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Gounding Question

Discussion in 'Hardware, Setup & Repair [BG]' started by Pacman, Mar 10, 2001.


  1. Pacman

    Pacman Layin' Down Time Staff Member Gold Supporting Member

    Apr 1, 2000
    Omaha, Nebraska
    Endorsing Artist: Roscoe Guitars, DR Strings, Aguilar Amplification
    Not sure where else to post this.

    I've got a TRB6P with Bartolini p/us and the Aguilar OBP-1 preamp. took it to the gig tonight, it buzzed through the amp. if I touch the pots, buzz goes away. if I touch the barrel of the cord, buzz goes away, if I touch the *rack mount pre*, buzz goes away. Didn't seem to buzz with the neck pickup pulled down.

    ok, guys, I'm pretty damn good with electricity.....but what did I miss?!?
     
  2. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.
    Pacman, have you checked to make sure the bridge is grounded? sometimes corrosion will build up or the wire will mash down into the body enough to cause a poor connection.

    In order to check the bridge ground, just hook a temporary jumper wire from the bridge to the ground on the output jack. If the hum changes or goes away just remove the bridge, locate the ground wire and clean the connection. A very thin coat of vaseline between the bridge and the body, just where the ground wire contacts the bridge, will afford some protection against oxidation at the connection

    Pkr2
     
  3. Pacman

    Pacman Layin' Down Time Staff Member Gold Supporting Member

    Apr 1, 2000
    Omaha, Nebraska
    Endorsing Artist: Roscoe Guitars, DR Strings, Aguilar Amplification
    well, it's a piezo bridge (that's not hooked up), so it's probably not grounded. however, touching the bridge (like touching the other parts described above) does not change the hum in any way.

    the thing about the whole situation that struck me as funny was touching my rack changed the buzz....weird.
     
  4. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.
    Pac, I would still try temorarily grounding the bridge. You are dealing with a ground loop and they don't always respond to logic in diagnosis. :)

    An ungrounded bridge acts sort of like one plate of a capacitor with the pups and wiring acting as the other plate. The strings act as an antenna, capacitively coupling stray A.C. hum to the pups and wiring.

    You mentioned that the problem showed up at your gig. Do you still have the hum when you try it at home? Line voltage at clubs are notorious for being nasty. Neon lights are often nothing more than accidental noise generators and the offending neon can be physically located anywhere in a shared A.C. circuit.

    If you don't have a jumper wire, one can be fabricated by straightening out a wire coathanger and touching the bridge with one end and the ground point with the other end.

    The fact that the piezo pup isn't hooked up probably doesn't elimenate an ungrounded bridge as the culprit.

    Pkr2
     
  5. Pacman

    Pacman Layin' Down Time Staff Member Gold Supporting Member

    Apr 1, 2000
    Omaha, Nebraska
    Endorsing Artist: Roscoe Guitars, DR Strings, Aguilar Amplification
    I'll give this a shot today....I didn't notice any noise problems at home....but I didn't plug the bass into a bass rig, just a small board....

    would bad a/c in a venue react with a grounding issue? could this be why touching my rack components stopped the noise? I'm going on the assumption that it was something I did.

    thanks for the help, dude
     
  6. I would guess a bad AC outlet. As PK said, does it do it now that you are home? If everything is ok in a different location, it'd rule out your equipment pretty much.

    On old amps, there was a polarity switch. This switch tied one side of a capacitor to either wire coming in from 120AC, and the other side was hooked to ground. If you had the switch flipped so that it was hooking the capacitor to "Hot", there is a nasty hum that goes away when you touch any ground, usually with a pop when you first touch it. When you flip the switch so that the cap goes to neutral, No Hum. So my first guess is that the outlet was wired reverse, and your amp was picking up line noise because of it.

