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Thread: Help with Amplifier Power Supply, Dropping Resistors

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    Forum Member jeru's Avatar
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    Help with Amplifier Power Supply, Dropping Resistors

    My dad always said to do the hard work, but know when it's time to ask for assistance. Well, I've tried to find a satisfactory
    answer to this on the Online, but either it's not always so clearly explained or I'm just not getting it. Anyway, for those of you
    who know more than I -- please try to 'splain this to me.

    I'm not sure how to determine the current portion of the calculation for resistance value and needed wattage rating of dropping
    resistors along the power rail. I absolutely need to be able to do this to get the right plate voltages to the right places when
    building these damn things.

    Varying articles (on the Online) say to add up the current draw for tubes 'upstream' of the resistor. Common assumption seems
    to be 1.1 Ma for 12aX7 triodes. Well, Ok.

    For power tubes, though, "current draw" for calculation purposes is unclear to me. If it's "plate current" on the data sheet then
    I'm confused. Looking at the GE 6L6 data sheet for two tubes in AB1, idle plate current is 88ma at 360v and 116ma at 450v, and
    maximum-signal plate current is 140ma at 360v and 210ma at 450v. Which current number to use..?

    Even better, can you run through an example for me -- say, using an AA864 schematic (link). For purposes of calculating the values
    (resistance/needed wattage rating) of each of the 1K/1W and 4.7K/1W dropping resistors (if we didn't know them or, say, needed to adjust
    for high/low B+), what is the 'current' in the R=V/I equation?

    Thanks for any insight you'd care to offer.
    Last edited by jeru; 10-07-2015 at 11:43 AM. Reason: Can't type worth a damn

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