would not be what I wanted anyway. So one day I discovered the hammond enclosure and decided to make a OEM like PS for it. Now it is a complete 700W linear instead of a incomplete project to trip over in the shack.

    John
     
    KD2ACO likes this.
  5. N8FVJ

    N8FVJ Ham Member QRZ Page

    HV linear amp transformers are expensive new such as the Peter Dahl products. Heathkit SB-220 used is only 23 lbs and the L-4B is 29 lbs that is just large enough for 1500 watts PEP out. Antek makes 950 volt 36 lb 1500va (1800 va on 60Hz) transformer for $195 that would likely make 1.5 watts PEP out for two tubes ICAS duty. The 36lb transformer is 6 lbs heavier than the other 1500va transformers so it is likely about 2000va on 60Hz power. It may be too large for the Drake power supply.
     
  6. WA7PRC

    WA7PRC Ham Member QRZ Page

    (Re)winding a transformer is dirt-cheap, easy, and (in a nerdy way), fun. Often, a suitable core can be had for free or very low cost. Then, most of the cost is a few pounds of wire.
    My free > 4 VA xfmr came to me in a scrapped medical power supply. The split bobbin gave me excellent pri-sec isolation (and no need to touch the two tapped primary windings).
    [​IMG]
    The new tapped secondary gives me 3338, 3581, 3827 or 4073 VDC at up to 1.5A from a full-wave doubler rectifier. I spent about $140 on polyimide (Kapton™) coated wire + NMN (Nomex-Mylar-Nomex) interlayer "paper". I have enough wire & paper leftover to (re)wind another. :)
     
  7. HS0ZED

    HS0ZED Ham Member QRZ Page

    Have you looked to see if the line side voltage sags in sympathy with the HT side sag. A few quick measurements and some simple arithmetic might suggest the transformer is actually not too bad. Line sag is a big problem for me. House wiring here is all 1mm, lights' power, the lot. A kW amp pulls the supply side down quite a bit if you try to run on that even on 220v.
     
  8. W9JSW

    W9JSW Ham Member QRZ Page

    I ran a 220 line myself. Used 10ga wire with a 25ft run to the panel. Not sagging.

    Running Duncan's PS designer with accurate pri/sec ohm readings and offload voltages, it predicts the sag almost exactly.
     
  9. HS0ZED

    HS0ZED Ham Member QRZ Page

    Okay that's one thing sorted then for sure :)
     
  10. WA7PRC

    WA7PRC Ham Member QRZ Page

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