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Covers over poles of pickups

Discussion in 'Luthier's Corner' started by Means2nEnd, Dec 23, 2014.

  1. Dec 23, 2014 #1
    Means2nEnd

    Means2nEnd Supporting Member

    Jun 27, 2010
    Prana Custom Guitars - Mountville PA
    So I am in the middle of two separate builds and one will have Delano large open pole jazz type single coils. I was over the weekend helping my brother in law make a knife holder for his kitchen with some scraps of wood in my shop. Did a padauk slab with Purpleheart band around it. Anyway I used a forstner bit and recessed the neodymium magnets about 20mm wide into the wood about 2mm below the surface. I then dabbed black epoxy over the magnets.

    They are wicked strong and before I put them in one magnet could hold a pretty large 10” chef knife. We did a random pattern with some grouped together for a butcher knife and heavier stuff. Anyway after the epoxy cured three together would not even come close to holding a medium knife up. I am sure the epoxy was only 2mm max thick. After some head scratching I sanded the whole thing down with a hand belt sander to expose the magnets and they were again insanely strong and could hold up 5 butcher knives. So this leads to my question about wooden pup covers and how much they may or may not impede magnetic fields. You may think oh wood is less dense than epoxy I’m not 100% sure of the when compared to some of the harder exotic woods sometimes used and it was thin. I would venture to say they lost 50-70% of their magnetic pull from that little bit of epoxy. Even if it’s not as much the difference was more than huge. I don’t think I would ever cover them up it would have to do something pretty drastic I would think. What’s the common consensus here on covering up pole pieces on say passive pickups?

    FWIW I used Odie’s Oil because it’s all natural and food safe and the knives wouldn’t get anything funky for food on them. The stuff is insane excellent and used for guitars and fretboards by other builders I know of over on Luthiertalk.com so tried it and man it’s a great product.


    http://www.odiesoil.com/odies-oil-original/
     

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  2. Dec 24, 2014 #2
    JustForSport

    JustForSport

    Nov 17, 2011
    It was all due to the distance from the magnets, regardless of the material used, unless that material is nickel, cobalt, or a ferrous metal which disrupts the magnetic field.
    There's a formula for the distance vs strength for magnetic attraction (which I don't have at hand).
     
  3. Dec 27, 2014 #3
    pilotjones

    pilotjones

    Nov 8, 2001
    US-NY-NYC
    Yes, it's the gap. Whether filled with epoxy, wood, or just air, or even a vacuum, the attractive force falls of rapidly with distance.

    Try sticking a knife on the now-flush magnets. Then place a business card, or a sheet of veneer on the magnets and try again - you'll see a huge difference. Then try two business cards or veneers, and then three - you'll see what's going on. And you will also see the same effect if you place the shims to either side of the magnets, forcing the same gap but with nothing but air between the magnets and the knife.
     
  4. Dec 27, 2014 #4
    Means2nEnd

    Means2nEnd Supporting Member

    Jun 27, 2010
    Prana Custom Guitars - Mountville PA
    Thanks I do get the science behind it I even downloaded the mathematical equation for the gap. My point was merely a small amount of plastic or wood or anything makes a huge difference that to me it would seem anything between the open poles and strings is changing the dynamics and response of the pickup by a good bit. Those beautiful wood covered pickups I see a lot of makes me think it may be pretty but most likely is pretty stupid sort of like my first girlfriend.
     
  5. Dec 28, 2014 #5
    MPU

    MPU

    Sep 21, 2004
    Valkeala Finland
    If the top of the wood cover is the same as in plastic cover there is no difference in sound. To be exact if the distance from the poles to the strings is the same there is no difference if the gap is filled with plastic, air or wood. Playability is of course different if the strings lay on the pickup cover.
     
  6. Dec 28, 2014 #6
    Bruce Johnson

    Bruce Johnson Gold Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    Yes, the main thing is to keep the decorative cover thin. Wood, plastic or glue have no effect on the sound, as long as the same distance to the magnets is maintained. Brass and aluminum may slightly trim the very high end, due to eddy currents. It depends on the size and design of the plate. A steel or iron cover plate will mess with the sound, mostly making it muddy.

    I use thin Macassar ebony caps on the pickups that I build for my basses, matching the look of the fingerboard. My pickups are curved on the top, to match the fingerboard radius. I epoxy on a thicker chunk of the ebony, that's pre-curved on the underside, then belt sand it down to about 0.030" thickness.

    The caps are mainly for decoration, but they have one practical use. I cast my pickups permanently in black epoxy, setting the coil assemblies down into a silicone mold. In the finished casting, the faces of the magnets and bobbins are right up against the top surface, and look uneven. The ebony cap covers them and adds some extra class.