FL2100Z - using 811A

Discussion in 'Amateur Radio Amplifiers' started by VA3QY, Jul 7, 2020.

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  1. W1VT

    W1VT Ham Member QRZ Page


    The 572b can take significantly higher plate voltages than the 811A. So some means of lowering that without changing the other voltages is required.

    This could be an excellent use of a re-wound plate transformer. It may be hard to duplicate the original windings--a lower voltage winding will be easier to fit in the original space available on the transformer core.

    Zak W1VT
  2. AE5SB

    AE5SB Ham Member QRZ Page

    Have done this in FL-2100B
    move tap from 850 to 620
    plug in the 811's
    limit your drive
    there you go
    N8VIL likes this.
  3. G3YRO

    G3YRO Ham Member QRZ Page

    But the Pi-Tank will be all wrong for correct Loading at lower power . . .

    Why anyone would ever consider putting 811As in an FL2100 is beyond me !

    Roger G3YRO
  4. K7JOE

    K7JOE Ham Member QRZ Page

    The story on the 572b tube supply out of china is three-fold. First, there are tariffs on imported goods and those caused a modest (10%) price increase from late in 2019; COVID disrupted the supply chain in China from Dec - March causing restocking delays or stock outs across the globe. Then, in August a building adjacent to the factory that produces 572B and other tubes burned down - the factory / production is closed; The Changsha city government demands the factory relocate too due to health safety and EHS concerns.

    The tube supply that is in the market is old stock and/or possibly some factory seconds that are being resold now that supply has dwindled. Expect the prices on many of the Chinese produced (Psvane, Shuguang,Taylor) tubes to continue to increase, who knows if the factory in China, when or if relocated will continue to produce 572B vacuum tubes.

    The audio guys were discussing this on some forums, and my friends in China confirmed the situation. https://www.audiokarma.org/forums/i...-that-shuguang-tube-factory-is-closed.896506/

    To answer Roger's question -- if you can't get 572B you are not left with many choices. Just like those owning amplifier using 8873 / 8874 / 8875 and a few of the old Henry amps that use 3-400Z;
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    Seems like the U.S.A. could build them again.
  6. K6BSU

    K6BSU Ham Member QRZ Page

    Reasoning for using the 572 in place of 811A is longevity, and not more power output. As noted, using 572's to get more power in an 811
    a amp involves impedance matching of the output networks.
    K4AGO likes this.

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