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Pro Jr Point-to-point hum issue

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  • #46
    Originally posted by nickb View Post
    I think that puts us back to square one. Focusing on the 100Hz hum, try this. Hook it all back up the way it should be, screen to lug 3 and inner to lug 2 of the volume control, set vol to '1' and note hum level. Now disconnect the negative end of 22uf that goes to node "1" on the layout diagram and connect it to node "2" instead. Any better?
    Thanks. I redid the 'original' wiring. Strange thing is that there is more hum than I had a few days ago... Volume down gave 55dB and volume up all the way lowered it to 41dB. Now the numbers are almost 70dB. Although I know the absolute values don't say so much, relatively it can give a hint.
    I decoupled joint 1 and connected the negative end of 22uF to point 2. But unfortunately no difference.
    Also when I poke around with a chopstick, nothing seems to change the hum.
    What else can I do now?

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    • #47
      Is the screen reconnected? Is there a wire from vol lug 3 to ground? A 20db change is huge but I don't know how you are measuring this. Is it possible that there is a gain adjustment in the path between your amp and the measuring device that would account for the difference? Is it actually sound 20dB more hummy?

      What I would now do is disconnect the AC heater supply at the transformer and feed the heaters from a DC supply to isolate the heater hum from power supply hum.

      PS: Can you measure the hum in ACV at the speaker using a DVM? Specify meter make and model too please.
      Last edited by nickb; 04-13-2020, 12:33 PM.
      Experience is something you get, just after you really needed it.

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      • #48
        Originally posted by nickb View Post
        Is the screen reconnected? Is there a wire from vol lug 3 to ground? A 20db change is huge but I don't know how you are measuring this. Is it possible that there is a gain adjustment in the path between your amp and the measuring device that would account for the difference? Is it actually sound 20dB more hummy?

        What I would now do is disconnect the AC heater supply at the transformer and feed the heaters from a DC supply to isolate the heater hum from power supply hum.

        PS: Can you measure the hum in ACV at the speaker using a DVM? Specify meter make and model too please.
        I resoldered the grid wire, but also 80% of all other connections. The 22uF from point "1" to "2" was actually giving a tiny bit less hum. So I put another wire there. And... I installed another tube for V1 - and now it's... fine. The v1 tube appeared to be faulty. I put that one in a few days ago... Apparently then it went to 70dB... Now the hum is the same as the stock Pro Jr. that I will transform as well. But the 50Hz buzz and high freq noise is much and much less. So far so good?!

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        • #49
          Originally posted by klooon View Post
          I resoldered the grid wire, but also 80% of all other connections. The 22uF from point "1" to "2" was actually giving a tiny bit less hum. So I put another wire there. And... I installed another tube for V1 - and now it's... fine. The v1 tube appeared to be faulty. I put that one in a few days ago... Apparently then it went to 70dB... Now the hum is the same as the stock Pro Jr. that I will transform as well. But the 50Hz buzz and high freq noise is much and much less. So far so good?!
          Hooray! When another fault gets introduced in the middle of debugging it gets confusing very quickly. You have to start over.
          Experience is something you get, just after you really needed it.

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          • #50
            Originally posted by nickb View Post
            Hooray! When another fault gets introduced in the middle of debugging it gets confusing very quickly. You have to start over.
            Thanks so much for your help and patience!

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