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5F6A Hum...

Discussion in 'Amp Tech Center' started by MrCoolGuy, Aug 25, 2020.

  1. andrewRneumann

    andrewRneumann Tele-Holic

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    Hey sorry late to the party, but I DID skim the entire thread!!! @MrCoolGuy I know you have checked and rechecked the wiring from the B+ nodes of the filter caps. I’m curious how you are checking them since they are all black. Also they are all twisted together. How are you confirming the proper connections inside the chassis?

    You have something I have heard called “sticktoitiveness.” I salute you!
     
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  2. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Grounding any of those three spots slightly reduces the hum. But not significantly.
     
  3. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Grounding points along the ground bus, the closer I get to the input jacks (where the ground bus is actually grounded) the less it helps reduce hum.
     
  4. andrewRneumann

    andrewRneumann Tele-Holic

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    I’m concerned that maybe your B+3 feed which should be dead quiet, is picking up ripple from the B+1, since they are all twisted together. I know it should be 120Hz hum, but you might have some interesting harmonics, phase shifts, summations/cancellations going on in those feeds so maybe it does sound more like 60Hz hum.

    Way back last year when you started this thread, I gathered you grounded both grids of the V1A and V1B, and the hum remained. However, when you grounded the grids of the next valve in the chain, the hum went away. To my mind that means the hum is coming from the plate supply of V1 or the heater of V1 through the cathode (doubtful, and I think I read you tested it with DC on the heater).
     
    Last edited: Sep 10, 2020
  5. D'tar

    D'tar Friend of Leo's

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    Try reflowing all those connections.
     
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  6. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Lol thank you. And thanks for responding.

    So I checked them before and after soldering them with continuity (DMM).
    Also, they are cut to different lengths.
    And they are marked... Notice the dots on the cloth of the wires.

    The new wires since I turned the orientation off the filter caps, are yellow as you see.
     

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  7. andrewRneumann

    andrewRneumann Tele-Holic

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    You da man. Duh on the DMM check, I should have figured that.

    This is beyond a Hail Mary, but maybe run those wires without twisting them. You have one wire with a lot of current in a coil with 2 other wires. There’s no benefit as far as I can see in having them twisted as they do not contain inverted currents like heater wiring.
     
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  8. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Yeah, agreed. I will try. It simply made it easier to keep them together fishing them through the hole and along their way. I will try.
     
  9. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Yes, I sure will. Tomorrow! Wife and kids are telling me to go to bed.
     
  10. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Ah! B+3, indeed. You confused me there momentarily with the b+4.
    Ok, I will sure try in the morning. Thank you. More on your other comment then.
     
  11. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    So if I ground either grid of V1, the hum remains. If I ground pin 2 of V2, the hum is reduced. If I ground pin 7 of V2, the amp is silent.
     
  12. D'tar

    D'tar Friend of Leo's

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    Since we are trying to tidy things up a little. This here is something I would address and note that it is personal preference more than necessary.

    The connection from V2p6>100k>V2p1>V2p7 can be made using only the 100kr and its leads. Feel free to bend the socket solder tabs to your advantage. You can do whatever you like. run the lead from p6 through p1 and back over to p7. No extra wires needed. If the center tab is in your way, remove it.

    upload_2020-9-11_10-43-28.png

    Here is a photo posted by @Lowerleftcoast in another thread talking about similar circumstance.

    upload_2020-9-11_10-44-14.png
     
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  13. D'tar

    D'tar Friend of Leo's

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  14. andrewRneumann

    andrewRneumann Tele-Holic

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    Ok... I'm not an expert on DCCF's, so need more wisdom here--but if you ground pin 7 of V2, you are sending B+4 to ground directly through a 100K resistor. If B+4 is 325V, that's about 1W going through the load resistor that sits on the top of V2. I'd be very careful doing that.
     
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  15. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Yes, I did. And I reflowed... pretty much everything around preamp.
     
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  16. andrewRneumann

    andrewRneumann Tele-Holic

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    How about that Sprague 8uF on the board. Did you replace that cap when you did the 4 F&T's? It looks new... but possibly faulty? I just got one from AES that was crushed during delivery.
     
  17. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Yes, I replaced it. I have more, I will try another after I clean up V2 lead dress.
     
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  18. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Yes, agreed. It was very momentary.
     
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  19. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    I replaced the 8uf cap. I fixed up V2,
    Reflowed some more joints...
    Exactly the same. It sounds like something isn't grounded or there is a ground loop... I don't know.
     

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  20. D'tar

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    That 10K B+4 dropping resistor looks to have suffered under the V2p7 to ground moment.

    upload_2020-9-11_14-20-30.png
     
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