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Adding presence control in Bogen CHB20A?

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  • Adding presence control in Bogen CHB20A?

    I had just purchased a Bogen CHB20A PA head to convert to a guitar amp. Was gonna totally rebuild the preamp section to make it that of a Orange OR120 seeing them tonestacks are both the James Bandaxall style. And most pot values are the same too so less parts to remove and replace\buy. My question is about the negative feedback loop in this amp. It appears there is what I believe to be a 220k resistor feeding from the green/white secondary tap. What also has me confused is the 70v at the speaker outs. Wouldnt this cook your speakers? Also the feedback resistor not feeding off the transformer speaker impedance taps has me confused as well, what purpose does the second set of secondaries (for lack of a better term.) serve?

  • #2
    Schematic?
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    • #3
      Here is the unmodded schematic. I wonder if it would be major hassle to ditch those ECL86s for a pair of 12ax7s and some EL84s... But maybe Im getting ahead of myself.

      https://www.dropbox.com/s/6fa42guvm5...agram.pdf?dl=0

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      • #4
        The 70V output is meant to feed a 70V sound distribution system (:https://en.wikipedia.org/wiki/Consta...speaker_system) where each speaker needs to have its own step-down transformer.

        Your speaker(s) should be connected to the appropriate impedance OT tap.

        Regarding presence control you may use a 470R linear pot in series with a 1F cap wired across/in parallel to R15.
        Last edited by Helmholtz; 03-16-2020, 11:48 PM.
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