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Gibson BR-9 OT

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  • Gibson BR-9 OT

    So I got this Gibson BR9 in a big batch of parts I acquired a while back. No tubes, no power cord, no cabinet which also meant no speaker and thusly no output transformer. But the PT is good and everything else seems to be there so Iím trying to get it running. Iíve replaced the field coil connection with a 10w resistor, but I have no idea how to find an output transformer that will work. There arenít any popular amps that use this setup that I can think of.

    Itís the double single-ended version, not the push-pull version. It exactly matches this schematic I found on the web:

    https://images.app.goo.gl/teGR2AjbwcmTT5H37

    Any suggestions would be appreciated and if anyone would like to explain how one goes about sorting out which transformers would work Iíd appreciate that as well. Thanks!

  • #2
    I would convert it to push-pull using an 8k primary/20W PP OT like a Fender Princeton replacement OT. But that will require the addition of a PI tube (or an interstage transformer).

    Other possibility is a 12W SE OT with a 4k to 4.5k primary.
    Last edited by Helmholtz; 06-03-2020, 10:31 PM.
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    • #3
      Thanks! Iíd like to keep it as original as possible, and I donít want to put that much work into it, so Iíd probably go with the second suggestion. How did you arrive on the primary winding rating?

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      • #4
        How did you arrive on the primary winding rating?
        Optimum plate load for SE class A operation is specified in the 6V6GT datasheet as 8.5k. Two tubes in parallel means half that value.
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        • #5
          Thanks-

          Another question regarding the schematic in the original post; is there an error with the speaker connection? Doesn’t one side of the OT secondary need to be grounded? This seems to be the arrangement in every other SE amp I’ve looked at. Or does it have to do with the fact that the original design used a field coil speaker? I’ll be using a PM speaker as the original is long gone. Thanks.

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          • #6
            Originally posted by MikeH View Post
            Thanks-

            Another question regarding the schematic in the original post; is there an error with the speaker connection? Doesn’t one side of the OT secondary need to be grounded? This seems to be the arrangement in every other SE amp I’ve looked at. Or does it have to do with the fact that the original design used a field coil speaker? I’ll be using a PM speaker as the original is long gone. Thanks.
            While the amp will work ok with a floating output, one OT secondary lead should be grounded for safety reasons. I don't see a relation to the field coil design.
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            • #7
              I replaced a couple of single-ended parallel output- output transformers in Gibson amps with an Allen Amps TO-11C transformer. He used to list them on his transformer page but I don't see them there now. The amps sound great and I am very pleased. He does list the TO-11S which is the same OT but mounted on its side instead of vertically. Give David Allen a call. He has always been extremely helpful!

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