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  • Pro Jr Point-to-point hum issue

    Hello, it's been a while ago since I posted, but it was time for new build project now. I have been converting a Pro Jr. Amp into a version with an eyelet board - see the pictures for the schematic and actual amp.
    I am having a serieus issue with it. I have followed the same design guideline as other builds and this one hums like crazy. Could you please help me with a structured way of trouble shooting this situation?
    Hums seems like 60Hz (or double?) noise... It goes away when v1 tube is pulled. Also the grid wire v1A in the schematic seems to pick up a lot of noise. When fiddling around with this wire it becomes stronger or less, but never seems to go away entirely. Seems to me that is not the source of the hum.
    Who can advice me with a troubleshooting "plan"?
    Help is greatly welcome.
    Regards from the Netherlands,
    Mark
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    Last edited by klooon; 03-30-2020, 12:27 PM. Reason: Pictures uploaded

  • #2
    Since it's changing with moving the wires around, I'd say it's being picked up by a signal wire somewhere at either grid of V1. My first suspect would be the second gain stage, as it's running right close to the heater wiring. Possibly that other long wire, even though it's going to "not-V1."

    I'd move those as far from the heater wiring as possible; also check that any other signal-carrying wires are well away from high correct AC sources. Looks like some of your PI wiring or parts may come close, too? Maybe even use shielded wire there also, if you must. Someone else may jump in with the "ground the grid" trick to possibly help isolate which stage it's coming from.

    Personally, and I know it's a little late now, when I've rebuilt amps, I'm now a proponent of laying the heater wiring along the chassis like Ampeg & Marshall; as I build True PTP it gives me more freedom to fly wires high and I've had good luck with it.

    Justin
    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

    Comment


    • #3
      Try grounding V1b grid, then V1a in turn - does the hum disappear?

      Comment


      • #4
        Also, I'm having some trouble seeing it between the layout & photos; check your wiring, around the Volume pot & rhe far right side of the board. That resistor hanging off the middle terminal looks like it should be connected to a different resistor on the board? Also V1 to board.

        Justin
        "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
        "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
        "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

        Comment


        • #5
          I suspect your hum is 50Hz or 100Hz rather than 60. But it is important to determine which.

          I see you used a Cliff type jack as input. Does it have a chassis connection at the jack?

          get your heater wires AWAY from any signal wires. That red wire going to one of the controls for example. And try when wires come near each other the signal wires sit at 90 degrees to the heater or other AC wires.
          Education is what you're left with after you have forgotten what you have learned.

          Comment


          • #6
            Originally posted by Mick Bailey View Post
            Try grounding V1b grid, then V1a in turn - does the hum disappear?
            Grounding v1B (first stage=first half of 12AX7) did not do anything - hum is still there. Grounding v1A (second stage=second half of 12AX7) is doing 'something': the hum is less but still there. Grounding either v2A or v2B is very effective = no hum at all.

            Comment


            • #7
              Originally posted by Justin Thomas View Post
              Also, I'm having some trouble seeing it between the layout & photos; check your wiring, around the Volume pot & rhe far right side of the board. That resistor hanging off the middle terminal looks like it should be connected to a different resistor on the board? Also V1 to board.

              Justin
              Noted very well! I have been running the connection of v1A via the 100R on the board to the pot, but I have chosen to be a bit more 'flexible' and run a separate wire to play around a bit. Moving away from the heaters doesn't seem to affect the hum. Moving it to the board and off the board, left to right, etc. is changing the amount of hum, but never dead silent. I have been trying to do the v1A with a shielded wire as well, but did not change a lot: it still picks up hum/noise as I am moving it around?!

              (Note that I switched v1A and V1B compared to the schematic)

              Comment


              • #8
                Originally posted by Enzo View Post
                I suspect your hum is 50Hz or 100Hz rather than 60. But it is important to determine which.
                Yes, I am in Europe so it should be 50 or 100. Thanks.

                Originally posted by Enzo View Post
                I see you used a Cliff type jack as input. Does it have a chassis connection at the jack?
                First I had the Switchcraft, but I swapped it to avoid touching the chassis to avoid a ground loop... But it did not do anything to the hum...

                Originally posted by Enzo View Post
                get your heater wires AWAY from any signal wires. That red wire going to one of the controls for example. And try when wires come near each other the signal wires sit at 90 degrees to the heater or other AC wires.
                Now, it's just for playing around I have put the red wires in different places, moving it around. More away from the heaters, closer to the heaters, but not change in hum... Only thing that is changing the level of hum is the v1A grid wire (100R to pot).

                Comment


                • #9
                  Originally posted by Justin Thomas View Post
                  Since it's changing with moving the wires around, I'd say it's being picked up by a signal wire somewhere at either grid of V1. My first suspect would be the second gain stage, as it's running right close to the heater wiring. Possibly that other long wire, even though it's going to "not-V1."

                  I'd move those as far from the heater wiring as possible; also check that any other signal-carrying wires are well away from high correct AC sources. Looks like some of your PI wiring or parts may come close, too? Maybe even use shielded wire there also, if you must. Someone else may jump in with the "ground the grid" trick to possibly help isolate which stage it's coming from.

                  Justin
                  I have been playing with the wires, but I don't get much improvement. I also think the problem might be around V2 or V1 "towards" V2, from V1 to the grid of V2B. When I touch the 0.01uF cap, it is 'microphonic'/highly sensitive. Any ideas how to improve? Is there any other way of 'investigating' next to the grid to ground methode?

                  Comment


                  • #10
                    Capacitor across R4

                    Comment


                    • #11
                      I have been running the connection of v1A via the 100R on the board to the pot
                      The 100R resistor is a grid stopper and should be soldered directly to the tube socket.

                      Is the inside surface of your chassis bare metal or coated?

                      Do the pot cases connect to ground?

                      PLease post the complete schematic showing the heater supply.
                      Last edited by Helmholtz; 03-30-2020, 03:51 PM.
                      - Own Opinions Only -

                      Comment


                      • #12
                        Originally posted by mozz View Post
                        Capacitor across R4
                        Then the hum will be gone?

                        Comment


                        • #13
                          Originally posted by Helmholtz View Post
                          The 100R resistor is a grid stopper and should be soldered directly to the tube socket.
                          OK - I will do that.

                          Originally posted by Helmholtz View Post
                          Is the inside surface of your chassis bare metal or coated?
                          Good point! I thought it would be bare metal, but looking at it closer it could be coated...


                          Originally posted by Helmholtz View Post
                          Do the pot cases connect to ground?
                          I do not use the pot cases as ground. But I have not isolated the speaker jack, yet. Everything else is not connected to ground. The only ground connection to the chassis (next to the speaker jack) is through the point close to the input jack (see picture of my layout).

                          Comment


                          • #14
                            I do not use the pot cases as ground.
                            Yes, but the pot cases act as shields and should connect to ground. Does hum increase when you touch them?

                            Please post the complete schematic showing the heater supply.
                            - Own Opinions Only -

                            Comment


                            • #15
                              Originally posted by Helmholtz View Post
                              Yes, but the pot cases act as shields and should connect to ground. Does hum increase when you touch them?

                              Please post the complete schematic showing the heater supply.
                              I put the 100R at the tube post. Ran the wire nicely on the bottom of the chassis. That improves it. Still hum is there.
                              If I am close (with my hand) to the pots with volume open, then it is giving much more hum. Also when I tap on the 0.01uF caps I hear a 'microphonic' noise.
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