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M3 Percussion Distortion

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  • M3 Percussion Distortion

    M3 AO-29 which distorts at mid-full pedal. Removing V9 12AU7 eliminated problem. As a friend gigs with this organ and due to time constraints, I swapped out the AO-29 with another from a M101.

    On the bench, it appears there is to much gain through V9 & T6. Voltages and T6 resistance look ok.
    Does anyone know what the gain of this stage should be?
    Is there a procedure for calculating the value of R63?
    Thanks,
    Jim

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  • #2
    Hello,
    As the gain varies from MaxValue (Percussion start VGrid=25V) to Zero (12AU7 is at CuttOff because VGrid-Vcathode =37-15 =-22V) it is difficult to calculate it.
    But it is possible to calculate MaxValue only by consulting the charts of the 12AU7 (Working Point Va = 205-37 = 168V VG = VGrid-Vcathode =37-15 =-22V) and the transformer.
    But hammond did it perfectly.

    i think the bias of the 12AU7 is not correct.
    i suggest you check V=37Volts (12AU7), V=32Vots(Percussion Cutoff), And Percussion CuttOff potentiometer is working.

    Read Also Benton : 'Servicing the Hammond B3 Preamp'

    JP

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    • #3
      Thanks JP
      I've read the perc section several times. V9 cuts off as described. I dug out another AO29-7 and after getting it in working order compared the 2 for transformer resistance and voltages - checked out ok. I also compared the gain through T6 by injecting a signal - ok. The input to the preamp 12AX7 as measured on terminal D is almost twice that of the 6AU6 outputs. Both amps do this.

      I swapped out the speaker dummy load with an actual speaker and realized the amp wasn't being overdriven - but oscillating. I believe the higher prec output causes this. I'm going to start a new thread.

      Thanks again,
      Jim

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