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Ampeg B3 with crossover distortion

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  • Ampeg B3 with crossover distortion

    Just replaced the two fried output transistors on an Ampeg B3 bass amp of the high freq section. After that the amp is working but I have crossover distortion on the output and it's audible. There's no any trimmer that I can bias it, which resistor(s) do I have to replace to bias it properly?

    Schematics attached.
    Attached Files

  • #2
    Are you getting correct DC at TP's 7 & 8 ?
    Note that they are different than the values given for TP's 5 & 6.
    Those voltages are what sets the bias.
    The only difference between the two circuits are R4 & R47 being different values from R23 & R28.
    So I would think those are the resistors you want to look at.
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


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    • #3
      I get slightly lower values at TP 7 & 8.
      Yes, those resistors I thought too but I wasn't sure.

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      • #4
        Have you actually measured the bias across the emitter resistors?

        I ask because something else may be causing the distortion.

        If you blindly hack into the bias circuit, you may mask the original problem.

        I am thinking maybe you could lift D32 & D33, as the protect transistors may have taken a hit.
        If the xover distortion goes away then you can inspect each half circuit.

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        • #5
          It's time to bring this amp back to my workbench. I haven't found the problem, yet. Crossover distortion is still there.

          Today I replaced those protect transistors and the emitter resistors but with no luck.

          Voltages across emitter resistors are around 0.8mV and at the low frequency amp section 1.8mV.

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          • #6
            Finally, no crossover distortion!

            I messed around with those resistors, R4 and R47. I started with 110K and on my scope the crossover distortion was less than before, also voltages at the emitter resistors where a little bit higher. I connected a bass and a speaker and was less audible, too.

            I ended up with 133K (100K + 33K) resistors and now I have across all emitter resistors, both low and high amp section, around 2mV. Amp sounds awesome! Hope I didn't mask the original problem. The amp is at full power right now almost an hour without any problems.

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