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5F6A Hum...

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

  1. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Hello, so I'm working on 5F6A clone. The filter caps used to be located inside the chassis right above the power tubes. I wanted to install new F&T caps, but they are too big to go inside the chassis so I mounted them outside, behind the chassis where Fender used to do it.
    Now there's a hum. Sounds like 60 cycles and it increases with volume on either channel and is also affected by the tone controls (treble, mid, presence, bass).
    I feel like it's because of the proximity of the B+ wires to the tone pots/ wires.
    But that's how Fender did it, correct? Am I missing something here? Is there a better way to run these wires?
     

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  2. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Ok... I rerouted those b+ wires through a new hole in the chassis, away from all the tone controls... still humming. What else could it be? When I changed the filter caps, I changed all the electrolytic caps. I noticed one thing. This first bypass cap on V1(or something very close to it) is slightly microphonic. It's not the tube in V1. Seems like it has to be either that 250uf cap or it's 820R resistor...
    Could that be why this is happening?
     

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  3. dan40

    dan40 Tele-Afflicted

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    The problem must be occurring very early in the circuit since the volume and tone controls affect it. Did you move any of the wires around the v1 socket or disturb any of the solder joints when you did your work? Have you tried a new tube in v1 yet?
     
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  4. D'tar

    D'tar Friend of Leo's

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    Are you positive on the 60Hz? Did you disturb the heater center tap in the process?

    120Hz? How did you wire the B+ string and where is your PT center tap connected? And the rest of your ground scheme?

    More pics please. Can include the filters.
     
  5. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Here's how I have the wiring for the electro caps. Through a hole in chassis. The center taps are both grounded with the power amp ground
    (Power tubes, stand by, first three filter caps).

    Everything else (including the fourth filter cap that goes to phase inverter) is grounded to an input Jack sleeve via ground bus. The power chord is grounded alone on the closest PT bolt.
     

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    Last edited: Aug 25, 2020
  6. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    ...
     

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  7. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Yes, I tried a different tube in V1. No luck.

    I also swapped out that bypass cap that was microphonic... didn't help.
    I've tried several tubes in V1.
    The wire that goes freon the eyelet board to V1 pin 2 (grid 2) seems to be the most microphonic. What could be causing this?
     
  8. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Pretty sure it's 60 cycle.
     
  9. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Input
     
  10. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Input section
     

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  11. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Heater wiring/ tube sockets
     

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  12. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    It's not a very bad hum. At normal playing level you can't even really hear it. It's just annoying because it used to be dead silent.
     
  13. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Hmmm....
    Yeah, I don't know either.
    I've been moving around center tap grounds... The only other thing I changed...
    It already had a 1 ohm resistor between 1 of the power tube cathodes and ground (for biasing). I added one to the other tube.
    But it's gotta be something close to V1 or a grounding issue... so frustrating.
     
  14. timfred

    timfred Tele-Meister

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    Try moving PI filter cap ground off of the input signal ground...maybe directly to the same place the other filter caps are grounded. I try to keep power and signal grounds separate.

    Do the green/black wires in the pic go to the OT or the speaker? Check proximity there. If one of the black wires is the preamp power ground and it's getting EMF coupled to the OT in some way that could be a problem.

    Moving the caps out of the chassis means new opportunities for EMF interference. Are you planning on building a "dog house" for them?
     
  15. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    In order of my thought.

    You have soldered in several caps. Check/reflow the solder joints. Did you use a heat sink on the caps when soldering? Did any get damaged?

    I do not like the white ground wire from the filter caps placed through the same hole with the B+ wires. That white *ground* wire is not quiescent.

    Like @timfred I too am concerned with the filter cap wires running near the OT and OT wires.

    Try clipping in a 10uF to 22uF cap between the rectifier pin8 B+ and the HT CT grounding terminal. Keep the leads as short as possible. Your hum may be ripple current from the rectifier. If this reduces the hum the location of the HT CT and first filter cap will need attention.

    Since you are noticing V1, I am concerned with the lead dress. I hope you don't mind the nit picks. The board to socket and heater wire routing could be better. (You said it was quiet before so this may not be an issue.)
     
  16. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Great. Some good ideas here. I'll get on it this afternoon. I don't mind the nitpick. I know it has problems. I built it a long time ago. It was the first amp I built.
     
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  17. D'tar

    D'tar Friend of Leo's

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    If you pull V1 does the humm stop? if you ground V1 pin2 does the hum stop? With nothing plugged in, ohm V1 pin2 to chassis. Should be 0.01ohm.

    You need a dog house/cover/cage over the new caps in that configuration. Not sure if you have that or not?
     
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  18. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    I have a dog house cover off a super reverb... going to see if it fits. If not, I'll modify it or get one. And, yes, when I pull V1 the hum stops.
     
  19. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    With
    the plugged in, V1 pin 2 to ground...
    What do you mean "nothing plugged in"
    No tubes anywhere?
     
  20. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    I tried moving the PI filter cap ground to the power amp side. I did this first.
    Yes, the green and black wires do go to the OT. But the hum is the same as it was before the b+ wires were there. I don't mind moving them back to the way they were before. But are they ok running up beside the tone pots?
    I mean... it seems that's the way fender did it.
     
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