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Thread: Pittbull 45 112 Combo Biasing Info

  1. #1
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    Default Pittbull 45 112 Combo Biasing Info

    I need to replace the output tubes in a Pittbull I just bought (1997 model). Does anyone have the biasing information? I can do it myself (I've done my Marshalls, Riveras and Orange) but can't find anything on the Pittbull. No owner manual, no service manual, no service sheet... nothing... Great amp with no information... Can anyone help me out? Thanks.

  2. #2
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    on the 2 Fryette amps ive done it with, the info was printed on the circuit board
    Ultralead
    D120H
    Sig X
    Fat Bottom 4x12
    D412
    Valvulator I

  3. #3

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    I've biased several Pittbull 45 amps. I use a bias probe (Weber) which is convenient because it gives both the plate voltage and the cathode current readings. You would want to make sure that the product of these was at or below 8.4 Watts. The rule of thumb for Class AB operation is to bias your tubes so the idle dissipation is no larger than 70% of the maximum that the tube can handle - which is 12 Watts for an EL84. There is an obvious bias potentiometer inside the chassis. I also checked the idle dissipation for Class A operation (cathode bias), and with the stock cathode resistor, the idle dissipation was quite a bit higher than 12 Watts that is used as a benchmark for biasing EL84 tubes in cathode bias Class A operation. If I remember, they were idling at more like 15 Watts or a bit more. I switched to a larger cathode resistor to bring down the idle dissipation a bit, and the sound is still fine. The tubes will last longer if the amp is not running quite as hot, but then again, some people might be willing to sacrifice some tube life to get the sound that an amp biased on the hot side might produce. I didn't feel it was necessary for the sounds I wanted from the amp. I guess that is a personal decision.

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