    Just a guess, though. You definitely need to plug into another outlet in another building and see if it still does it.

    chris
     
  7. I jsut saw you are in Atlanta. cool, I'm only an hour away down I-85.

    Chris
     
  8. Pacman

    Pacman Layin' Down Time Staff Member Gold Supporting Member

    Apr 1, 2000
    Omaha, Nebraska
    Endorsing Artist: Roscoe Guitars, DR Strings, Aguilar Amplification
    I just plugged this into my board here at home. Here's the deal, there's some interference, but I think that's my computer monitor....just turned off both monitors and it's dead quiet. My rig is still in the car and we're having a 3 year old birthday party today, so I can't check it in my amp until later.

    so...if the house wiring was at fault, how do I solve that at a gig? ( I don't have polarity on my amp - demeter pre to crown power)
     
  9. -yeah, I've always wondered why they just stripped the wire and left it under the bridge.
    I've taken the grounding wire and tinned it with solder to give a more reliable contact with the bridge. no problems with oxidation yet.
     
  10. Pacman

    Pacman Layin' Down Time Staff Member Gold Supporting Member

    Apr 1, 2000
    Omaha, Nebraska
    Endorsing Artist: Roscoe Guitars, DR Strings, Aguilar Amplification
    been thinking about this today....if I built a short extension cord and wired it so that it would reverse hot and cold, would that help in a situation like that?
     
  11. What you describe would work, but I don't know if it should be recommended. If it were me, I'd go down to the hardware store and buy an outlet checker. It's a little plug in thing with lights that tells you if you have a true ground, if Hot and Neutral are reversed (>>>but if there is no Ground, it won't be able to tell you if hot and neutral are swapped.) But it's better than nothing. If I was ABSOLUTELY SURE that Hot and Neutral were swapped and that was the only thing wrong with the outlet, and I had a good ground, I would probably use the cord of which you speak.

    Once again, legality and the US court system being what it is -- kids, don't try this at home.

    Chris
     
  12. Three words...Furman Power Conditioner. I never leave home without it. :)
     
  13. Pacman

    Pacman Layin' Down Time Staff Member Gold Supporting Member

    Apr 1, 2000
    Omaha, Nebraska
    Endorsing Artist: Roscoe Guitars, DR Strings, Aguilar Amplification
    update:

    took the bass to another gig this morning - quiet as a church mouse. good call on the a/c guys. no where's my furman catalogue?........
     
  14. Pacman

    Pacman Layin' Down Time Staff Member Gold Supporting Member

    Apr 1, 2000
    Omaha, Nebraska
    Endorsing Artist: Roscoe Guitars, DR Strings, Aguilar Amplification
    OK. to follow up, (and this should be in amps, but since i started it here....)

    could the problems I've had be caused by a tube going bad?

    I tried the bass thru the PA tonight, went fine, but thru the amp I got popping, cracking, buzzing like crazy.... this **** is driving me nuts. I'd like to get this fixed because Patitucci may be using my bass this month (woohoo!!!!)

    Oh yeah, and two basses did this stuff thru my amp, so I'm guessing, not the bass...........

    you'd never know I was a sound guy, huh?
     
  15. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.
    Pacman, a problem like you are describing very well may be tube related or even a bad tube.

    A dirty tube socket can cause exactly what you describe. A microphonic tube can too. A tube with an intermittent inter-element short in the tube itself can act as you describe.

    Usually that kind of problem is pretty easy to isolate, but it does require that you open the amp up to gain access to the tubes. With the tubes accesable, warm up the amp and tap the glass envelope of each tube gently with something like the eraser end of a pencil. A microphonic tube will make itself obvious by the noise it generates through the speakers as you tap it.

    If you can recreate the problem while you can see the tubes, keep a sharp eye for any internal arcing in the tubes. Particularly the output tubes, although any tube in the amp could cause the prob.

    A tube sockets contacts are usually made from a phosphor/bronze alloy that loses its temper when a tube overheats a few times. If cleaning the socket clears up the problem but the problem comes right back in a day or two, a tube socket replacement is about the only cure. If replacement is required, it's worthwhile to locate ceramic base tube sockets which seem to hold up much better than the phenolic sockets.

    Don't even bother with the various methods of retensioning the socket contacts. Once the temper is gone from the contacts, the socket is useless.

    Just be systematic and patient. As with any sort of intermittent problem, the worst thing that can happen is to temporarily fix the problem and not be able to make the problem reocurr. Go gently untill you KNOW what causes the problem. The repair is the easy part. Knowing what to repair is the hard part.

    Good luck. Keep us abreast as you go along.

    Pkr2