Hum coming through tube system, most likely culprit?

Discussion in 'Audio Hardware' started by seed_drill, Apr 12, 2019.

  1. seed_drill

    seed_drill Forum Resident Thread Starter

    Location:
    Tryon, NC, USA
    It's an HH Scott 130 preamp.

    Since I get the hum through both the phono and tuner outputs, I don't think it's coming from the turntable. But when I go to the "extra" output, which currently has nothing going into it, the hum vanishes. There is also no hum when the preamp is off, but the amp is on.

    I haven't checked all the tubes yet, because it's a major PITA. I just replaced the 6X4 a few months ago.
     
  2. Dennis0675

    Dennis0675 A force of nature

    Location:
    Ohio
    A tube is always going to be the most likely problem. That being said, I had a right channel hum in my fisher kx200 and it was a bad solder joint.

    If you can get an external phono or just use that AUX with any line level output, that should be a good clue on what to look at next.
     
  3. Thorensman

    Thorensman Forum Resident

    Always best to check voltages.
    Then if ok try a new or good tube.
    A loose fitting RCA phono lead/ will cause a hun as ground is poor.
    Some tube tuners are double insulated.
    And so earth would be through interconnect

    Additional earth will cause hum,
    My Leak touchline is built that way.
    Ate the interconnects screened.
    I ask because a lot of high end cables
    Are not screened as impedance us low.
    Impedance is high on a phono stage.
    Mogami are good in regard to screening.
    The casing of pre may not be ground.
    Try a wire to mains earth.
    This cured hum on a passive pre a few years ago
    Generally case of pre is ground through
    Interconnect

    Try a different interconnect
    Good luck.
     
    KT88 likes this.
  4. KT88

    KT88 Forum Resident

    It sounds like a ground loop. Unplug the phono and tuner and it should be silent, ...well less noisy. Maybe the tuner has a grounded plug?
    -Bill
     
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  5. nm_west

    nm_west Forum Resident

    Location:
    Abq. NM. USA
    Have you tried flipping the plug polarity?
     
  6. seed_drill

    seed_drill Forum Resident Thread Starter

    Location:
    Tryon, NC, USA
    When I had these things restored they put three prong plugs on them.
     
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  7. nm_west

    nm_west Forum Resident

    Location:
    Abq. NM. USA
    I know it's circuit dependent, but I had to go back to the two prong when I tried it with a citation ii.
     
  8. seed_drill

    seed_drill Forum Resident Thread Starter

    Location:
    Tryon, NC, USA
    These have been on for well over a decade now. This isn't my first episode with hum, but it's certainly not a consistent problem.
     
  9. SandAndGlass

    SandAndGlass Twilight Forum Resident

    Oddly enough, that can sometimes be the source of the problem.

    Try ungrounding from the AC socket.
     
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  10. seed_drill

    seed_drill Forum Resident Thread Starter

    Location:
    Tryon, NC, USA
    Would using a cheater plug into a two wire outlet have the same affect? I'm not competent to do anything more involved than swapping out a tube.
     
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  11. SandAndGlass

    SandAndGlass Twilight Forum Resident

    Yes it would.

    Most common causes of hum is that it is not grounded or that it is grounded.

    Easy things first!

    Also, is the hum coming out of both speakers?

    This is usually a main ground problem.

    If the hum is coming out of one side, then it is usually not.

    If the hum is coming from one channel only, then remove the tubes from one channel and place them in the other channel.

    In other words, reverse the tubes.

    If the hum is now coming out of the other channel, there is an excellent chance that you have a tube issue.

    If a tube issue, switch one tube with the tube on the other channel, until you figure out which is the tube that is causing the hum issue.
     
    Last edited: Apr 12, 2019
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  12. seed_drill

    seed_drill Forum Resident Thread Starter

    Location:
    Tryon, NC, USA
    Yeah, I've never known what to do with the turntable's ground wire, since this preamp doesn't have a screw for it. I usually just attach it to the chassis. I do have a decent phono preamp, but it's being used in my home theater amp that lacks a phono input at all. If I steal it from there, I lose my upstairs turntable.
     
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  13. Ghostworld

    Ghostworld Forum Resident

    Location:
    US
  14. seed_drill

    seed_drill Forum Resident Thread Starter

    Location:
    Tryon, NC, USA
  15. SandAndGlass

    SandAndGlass Twilight Forum Resident

    Please note that I did add to my prior post.

    Odd that that preamp does not have a ground connection. I have some old Scott's and Fisher's and all of them have a phono ground screw terminal.

    Usually, grounding the TT ground wire to the chassis does the trick.

    But, you might "borrow" your TT preamp from your other system, just to see if it might be a grounding problem.

    Must system hum problems that I have had are related to turntables and their grounding issues.

    I do however have two tube amps that have hum issues that need to be resolved.

    One of them is a used Rogue Stereo 90 and the other is a vintage Scott 222C.
     
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  16. Ralph Karsten

    Ralph Karsten Well-Known Member

    Location:
    St. Paul MN
    What is peculiar here is that there is no hum on the 'extra' input. Now each input has its own input level control. Its worth it to see if the 'extra' inputs have the levels turned all the way down.

    Does it hum if the main volume control is turned all the way down?
     
  17. Thorensman

    Thorensman Forum Resident

    I think from what KT 88 says there is a
    Ground loop, somewhere.
     
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  18. SandAndGlass

    SandAndGlass Twilight Forum Resident

    There is always something to that somewhere kind of thing. Its trying to find where that somewhere is.
     
  19. TarnishedEars

    TarnishedEars Forum Resident

    Location:
    Seattle, WA
    How recently was this unit recapped? That unit is at least 60 years old now, but electrolytics typically begin to dry-out after about 25 years or so. So assuming that it was recapped 30 years ago, it is likely due for another recapping of the power supply at least.
     
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  20. seed_drill

    seed_drill Forum Resident Thread Starter

    Location:
    Tryon, NC, USA
    I don't recall. No more than 15.
     
  21. seed_drill

    seed_drill Forum Resident Thread Starter

    Location:
    Tryon, NC, USA
    Given that it's still audible at that level, yes, a little. Plus it comes through on needledrops, which I tend to do with the volume all the way down since the speakers are behind the turntable. A less than ideal arrangement, but necessitated by the room layout.
     
  22. TarnishedEars

    TarnishedEars Forum Resident

    Location:
    Seattle, WA
    15 years seems a little too recent for cap failures to be very likely. But its just old enough that they can't be ruled-out completely.
     
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  23. seed_drill

    seed_drill Forum Resident Thread Starter

    Location:
    Tryon, NC, USA
    I just double checked and seem to have been mistaken about the tuner setting. The hum is only coming through the phono input.
     
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  24. Thorensman

    Thorensman Forum Resident

    Try, if possible another turntable.
     
  25. KT88

    KT88 Forum Resident

    Well now... That rules out the global ground loop. It looks more like a poorly grounded tonearm or cartridge if it stops when the input is disconnected. If the preamp hums on phono input without the table connected, then there is either an internal fault or an external interfering field.
    -Bill
     
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