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2A3/ VT4-C Amplifier Design help

Discussion in 'Audio' started by Y2KEDDIE, Aug 18, 2019.

  1. Y2KEDDIE

    Y2KEDDIE

    252
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    Sep 23, 2012
    I want to build/rebuild and amplifier using (2) 2A3 Tubes driving (2) VT4-C Tubes.

    This is a rebuild of a AM Modulator, similar to an Audio Amplifier.
    I found this inoperative chassis , with this tube line-up, but the bias circuits are missing, and I have no schematic.
    The audio input is a 1:3 xfmr, then 2A3'S, another 1:3 driver xfmr, VT4-C's, and then Modulation xfmr. The Power supply uses 866 mercury vapor rectifiers for the VT4-c supply, and a 5U4 for LV Supply feeding the 2A3's. There is an 0B3 connected to the 2nd interstage Xfmr. Center Tao to ground, ( no limiting resistor found)
    What I need is a schematic showing the correct bias circuitry , as these components are missing on my chassis.
     
    Last edited: Aug 18, 2019
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,306
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    Nov 17, 2011
    Does the chassis bear any indication as to manufacturer and/or model number? That would come in really handy.
     
  3. Y2KEDDIE

    Y2KEDDIE

    252
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    Sep 23, 2012
    It appears to be homebrew. It’s built on an old radio receiver chassis. Definitely not a commercial product.
    Vintage components circa 1940-50’s. I bought it at an estate sale, paired with a Johnson Invader 2000.
    The Invader 2000 was a Single Sideband/cw transmitter which originally came stock with a separate power supply. It appears this was an attempt to provide a power supply and external high level plate modulation.
    It was crudely built and half finished, very much heavy iron. Transformers and chokes ; Stancor and Thoradson.
     
  4. Y2KEDDIE

    Y2KEDDIE

    252
    15
    Sep 23, 2012
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    9,306
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    Nov 17, 2011
    The website you linked to requires login :(.
     
  6. Y2KEDDIE

    Y2KEDDIE

    252
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    Sep 23, 2012
    I can not seem to upload my diagram. tried resizing, still get error message
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

    9,306
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    Nov 17, 2011
    Less than 100 kB should work. I do not know the exact limit.
     
  8. Ylli

    Ylli

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    Jun 19, 2018
    The data sheets on the tubes give an approximate bias voltage requirement. You really think this thing is worth rebuilding?
     
  9. Y2KEDDIE

    Y2KEDDIE

    252
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    Sep 23, 2012
    Yes,I think so. The 1200 V 500 mA power transformer, two mercury vapor rectifier tubes, and 500Ma 10Hy choke, power supply is intact .. The Stancor modulation transformer is rated at 375 watts. All the components on this chassis all have good part numbers on them.

    I'm sure some old timer must have used these components, and has a schematic. Possibly a 1940-50's ARRL handbook. It's just that these parts are so old no one is familiar how to build the circuit.
    I'm tempted to put a 6000 ohm resistor on the output, ground the center taps of the interstage transformers, put -100 V bias on the grids and fire it up. It's just that with that big of a power supply I'm hesitant for obvious reasons.

    I only have ARRL handbooks from 1960 on.
     
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