Repair Question - Odd Humming Amp | The Canadian Guitar Forum

Repair Question Odd Humming Amp

Discussion in 'Amp Building/Technical/Repair' started by dtsaudio, Jan 3, 2020.

  1. dtsaudio

    dtsaudio

    Joined:
    Apr 15, 2009
    Location:
    Hamilton, ON
    This is a weird one.
    A dealer gave me a call saying his vintage Ampeg GS12 amp was humming. Went and had a look at it, and when you turn the volume control all the way down the amp hums. Turn the volume up and at about 9:00 position the hum suddenly stops and the amp is quiet.
    So I bring it back to the shop thinking bad cap, bad pot, bad ground, any of which may cause this.
    And this is where it gets weird.
    Fire it up, and no hum, no matter where the volume control is. I tried changing the grounds, removing the death cap, grounding the chassis and a few other things. I could not get the humming to return.
    Bring it back to the dealer and the hum returns. It's like a switch too. Reach a certain point on the volume control and it suddenly stops completely.
    He's going to try the amp himself at a different location and see what happens.
    Anybody else seen this happen?
     
  2. oldjoat

    oldjoat

    Joined:
    Apr 4, 2019
    Location:
    lost in time and lost in space RHPS
    bad wall plug ? (arcing inside the box ) ... as the power draw becomes higher, the full connection is made between the outlet receptacle and the supply wires .
    ( usually happens on a pig tail ) ... try a completely different circuit outlet .

    or the circuit is subject to interference from a ballast going bad ( somewhere along the supply wires )

    possibly the tip grounding lug on the input jack is oxidized , and you're picking up stray 60HZ till the power level reaches the break over point
    jack cord with one end tip/ground shorted and plugged in should eliminate this as a problem source.

    try tuning an AM radio between stations ( white noise ) ... then move it around the area to see if you pick up any enhanced buzzing.

    just suggestions of course .
     
  3. nonreverb

    nonreverb Gold Member

    Joined:
    Sep 19, 2006
    Location:
    Ottawa, ON
    I had a Marshall JTM reissue in once were the customer was complaining about really bad hum and noise....Of course when I got it on the bench, it was dead quiet. Went back to the customer only to be returned...he even sent me a video of the noise. I finally asked him if he had tried it in a different circuit which he did....that solved to problem. I can only suspect there were serious wiring issues in his house which I indicated to him.
     
  4. knight_yyz

    knight_yyz Gold Member

    Joined:
    Mar 14, 2015
    Location:
    Hamilton, Ontario
    older houses with knob and tube wiring may have a 3 prong wall jack but no ground..
     
  5. dtsaudio

    dtsaudio

    Joined:
    Apr 15, 2009
    Location:
    Hamilton, ON
    Checked that. His store wiring is properly wired three prong.
     
  6. nonreverb

    nonreverb Gold Member

    Joined:
    Sep 19, 2006
    Location:
    Ottawa, ON
    The supply might be noisy. Depends on what else is on that circuit....You never know...I've seen the results of some horrendous wiring jobs in my time.
    Also, like a cell phone that's close to an amp, there might be something near by that's emitting interference.
     
  7. WCGill

    WCGill Gold Member

    Joined:
    Mar 27, 2009
    Location:
    MH,AB
    Sure sounds like a grounding issue to me. High impedance tube circuits can misbehave when stability is "just so" compromised.
     
    Lincoln likes this.
  8. dtsaudio

    dtsaudio

    Joined:
    Apr 15, 2009
    Location:
    Hamilton, ON
    I'm think the noise is on his hydro in the store. He's supposed to try it at a different location and see what happens.
    I could probably eliminate the issue if he's agree to re-cap it and add a three prong power cord. But since it is in good physical condition he wants to sell it as factory as possible. He's even got NOS tubes for it.
    Personally, I'd rather do the re-cap and power cord. Chassis is quite live if plugged in wrong. But he wants is vintage.

