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Would shielding the inside of a P90 plastic cover help with hum?

Discussion in 'Just Pickups' started by JUSS10, Oct 29, 2020.

  1. JUSS10

    JUSS10 Tele-Meister

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    SO for starters, I know P90s hum, lets get that out of the way. This is more about what can best be done to minimize the hum. Recently built a guitar with a bootstrap overwound pretzel pup in the bridge and their mean 90 in the neck position. I've used their clean 90s in another build and was quite happy and rather surprised how quiet they were. This mean 90 is a bit noisier (its would higher so I suppose thats to be expected). I have the cavity its mounted in shielded but I was curious if I'd line the inside of the plastic cover with foil and fold it over so it makes contact with the cavity shielding if that would maybe help? Anyone tried that?
     
  2. tubelectron

    tubelectron Tele-Afflicted

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    @JUSS10,

    Here's what you can do : not shielding the cover, but "wrapping" the pickup, as below... It's what I did with full success on the Jazzmaster-style pickups of my Harley-Benton JA-60 LH OW. No more adverse noises, hum, buzz and the like... :cool:

    The PUs, as they were stock :

    [​IMG]

    Shielding with adhesive GND'ed aluminium foil, and grounding the magnets :

    [​IMG]

    The adhesive aluminium foil that I used, available in Weldom-like markets...

    [​IMG]

    The P-90 being similar to the JM piclups, it should work successfully too... ;)

    -tbln.
     
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  3. tubegeek

    tubegeek Friend of Leo's

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    I did this wrapping operation with copper foil on a Melody Maker pickup, it was so long ago I can't say whether the result was quieter, but it did not make it very quiet though.
     
  4. JUSS10

    JUSS10 Tele-Meister

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    The wrapping idea is good though I hate to cover a brand new pickup in foil tape. I feel like lining the whole inside of the cover and the bottom of the cavity would yield similar results. I could also add a ground wire across the bottom and see what that does. Thanks for the tips everyone. Its not an unbearable hum, I just take pride in making my single coil guitars as silent as possible.
     
  5. Antigua Tele

    Antigua Tele Friend of Leo's

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    The shielding will probably cause eddy currents and darken the pickup unless you put cuts in the shielding to break up the circular continuity. This page has some examples of how it could be cut https://kenwillmott.com/blog/archives/246 The eddy currents move in circles around the pole pieces and around the perimeter of the whole pickup, so you want to have at least one cut from the center to the outside, as well as cuts leading away from the individual pole pieces.

    IME shielding doesn't usually make much of a difference, as compared with humbucking. I don't think it's worth the hassle to shield anything. A large portion of the noise is magnetic in nature, and is picked up by the pickups through any shielding.
     
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  6. hopdybob

    hopdybob Tele-Afflicted

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    Bill Lawrence would advise you not to use aluminium
    http://billlawrence.com/Pages/All_About_Tone.htm/TeleLovers.htm
    http://billlawrence.com/Pages/Pickupology/External Interference.htm

    some may say his writing is humbug ;-) but i believe him and therefor i would do this with copper tape and shield the control cave with the alu
     
  7. Danjabellza

    Danjabellza Friend of Leo's

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    There are a couple options out there for P90 nickel covers, which you could solder to the baseplate like on a PAF style pickup, that will take care of the grounding part of what makes shielding effective. However, there’s a long held debate about whether or not covers will suck tone.
     
  8. Antigua Tele

    Antigua Tele Friend of Leo's

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    Aluminum and copper both conduct electricity really well, so both will cause eddy currents, if nothing is done to disrupt the electrical path. Copper is more conductive than aluminum, but they attenuate by about the same amount in experiments.

    Most of the eddy current damping in a P-90 is caused by the screws, so no matter what, part of the pickup's sound owes to damping, but the conductive cover makes a P-90 very woofy sounding. I bought an Epi Casino with brass chrome covers, it was a dark, muddy guitar. I replaced them with black plastic covers and the guitar really brightened up. I'm far more concerned that the guitar no longer looks stock than I about the lack of shielding, though.
     
  9. 11 Gauge

    11 Gauge Doctor of Teleocity

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    How hard would it actually be to make convincing looking faux chrome or nickel covers out of plastic? The only ones I've ever seen were the chintzy cheap stuff like you'll find on kids' toys, but I'd like to at least imagine that there would be some 'premium' way to do it.

    I mean, we've got chromed plastic on cars, and some of that looks pretty decent, and laptops seem to have all sorts of variations of metallic finish plastics on them.

    I bring this up because I also am hopelessly obsessed with having my guitars at least look as close to factory-like as possible, especially with the pickups.
     
  10. Zepfan

    Zepfan Doctor of Teleocity Gold Supporter

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    I agree with Antiqua Tele about putting shielding inside the covers and that it would effect the magnetic field and change the sound of the pickup. I would line the cavity in the body with the aluminum shield tape because it can reduce or completely get rid of 60 cycle sum as Bill Lawrence stated. I've done it and it works. The pickups will still sound great without loss of their usual tonal character.
    Wrapping the bottom of the pickup with foam will get rid of feedback squeal and works great on hollow body guitars.
     
  11. Antigua Tele

    Antigua Tele Friend of Leo's

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    They say Bill Lawrence offered something like this, but I never saw it myself. Mojotone has some fake chrome/metal for their noiseless "single" coils, it looks pretty cheesy up close, but fine from a distance.
     
