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slippery C3 hum issue

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  • slippery C3 hum issue

    Hiya folks. I've had a 60 cycle hum problem in a C3 that I've been working with a tech on. Bad enough to render it unrecordable.

    The tech brought another A028 from his shop and put it in my organ. Dead quiet. So he took my amp to his shop to scope it and it checks out fine. Put it in his organ there and it was perfectly quiet. He's stumped.

    To summarize. My preamp hums in my organ but not in another. A test preamp does not hum in my organ.

    What's being missed here?

    Many thanks.

  • #2
    Did the preamp get reinstalled in your organ after being tested in the tech's organ?

    Were any tubes swapped during the preamp exchange?

    How are you listening to the C3 (tone cab / Leslie type), and where is the tone cab and/or Leslie located in relation to the organ?

    Comment


    • #3
      No, my preamp is still away.

      No tubes were swapped initially. When we started trading preamp tubes there was a minor difference in the hum level, but trading all the tubes did not eliminate it.

      Listening through a 145, about 10 feet from the organ. Thanks muckelroy.



      The only other thing I noticed (besides being dead quiet) was that the test preamp did not quite have the mojo mine has. It's a bit tamer sounding,even loud. But not a huge difference.

      Comment


      • #4
        If it reoccurs on swap, see if the hum is or is not changed by the expression pedal. Also, is the hum changed, or not changed by altering the tone control pot?

        Are there any accessories in the picture? Is this a stock config or a chop / re-cased C3? What year was your C3 and preamp manufactured?

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        • #5
          My first guess would be poor ground continuity on one of the can capacitors. With enough handling of the preamp, the can may have wiggled enough to 'clean' the ground connection.

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          • #6
            Check how the heater center tap is terminated. And if the transformer has been replaced. If stock,
            check the flux band is installed and continuous.

            Comment


            • #7
              Originally posted by muckelroy View Post
              If it reoccurs on swap, see if the hum is or is not changed by the expression pedal. Also, is the hum changed, or not changed by altering the tone control pot?

              Are there any accessories in the picture? Is this a stock config or a chop / re-cased C3? What year was your C3 and preamp manufactured?
              The hum had been increased with the expression pedal. I'll check it again when it's back together and check the tone pot.

              No accessories. Stock config, 1957. Thanks.

              - - - Updated - - -

              Originally posted by Tim_at_Jonas View Post
              My first guess would be poor ground continuity on one of the can capacitors. With enough handling of the preamp, the can may have wiggled enough to 'clean' the ground connection.
              Will check. Thanks.

              - - - Updated - - -

              Originally posted by sae View Post
              Check how the heater center tap is terminated. And if the transformer has been replaced. If stock,
              check the flux band is installed and continuous.

              I'm pretty sure it's the original transformer. I'll mention the flux band to the tech. Thank you.

              Comment


              • #8
                Originally posted by dweisskeys View Post
                The hum had been increased with the expression pedal. I'll check it again when it's back together and check the tone pot.

                No accessories. Stock config, 1957. Thanks.

                Ok. If it's a preamp issue (which so far, it's unclear if it is or not), then this tells you the hum is originating before the expression pedal.

                I've personally experienced excessive hum from a 12AX7 tube in the preamp intermediate amplifier section (V4). The tube was not microphonic, but somehow it was picking up heater hum.

                You can also try removing (one at a time, while organ is off) the tubes that feed signal to the intermediate amplifier, which would be V1 (6AU6, the non-vibrato channel), V2 (6AU6, vibrato channel), V7 (12AU7, the percussion channel).
                -- If, upon removing a particular tube the hum disappears, it may not necessarily mean that the tube is bad, but it may point to a problem in that section of the preamp circuit.

                Comment


                • #9
                  Originally posted by muckelroy View Post
                  Ok. If it's a preamp issue (which so far, it's unclear if it is or not), then this tells you the hum is originating before the expression pedal.

                  I've personally experienced excessive hum from a 12AX7 tube in the preamp intermediate amplifier section (V4). The tube was not microphonic, but somehow it was picking up heater hum.

                  You can also try removing (one at a time, while organ is off) the tubes that feed signal to the intermediate amplifier, which would be V1 (6AU6, the non-vibrato channel), V2 (6AU6, vibrato channel), V7 (12AU7, the percussion channel).
                  -- If, upon removing a particular tube the hum disappears, it may not necessarily mean that the tube is bad, but it may point to a problem in that section of the preamp circuit.
                  Thanks for the suggestions. We did do the tube by tube swap. No meaningful difference. It's just so odd that my pre hums in my organ but in the tech's. Could there be some interaction between my pre and the other organ electronics? Well, I'll look at all the suggestions above when it gets back here. Maybe something did get bumped on the way to his place that altered a ground. It's a funny one.

                  Comment


                  • #10
                    What I proposed earlier wasn’t a tube “swap” but a tube “removal” one at a time to see if pulling a specific one eliminated the hum.

                    There are many grounds in the organ. Check all of them. Corrosion at a ground point could cause hum, if there’s enough of it.

                    Comment


                    • #11
                      Hello,
                      Is your organ connected to the ground by the main 3 poles plug ?
                      JP

                      Comment


                      • #12
                        Originally posted by muckelroy View Post
                        What I proposed earlier wasn’t a tube “swap” but a tube “removal” one at a time to see if pulling a specific one eliminated the hum.

                        There are many grounds in the organ. Check all of them. Corrosion at a ground point could cause hum, if there’s enough of it.
                        Sorry if I was unclear. We did do one tube at a time.

                        Comment


                        • #13
                          Originally posted by Jyvoipabo View Post
                          Hello,
                          Is your organ connected to the ground by the main 3 poles plug ?
                          JP
                          Sorry for my lack of knowledge but do you mean the plug in the wall? It's only got 2 poles.

                          Comment

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