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Thread: Rough bias current ballpark for a pitbull ultralead?

  1. #1
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    Default Rough bias current ballpark for a pitbull ultralead?

    Got some kt88's coming, nice set of svetlana winged C's, but am wondering what the general bias range for these tubes is in a UL?

    I mean, I could see what they're currently running at I suppose, but if I know what ballpark I am looking at it'll mkake my life a bit easier whilst biasing!

  2. #2
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    Posted a while ago by Steve Fryette.....

    To Ralph and all concerned,

    I hate to see you guys worrying about these superfine differences in plate current readings considering that you will typically see a significant line voltage variation in the half hour to hour you spend ultra-tweaking the bias, not to mention the transformer voltage variations that occur as the transformer heats up and cools down.

    Regarding plate voltage, if you are correctly measuring this at the plate of each tube, you will have the same voltage for all four, give or take a very small difference between the right and left pair, say 2 volts tops. There's no way you're legitimately getting a 30 volt difference in plate voltage readings between 2 of four tubes no matter what position they are in unless you have a seriously off spec tube in one or the other pair. Even then, that will pull down the whole supply, not just one side of the primary voltage drop.

    Keep in mind the best you're going to accomplish with these kinds of biasing tools is an approximation, not really too far removed from leaving the adjustment at the factory setting unless you are using a radically different tube. Any better accuracy requires a scope, signal generator, variac and a very accurate dummy load. Without that, trying to get less than an 8 to 10mA difference between pairs is unrealistic and largely unnecessary.

    My recommendation: Put the tubes in, let the amp idle for an hour, set the bias for 65% PD +/- 5% AVERAGED OVER THE NUMBER OF POWER TUBES, plug in your guitar and go peel some paint of the walls.

    Your friend,

    Steve

    P.S., Example: With the UL at idle plate voltage of 450V, a combined idle current of 232mA for all 4 tubes yields 65% PD. That's 58mA/tube AVERAGE. In other words if you have a pair in which one tube is drawing 60 and the other is drawing 56, you're within +/- 5%. No sweat. Therefore Ralphs 53 to 56.6 range averages out to 54.7 - essensitally 55mA, or 62% PD. Another 3mA/tube AVERAGE gives you 65%. Keep in mind this is just an example. Your plate voltage reading may be different. That's OK. Adjust the other variables accordingly.
    NO!!!!!
    I am not Steve Fryette....
    He just builds the most kick ass amps of all time....

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