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Bridge Ground Route

Discussion in 'Luthier's Corner' started by blipndub, Sep 8, 2004.


  1. Hi.

    I’m working on a Mighty Mite jazz body that doesn’t have any routing in the bridge area including no route from the bridge area to the control cavity for a ground wire.

    Does the ground from the input need to go to the bridge or can it ground somewhere else inside the control cavity (like to a metal screw, or copper plate)?

    Thanks in advance.

    jonno
     
  2. Scott French

    Scott French Dude Supporting Member

    May 12, 2004
    Grass Valley, CA
    The ground wire doesn't *have* to do anything but close the circut for whatever electronics you are using. The idea in making the bridge part of the ground system (by running that wire to it) is to ground the strings... and the idea behind grounding the strings (assuming they aren't some kind of nylon or elixer coated type) is to use YOU as part of the ground system (whenever you touch the strings that is). So if you are looking for the benfits of a bridge ground then yes, you need to run it to the bridge. If you ran it to a screw it would really only do something if you happen to be touching that screw all the time.


    [​IMG]
     
  3. Very interesting, I didn't know that, thanks.

    Are there any alternatives, what happens if the strings aren't grounded?
     
  4. Aside from the possible noise/hum issues, there's the distinct possibility that one could be seriously harmed without string grounding. The problem is that if you tangle with an out of phase circuit (mic, another amp, etc.) you become the ground connection instead of the voltage being shunted to ground through your amp.
     
  5. Scott French

    Scott French Dude Supporting Member

    May 12, 2004
    Grass Valley, CA
    Don't you mean there is possibility of being hurt WITH the string ground?!
     
  6. Tim__x

    Tim__x

    Aug 13, 2002
    Alberta, Canada
    Yes, without a ground wire you're isolated and pretty-well safe, with one you're a convenient conduction path to ground and not safe.

    Some people reccomend using a small 600v cap and a couple hundred kilo-ohm resistor in parallel from ground to the bridge instead of just a straight short.
     
  7. I don't understand, are you saying that having a ground is potentially more dangerous than not having one?
     
  8. Lyle Caldwell

    Lyle Caldwell

    Sep 7, 2004
    Memphis
    Having the strings grounded can be dangerous. If the strings aren't grounded, and no other metal parts of the instrument that you touch are grounded, then you can't be zapped.

    After getting my mouth burned by a club mic, I used to carry a voltometer to gigs to make sure the house electrical system was safe (if there's potential between the house ground and the stage ground, that potential goes through you- in my case, it went from the mic to my mouth through my body to my guitar to my amp to the stage outlet- I was really lucky to only have singed lips).

    About a year ago, I just ungrounded all my strings and did internal shielding jobs on my instruments with star grounding systems and telescoped shields. The circuits are dead quiet, and I won't be zapped any more.
     
  9. Tim__x

    Tim__x

    Aug 13, 2002
    Alberta, Canada
    Yup, this is, IMO, the best article you can read on the subject.

    Edit: On a side note, this site also contains a rather good article on what's important in cables.
     
  10. Thanks to all for input and advice. I hadn't fully considered the safety issue - talk about a newbie! :oops:

    What would a star grounding configuration look like for a jazz bass, all grounds to a single point? Where would that point be? ...and what are "telescoped shields"?

    the guitarelectronics site is full of great information, AND retail opportunities!
     
  11. Lyle Caldwell

    Lyle Caldwell

    Sep 7, 2004
    Memphis
    Ok, for a Jazz, here's how to approach it...

    Rather than this component being grounded to that component, and that component to another component (serial grounds), all components have individual grounds run to a central ground lug.

    What I do is shield the cavities with copper foil, tack soldered at the overlapping seams. You can use shielding paint if you prefer. The copper gives a better shield but is more difficult to work with.

    Anyway, once the cavities are shielded, screw a ground lug to the inside of the cavity through the shielding material. A ground lug is just a ring of conductive metal that has an attached eyehole for a screw.

    The ground wire from each pickup goes to this ground lug. the ground wire from each pot goes to this ground lug. The ground wire from the output jack goes to this ground lug. The bridge ground, should you choose to use it, also goes to this ground lug.

    At this point, you can use a voltometer set to read DC resistance to check from any ground point in the circuit to any other ground point, including the shielded cavity/cavities. You should have a potential of 0 ohms with copper, a potential of a few hundred milliohms max with paint.

    Note: on a well shielded star-grounded guitar, the bridge ground is unneccessary and can be unsafe.

    The only connections between pickups and pots will be the "hot" wires. If you currently have the tone cap running between the tone pot and a volume pot, set it so the cap grounds to the tone pot, and then the tone pot is grounded to the ground lug in the cavity.

    A telescoped shield is a shield surounding a central conductor, but the shield is only attached to ground at one end. For example, the hot wire running from the volume pot to the output jack is usually a single unshielded wire. Replace this with a shielded wire, and connect the shield to ground at the volume pot or at the output jack, but not at both. This way all individual wires are shielded but there is no potential for ground loops.

    The best off-the-shelf wire I've found for this is George L narrow diameter cable. If you strip off the outer protective jacket, it leaves a very small shielded wire that is perfect for wiring guitars.

    Note: you'll want to shrinkwrap the shielding wire coming from the end of the telescoped wires, so the hot and ground can't touch together.

    If you're really careful, you can replace the hot leads of pickups with telescoped wires as well. Just don't damage the wire coming off the coil.

    As you can tell by reading this far, this isn't something for an inexperienced solderer to try, unless you don't mind investing some time in practice runs. But it really pays off. I can play a Jazz bass using either pickup by itself and have no hum or buzz unless I physically place the pickup within 6" of a computer monitor. But it's immune to real-world causes of hum and buzz.
     
  12. This is most excellent instruction...thank you for writing it up.