  12. schmee

    schmee Doctor of Teleocity Silver Supporter

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    I think working on the entire shield and grounding system is the way to go. I have P90's without much hum issue and have had in the past. I dont know how your system is set up, but one thing to consider is using a shielded wire from the Jack to the volume pot. Only ground one end of the shield. This Strat was no more hum than the standard SC's and neither way was noisy.
    [​IMG]
     
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  13. jvin248

    jvin248 Poster Extraordinaire

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    .

    I've lined Strat pickup covers before and it works to cut noise. No eddy current darkening problems (eddy currents require material thickness and the foil tape is pretty thin).

    Line the inside of the covers and make a flexible jumper to the shielded pickguard. Verify continuity with an ohm meter.

    If you've seen the tape wrapped around pickups, I've also seen pickups wrapped with cloth tape, copper tape, and then over wrapped with the regular cloth tape. One place was the PRS factory tour and the other was a Squier Strat I owned for a while.

    I'd use aluminum flashing tape (like shown in a previous post, Nashua brand is what I've used). Copper tape will be thicker and more possible to cut you or the bobbin wires. There are research reports for the cell phone and computer industry showing aluminum shielding performs better than copper, if you need validation check google. One roll of that HVAC tape will shield two dozen guitars for $7.

    Before you mash the pickup back in the lined shell make sure no bobbin wires are in danger of being cut or nicked.

    You can also perform a test before you start on the whole endeavor ... make 'a tinfoil hat' that you can put over the pickup cover under the strings and run a ground strip to the bridge or something easy. Does the hat make the guitar more quiet for you? If so then dismantle the guitar and pickup, tape it up, and test it out.

    I had two identical HH guitars, full cavity shielding and wiring to the jack but one without a cover was noisy while the guitar with a cover was quiet when in front of a PC recording. I did the 'hat trick' and verified it was the uncovered pickup causing the problem. With the hat, that guitar was as quiet as the covered pickup guitar ... and no loss of tone.

    .
     
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  14. dogmeat

    dogmeat Friend of Leo's

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    from Bill Lawrence....

    "Leo used a .015 aluminum plate under the pick guard of his 54 Strat to reduce some of the hum and the buzz and take a little bit of the edge from the pickup, resulting in a very musical, sweet tone.

    Aluminum has some strange properties, and it's the only commercially available metal I know of that can eliminate the buzz caused by light dimmers. An inch thick copper or brass shield cannot reduce the buzz caused by light dimmers but .003 thick aluminum foil can! This is known some thirty years and the reason why Belden introduced double shielded cable ( Copper braid plus aluminum foil). There is one problem for guitar cords -- the double shielding makes the cable too stiff . It helps quite a bit when you shield your guitar with copper and aluminum foil."
     
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  15. hopdybob

    hopdybob Tele-Afflicted

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    would nickel silver be an option if someone wanted a shiny cover?
     
  16. TimTam

    TimTam Tele-Holic

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  17. Antigua Tele

    Antigua Tele Friend of Leo's

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    Few people make them. Kinman may or may not. And if they do make them, you have to hope the pole piece spacing fits the P 90 you want to pair it with.
     
  18. JUSS10

    JUSS10 Tele-Meister

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    WOW thanks for all the response!

    So I ended up adding a little foil to the inside of the pickup. that said.... turns out there was a continuity break in the shielding of the cavity the P90 was in. Fixed that and its silent as can be.

    Makes a lot more sense as I've had great luck with the bootstrap P90s in the past and having them very quiet. Seemed odd that this one wasn't. Did I need the shielding under the cover? no clue, but the guitar is quiet and I'm happy with the tone so I'll take it. But man, I have some reading based on some of the articles and links posted above.

    As always, great wealth of knowledge here. Thanks everyone!

    Justin
     
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  19. tubelectron

    tubelectron Tele-Afflicted

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    :lol::lol::lol:

    Bill Lawrence never worked at ALSTOM and SIEMENS (below, "The Cathedral" THT (SHV) lab at Alstom Lyon - France) :

    [​IMG]

    Otherwise he would have known - like me - that aluminium is the best shield against RFI, electrostatic and magnetic induction, after µMetal, conversely to copper, which is much less efficient against magnetic induced hum.

    But I only speak about shielding properties of aluminium, OK ? :D

    -tbln.
     
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  20. Antigua Tele

    Antigua Tele Friend of Leo's

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    I don't know about that, I'd need to see some references. Shielding is ubiquitous with electronic devices, and this isn't an idea I've seen come up.

    In general, in order for a shield to block an electromagnetic wave, the EMI has to be of a high frequency, so that, per Faraday's law, the more rapid the rate of change, the more intense the resistance against that change, otherwise the shielding has to be very thick, in order to compensate for the lack of "rate of change" with a greater penetration depth. Many electronic devices operate in the megahertz and gigahertz range, so that thin shielding will block EMI, but audio is very low frequency, which is why it tends to not make much of a difference in electric guitars, and why humbucking is a much better noise mitigation.

    High frequency EMI could cause noise in the amp, but there's usually a filter cap at the input to prevent that anyhow. I think shielding guitars is mostly a placebo. For example, I can tell if I'm using a humbcuker, but if plug in a Fender, I have no idea if the cavity or pick gaurd is shielded or not, it's not a difference that is self evident. If it has single coils, it will be noisy no matter what.
     
    Last edited: Oct 30, 2020
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