Phase inverter noise. Probably not the tube.

Discussion in 'The Workbench' started by Matthews Guitars, Jan 28, 2020.

  1. Matthews Guitars

    Matthews Guitars Well-Known Member

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    I've finished the overhaul/total restoration of my 1959 Superlead from 1973.

    It's survived the "deck everything!" test. Full blast, everything on 10, utter insanity. Works fine...except I have found that it's got some bacon frying noise.

    Turns out that it appears to not be a tube. As long as I have ANY 12AX7 tube in the phase inverter socket, it's got this noise. Changing to a different tube doesn't change it.

    That's with NO tube in either V1 or V2, either. So it's not from any tube, or so it would seem.

    What are the most likely causes of the noise in this case?

    I'm thinking it's one of the three .022 coupling caps. But not one in the tone stack since adjusting the tone controls has no effect on the noise.

    I think that the one between the treble pot and V3A grid is most likely to be at issue.

    Do you agree? Or is it more likely that one of the plate couplers, being at a higher operating voltage, is more likely to be the culprit?
     
  2. Gunner64

    Gunner64 Well-Known Member Premium Member

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    Are the sockets new? Does Tapping or gently nudging the p.i. tube make the noise greater or quieter? I have had the same symptoms from a dirty tube socket.
     
  3. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Every socket is brand new with shiny clean contacts.
     
  4. South Park

    South Park Well-Known Member

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    Check all solder joints that you worked on . Is the grounding the same . Chop stick stick the amp . A bad ground will cause a loop . If the amp works that is good
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Do you have any freeze spray?
    Freeze the resistors one at a time and wait for it. You can freeze the capacitors.
    Well...carbon resistors can definitely get noisy being that the leads are glued into the carbon.
     
  6. Matthews Guitars

    Matthews Guitars Well-Known Member

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    I found the bias current set at retarded levels. 70 ma? Seriously? No wonder I could feel the heat coming off the power tubes! That was at 485 volts on the plates.


    Chop stick test reveals no loose connections. I think I have some freeze spray to try out. Or canned air. Turned upside down, that works. As long as it's not conductive.
     
  7. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Freeze spray helped. Looks like a mustard cap (.022 400v) needs resoldering. Will try.

    Since the amp was sitting on top of its 4x12 when completely dimed out, the vibration could have easily caused a questionable solder joint to cut loose.
     
  8. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    So where did you set the bias?
    How is it holding?
    How is it sounding?
     
  9. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Going for 30 mA at the moment, at 509 volts plate, and as for how it sounds, I don't know yet. I want to get this noise sorted out first.

    At full crank it was pretty apocalyptic, I'll say that!
     
  10. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Well make sure the PI anode resistors and coupling capacitors are good.
     
  11. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Looks like resoldering the coupling caps and a few other parts solved it. Sounding good, and QUIET, too.
     
    MickeyJ likes this.
  12. MickeyJ

    MickeyJ Well-Known Member

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    if the sound comes back, it may be normal. this is an older one but it might be the same.
    1:39
     
  13. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Would you mind telling us which parts in whole were resoldered?
     
  14. Matthews Guitars

    Matthews Guitars Well-Known Member

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    The coupling caps and pretty much all the rest on the board! Might as well check them all while I'm at it, right?
     
  15. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Freeze spray is your friend.
     
  16. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Freeze frame is not.
     
  17. Matthews Guitars

    Matthews Guitars Well-Known Member

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    It was one of the .022 coupling caps on the output of the phase inverter that was the culprit. Soldering its connections solved it.
     
    mickeydg5 likes this.
  18. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Yes. I got it.
    :headbanger: :headbanger::headbanger:
     
  19. Matthews Guitars

    Matthews Guitars Well-Known Member

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    I wanted to run my set of KT66 tubes in this one but the bias adjustment won't go low enough. I'll have to increase that 47k series resistor to maybe 56K.
     
  20. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Or increase the trimmer/potentiometer unless that is a no go on the vintage.

    Or use a 33k resistor and 50k trimmer series combination.
     

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