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Marshall 1962 Bluesbreaker (JTM 45) Reissue Hum

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  • Marshall 1962 Bluesbreaker (JTM 45) Reissue Hum

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ID:	958971 Need to bias power tubes and locate hum. Not able to record due to hum. 5881s reading(30mA/21mA) w/eurotubes bias probe. See a video on YouTube that guy is rotating power transformer to be facing like original jtm45. Says marshall designed incorrectly and is causing hum. Would like some opinions on that video. Thanks.

  • #2
    Originally posted by ca7922303 View Post
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Name:	Screenshot_20220422-180653.png
Views:	164
Size:	272.1 KB
ID:	958971 Need to bias power tubes and locate hum. Not able to record due to hum. 5881s reading(30mA/21mA) w/eurotubes bias probe. See a video on YouTube that guy is rotating power transformer to be facing like original jtm45. Says marshall designed incorrectly and is causing hum. Would like some opinions on that video. Thanks.
    https://youtu.be/UbynIuLdIuo

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    • #3
      Originally posted by ca7922303 View Post
      5881s reading(30mA/21mA) w/eurotubes bias probe.
      Swap the power tubes around and see if the higher reading stays with the tube, or the socket.

      "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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      • #4
        Originally posted by g1 View Post
        Swap the power tubes around and see if the higher reading stays with the tube, or the socket.
        Swapped 5881s, (30mA) reading stays with tube.

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        • #5
          Do you have a better matched pair? Try the 6L6's that matched up same in the Ampeg R-12.
          "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

          Comment


          • #6
            If your output valves, push pull in this case, are not balanced, hum will be in the background or forefront, depending how far out of balance they are.
            Always fit a matched pair.
            Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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            • #7
              Do any controls affect the hum?
              Remove the valve in V1, what effect does that have on the hum?
              Repeat for V2, then V3.

              Unless you enjoy stressing the rectifier valve, avoid using the stupid hot switching standby.

              https://el34world.com/charts/Schemat...e_45w_1962.pdf
              My band:- http://www.youtube.com/user/RedwingBand

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              • #8
                Originally posted by g1 View Post
                Do you have a better matched pair? Try the 6L6's that matched up same in the Ampeg R-12.
                Installed 2-6L6s both reading (32mA).

                Comment


                • #9
                  Originally posted by pdf64 View Post
                  Do any controls affect the hum?
                  Remove the valve in V1, what effect does that have on the hum?
                  Repeat for V2, then V3.

                  Unless you enjoy stressing the rectifier valve, avoid using the stupid hot switching standby.

                  https://el34world.com/charts/Schemat...e_45w_1962.pdf
                  With no signal to inputs, presence is only control having any effect on hum. On zero is scratchiness, at 10 it's it's full treble. Have sprayed pot with contact cleaner and worked pot.
                  V1 removed-no effect V2 removed-no hum but presence pot still scratches
                  V3 removed-no hum, no issue with presence pot

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                  • #10
                    Presence is always scratchy in such circuits, ie that put DC voltage across the pot. So not a repair / maintenance issue.

                    Has a known good valve been swapped into V2?
                    My band:- http://www.youtube.com/user/RedwingBand

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                    • #11
                      Originally posted by pdf64 View Post
                      Presence is always scratchy in such circuits, ie that put DC voltage across the pot. So not a repair / maintenance issue.

                      Has a known good valve been swapped into V2?
                      I did swap a good 12ax7 @,V2 and hum about the same.

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                      • #12
                        Originally posted by pdf64 View Post
                        Presence is always scratchy in such circuits, ie that put DC voltage across the pot. So not a repair / maintenance issue.

                        Has a known good valve been swapped into V2?
                        Filter caps?

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                        • #13
                          If you are following the schematic from EL34world, it is incorrect around the NFB tone control and more than likely, in other places.
                          Click image for larger version  Name:	Correct diagramme.png Views:	0 Size:	42.3 KB ID:	959072 Click image for larger version  Name:	Incorrect diagramme.png Views:	0 Size:	86.8 KB ID:	959073
                          Is the hum mains frequency or twice mains frequency. At least with that information we can determine whether mains hum or smoothing hum.
                          Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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                          • #14
                            Originally posted by Jon Snell View Post
                            If you are following the schematic from EL34world, it is incorrect around the NFB tone control and more than likely, in other places.
                            Click image for larger version Name:	Correct diagramme.png Views:	0 Size:	42.3 KB ID:	959072 Click image for larger version Name:	Incorrect diagramme.png Views:	0 Size:	86.8 KB ID:	959073
                            Is the hum mains frequency or twice mains frequency. At least with that information we can determine whether mains hum or smoothing hum.
                            I believe it to be 60hz by comparing it to an audio sample online.

                            Comment


                            • #15
                              Originally posted by Jon Snell View Post
                              If you are following the schematic from EL34world, it is incorrect around the NFB tone control and more than likely, in other places.
                              Click image for larger version Name:	Correct diagramme.png Views:	0 Size:	42.3 KB ID:	959072 Click image for larger version Name:	Incorrect diagramme.png Views:	0 Size:	86.8 KB ID:	959073
                              Is the hum mains frequency or twice mains frequency. At least with that information we can determine whether mains hum or smoothing hum.
                              What do you make of the thought that turning the power transformer to jtm45 original position can lower the hum?

                              Comment

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