    Could be also. But I think the issue is store wide wiring.
    Funny side story though. I did some work for another store, and all the amps were quiet, and worked fine, except on one receptacle. Whenever an amp was plugged into this one particular receptacle you would know when a cell phone was about to get a text or ring. The amp would start beeping if the volume was turned up even slightly. The receptacle checked out fine, as did all the others, except it was on another wall 30 feet from the other ones. Something on the line was picking up interference. This is the world we live in now.
     
    jb welder and greco like this.
  9. nonreverb

    nonreverb Gold Member

    Joined:
    Sep 19, 2006
    Location:
    Ottawa, ON
    Yup, that's what it sounds like. AC supply is only as good as the electrician who installed it.
     
  10. jb welder

    jb welder

    Joined:
    Sep 13, 2010
    Location:
    Melonville, Canada
    One store that I checked, the wiring was 'correct' but there was a substantial voltage drop between the panel and the outlet. And varying depending on the load. Caused some weird noise related issues, and occasional drop outs in the units being powered.
    Lots of weird wiring issues out there and hard to check them all.
     
  11. greco

    greco Gold Member

    Joined:
    Jul 15, 2007
    Location:
    Kitchener, Ontario
    What could cause this? Just curious and enjoy learning.
     
  12. oldjoat

    oldjoat

    Joined:
    Apr 4, 2019
    Location:
    lost in time and lost in space RHPS
    possibly 1 loose connection on one of the receptacle plugs along the way ( pwr drop)

    ontario code today is to use pigtails to bring power out to the plug ( neut, ground and power ) and the rest tied with marrets
    just so you don't ever break the neutral or ground when working in a box .

    as far as that "odd plug 30 feet away" ... perhaps a close harmonic wavelength for the 850 Mhz for the wire length .
    or that side of the building faced a cell tower in the area.
     
    jb welder and greco like this.
  13. jb welder

    jb welder

    Joined:
    Sep 13, 2010
    Location:
    Melonville, Canada
    Yeah, in the case I mentioned it was just a bad wiring job to the outlet.
     
    gtone likes this.
  14. dtsaudio

    dtsaudio

    Joined:
    Apr 15, 2009
    Location:
    Hamilton, ON
    I wouldn't doubt there's some bad connections somewhere. Building is a t least 75 years old, probably more.
    That too is another mystery that will not get solved. However I've seen stranger phenomenon when dealing with cell or broadcast towers, so what you say wouldn't surprise me.
     
  15. dtsaudio

    dtsaudio

    Joined:
    Apr 15, 2009
    Location:
    Hamilton, ON
    So finally got the amp back from the dealer. Sure enough it doesn't hum here, but does it at the dealers and in his home. I checked his house wiring, and it is actually better than what I have. New panel, properly grounded etc.
    That part is still a mystery. Must be some noise on his lines.
    However I did fix the problem. And it is almost as strange.

    On the volume pot there is the typical wire going to ground, and the signal coming in and out (from the wiper) The wire leading from the pot to the first tube is a shielded wire with the shield connected to the ground connection of the pot AND the back of the pot. I was reading an article a few days ago about induced noise, and it was mentioned that the shield itself could pick up noise if not wired properly. So just on a hunch I unsoldered the shield from the back of the pot but leaving it tied to the ground connection at the pot. Immediately the amp was picking up a bit of noise from my bench lamp which it wasn't doing before, so even that very short connection to the pot was making a difference.
    I packed the amp back up and brought it to the store, hooked it up and no more hum. What we had was a very small ground loop centered at the pot. The pot body is already grounded through the chassis from its own mounting hole, and the shield wire is grounded at the pots ground terminal. Connecting the body of the pot to that shield was inducing ground noise in the cable via a small ground loop. The amp is dead quiet now no matter where the volume control is.
    Very unusual.
    The moral 0f the story is - think about grounding whenever you have a noisy amp. Even the smallest incorrect connection can lead to problems.
     
    nbs2005 and DaddyDog like this.

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