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Questions about Swart AST V3 PI/tremolo tube

Discussion in 'Glowing Bottle Tube Amp Forum' started by sugarinthegourd, Sep 22, 2020.

  1. sugarinthegourd

    sugarinthegourd Tele-Meister

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    I have a Swart AST which I continue to love, and I am trying to understand its inner workings a bit better.

    V3 is a 12ax7 which I believe is the Phase Inverter and also controls the tremolo. Swart describes it thusly: "a 12AX7 does duty as the driver and bias regulated Tremolo."

    Are we basically saying the same thing? Is one half of the tube a cathodyne PI, and the other half part of an oscillator circuit that affects the power tube bias?

    If that's correct then the two triodes don't need to be matched in terms of gain or plate current, right? Have other AST owners found a favorite tube in this spot? I currently (no pun intended) have a really nice Bugle Boy in there but am wondering if that might not be the best use of a rare/valuable tube and intend to try some others. Anyone settle on something other than a 12ax7/ECC83?

    Thanks in advance,
    John
     
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  2. tubegeek

    tubegeek Friend of Leo's

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    Correct, they are doing jobs which do not require any match between each triode section - a cathodyne PI and a low frequency oscillator, that's an accurate statement. "Driver" isn't the term I would use myself, because of... well... the possibility of confusion, as we are seeing in this post! Not your fault.

    Using a different tube, if it reduced gain in the oscillator, the oscillator might not start up. There is a minimum REALIZED gain of - I think 27? - that is necessary to get a reliable self-oscillation going. (Realized gain is always lower than mu factor, so just because another tube has, say, mu of 70, doesn't mean the circuit's gain will be high enough.)

    I always think that the fancier tubes are most audible earliest in the circuit, so your impulse to use the Bugle Boy somewhere else makes sense to me.

    There's no harm in trying a swap - just be warned that, if your tremolo doesn't come on, that's why.
     
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  3. sugarinthegourd

    sugarinthegourd Tele-Meister

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    Thanks, tubegeek!
     
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