Necessary to ground pots if they are already touching shielding?

Discussion in 'Pickups & Electronics [BG]' started by Chad T, Sep 23, 2016.

  1. Chad T

    Chad T

    Feb 10, 2013
    I've got aftermarket preamps in both of my Ibanez basses. An Aguilar OBP-3 in one bass and a Nordstrand 3b in the other. The preamps have been installed for awhile and I don't really have any noise problems. But I was looking at diagrams and reading about how pot casings should generally be wired to ground. Neither bass is wired that way.

    They both have shielding paint in the control cavity. I just grabbed my mulit-meter, set it to continuity, placed one probe on the shielding paint, then touched the other probe individually to the backs of the pot casings. And I get continuity with every pot casing. In other words, it appears the pots are already grounded via their mounting hardware.

    In this situation is there any benefit or reason to ground the backs of the pot casings?

  2. it's never necessary to solder an earth to the pot casings if you have shielding paint or foil. however if the circuit was prewired, it may be relying on an earth connection that is made with a pot or worse, a jack. if there are wires or pot lugs soldered to the pots, or if the pre stops working with the pots and jacks removed and dangling,then this is the case, and I'd strongly recommend adding an earth wire or two.

    if the pre still works with the pots and jacks out dangling, there's no reason to solder the casings.
  3. Bruce Johnson

    Bruce Johnson Gold Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    The shells (rear covers) of almost all pots are electrically connected to the threaded sleeve around the shaft. So, when you mount it to any metal plate, or to any wood or plastic surface which has shielding on it (conductive paint, copper tape, etc.), then the shell will be grounded and will become part of the shielding. If you question the connection, check it with a multimeter, as you did. If you have continuity, then the shielding is good, and there's no need to solder a wire to the pot shell.

    Also, if you've shielded the whole cavity (which you should do) then the pots are already shielded by being within that cavity. Shielding their individual shells is duplicated effort; a shell within a shell.

    So, no, in almost all cases there's no real reason to solder to the pot shells. It's often done out of heritage throwback. Old instruments from way back didn't use shielded cavities. They used shielded audio wire running between the pickups and pots, and soldering the braided shields of the wires to the pot cans was a simple way to build up the harnesses on a production line.
    Grumry and ddnidd1 like this.
  4. It is common for mass production instruments to not have ground wires, because it cuts labor and costs. This does not make it a good practice. As mentioned, mechanical connections have the tendency to fail, while soldered connections do not. If a pot works loose, you can have intermittent issues, if you don't ground the pots properly.
    LoveThatBass and tlc1976 like this.
  5. iiipopes

    iiipopes Supporting Member

    May 4, 2009
    The original 1957-style Fender Precision Bass pickguards were anodized aluminum. So the pots weren't grounded with solder. They were bolted to the plate. But...the pickup ground needed to be soldered somewhere, and it was to the back of the casing, and the third lug of the volume control was also soldered to the jack casing and the bridge ground wire likewise.

    So: 1) it is not necessarily a cost decision; and 2) you may still need to solder something to a pot casing, if, like anodized aluminum, the foil or shielding paint is not conducive to being soldered to or screwed down to.
  6. ofajen


    Apr 12, 2007
    92.4W 38.9N
    This. The benefit is that you are taking a "belt and suspenders" approach and making sure you don't have problems crop up later.

  7. Bruce Johnson

    Bruce Johnson Gold Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    Maybe I should clarify: You should always use wire for the ground side of the pickup/control circuit. Wires should be soldered from the ground side of the pickup to the grounded side lugs on the pots, an on to the ground lug on the output jack. That's mandatory for reliable operation. What's not necessary is to solder leads to the cans of the pots, in any installation where the whole cavity is shielded.
  8. megafiddle


    May 25, 2011
    Cavity shielding alone does not eliminate the need for the pot cases to be grounded.

    If a pot with an ungrounded case has a metal knob (and metal shaft), noise will be be conducted into the cavity interior if you are ungrounded yourself and touch it.

    The noise can be quite noticable depending on the circuit, pot value, and pot setting.

    Last edited: Sep 23, 2016
    tlc1976 likes this.
  9. tlc1976


    Aug 2, 2016
    Simple continuity does not necessarily mean a great connection, just an acceptable connection. For example my van quit, and a wire showed continuity. But an actual numbers test showed it was only good enough to pass 9-10 volts, not 12. Ran new wire, problem solved.

    IMO a passive beater for the garage is one thing. But if I had a preamp in my bass, I'd be picky about having the best wiring that is within reason. (Nothing against passive basses, all mine are passive because I chose simplicity.)