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Gibson BR-6 output transformer

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  • Gibson BR-6 output transformer

    Hi,

    I have this old '47 or '48 BR-6 with the OT mounted to the 10" field coil speaker.

    Can I upgrade to a better OT and if so what can I use? Would a Deluxe Reverb OT work?

    Can someone point out what would work and the specs of such?

    Link to schematic

    Thanks
    Attached Files

  • #2
    Very surprised none of the experts here know any of this stuff.

    Thanks anyway.

    Comment


    • #3
      The schematic doesn't show the speaker voice coil impedance, so it's impossible to give you a definitive answer. You could have measured it with an ohmmeter.

      If the impedance is something sensible, or you intend to replace the speaker too, then any 5E3, Deluxe Reverb, or 18 watt Marshall style OT would do.

      They will all sound somewhat different.
      "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

      Comment


      • #4
        Originally posted by Steve Conner View Post
        The schematic doesn't show the speaker voice coil impedance, so it's impossible to give you a definitive answer. You could have measured it with an ohmmeter.

        If the impedance is something sensible, or you intend to replace the speaker too, then any 5E3, Deluxe Reverb, or 18 watt Marshall style OT would do.

        They will all sound somewhat different.
        What is the 1000 ohm then?

        Comment


        • #5
          The field coil.
          "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

          Comment


          • #6
            I doubt if you will get much of a difference in performance by "up grading" the OT on a 60+ year old field coil type speaker. If you replace the speaker with a modern type you will have to replace the field coil with a choke and/or a load resistor. If the amp has no collector value and is in good shape, I would consider just rewiring it in a more modern configuration, then replace the OT (if you really need to) and the speaker. Be aware that a lot of old amp's character comes from cheap and inadequate OTs at times.

            Comment


            • #7
              Originally posted by Steve Conner View Post
              The field coil.
              Hi Steve,

              Aren't we going circular here?

              Comment


              • #8
                Originally posted by olddawg View Post
                I doubt if you will get much of a difference in performance by "up grading" the OT on a 60+ year old field coil type speaker. If you replace the speaker with a modern type you will have to replace the field coil with a choke and/or a load resistor. If the amp has no collector value and is in good shape, I would consider just rewiring it in a more modern configuration, then replace the OT (if you really need to) and the speaker. Be aware that a lot of old amp's character comes from cheap and inadequate OTs at times.
                The amp has no collecter value but is solid and something I want to keep.
                What do you mean by "more modern configuration"? Do you mean dumping the field coil arrangenment and going permanent magnet?

                Comment


                • #9
                  Originally posted by pchilson View Post
                  The amp has no collecter value but is solid and something I want to keep.
                  What do you mean by "more modern configuration"? Do you mean dumping the field coil arrangenment and going permanent magnet?
                  Absolutely....By a more modern configuration I mean gutting it except for the PT, the chassis, and the tubes (maybe even the existing OT) and make a new tag or turret board with new components with the circuit and layout of a more modern 6V6 PP amp like a 6V6 18 watt Marshall or the various Fender designs. And of course using a speaker with a permanent magnet.

                  Comment


                  • #10
                    Well I'm not interested in gutting it. I like the circuit as it is and was wanting to know about the output transformer and you have answered that for me. Thanks

                    Comment


                    • #11
                      Given the 200-ohm cathode resistor, lack of cathode bypass cap, and running the plates and screens at the same voltage, my SWAG is that it was designed for class A, 10-watt service like in the RCA manual. The idle current for that spec, multiplied by the 200-ohm cathode resistor, gives a perfect +15 volts.

                      Any chance of hooking it up to a variac set for 110V and measuring the plate voltage?

                      If I'm right, the OT would have a primary Z of around 10K and be good for about 10 watts.

                      - Scott

                      Comment


                      • #12
                        Originally posted by ThermionicScott View Post
                        Given the 200-ohm cathode resistor, lack of cathode bypass cap, and running the plates and screens at the same voltage, my SWAG is that it was designed for class A, 10-watt service like in the RCA manual. The idle current for that spec, multiplied by the 200-ohm cathode resistor, gives a perfect +15 volts.

                        Any chance of hooking it up to a variac set for 110V and measuring the plate voltage?

                        If I'm right, the OT would have a primary Z of around 10K and be good for about 10 watts.

                        - Scott
                        No variac, sorry.

                        What is "SWAG" ?

                        Comment


                        • #13
                          Originally posted by pchilson View Post
                          No variac, sorry.

                          What is "SWAG" ?
                          "Stupid, wild-assed guess."

                          What kind of voltage are you seeing in there, just hooked up to the wall?

                          - Scott

                          Comment


                          • #14
                            The plates are running right about 295ish.

                            Comment


                            • #15
                              Originally posted by pchilson View Post
                              The plates are running right about 295ish.
                              Cool, then I'm pretty sure about my guess. That 295V would've been closer to 260-270V back in the day. Subtract off ~15V from the cathode, and you're pretty close to 250.

                              BTW, I'm definitely in favor of leaving it as-is as long as it works. If that OT fails, a Hammond 1609 would be a good replacement.

                              - Scott

                              Comment

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