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It is useful, especially with single coils and even more so with P90s with their large wide coils.
It won’t stop all hum, but it can cut down the level quite considerably.
But to be effective, the copper needs to be grounded and it needs to enclose as much of the electrics as possible. You can’t do too much about enclosing more of the pickup cavity than just the bottom and sides, but in the control cavity, you can also stick it on the underside of the cavity cover. On guitars/basses with pickguards that mount or surround the pickups, you can cover the underside of the pickguard as well.
The copper will be automatically grounded where it touches the pots, but for separate pickup cavities, you’ll need to run grounding wires out from already grounded shielding areas. I use thin wire and stick the bare ends down with more copper tape. For cavity covers and pickguards, you can ground by copper to copper contact, which means running the foil up and over the cavity recesses, and for pickguards, over the top of the body, but underneath the pickguard so it can’t be seen. Ideally run it on pickguards to screw areas so the copper surfaces are firmly held together at those points. Large areas of plastic tend to warp slightly, so trusting to a couple of small areas in the middle of a pickguard to make contact isn’t the most reliable of methods.
Because the shielding is grounded (it really doesn’t do much for noise if it isn’t) then beware of signal wires touching it with the result of no audio output. A single loose whisker of wire can result in silence. So be careful with your soldering and minimise the amount of exposed signal wire. Use heat shrink where possible. Use insulating tape over the foil where contact may be made once you put a cover or scratchplate on. Especially take care around the jack socket area as the spring contacts will move outwards when a plug is inserted, so it’s often safest to avoid shielding the hole around side-mounted jacks (and definitely use insulating tape in Strat jack socket cavities).
If you need more copper foil, you can find cheap rolls of shielding tape around 25-30mm wide on eBay or Amazon if you can’t find it locally. Also sold as slug-repellant tape (same product, different name). Just make sure it has conductive adhesive. The majority of tapes do now, but don’t assume and avoid anything that doesn’t state it explicitly, otherwise you may need to solder the strips together to get continuity.
And best to use a multimeter to check for continuity between strips. Even with conductive adhesive, I’ve sometimes had to run a crosswise strip of tape over the top of overlapping parallel strips of tape in order to get continuity from one end of a pickguard to the other.
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Regarding copper foil... I ordered a custom pickup one time and for some bizarre reason didn't get them to pot it in wax. (it was an option...) (D'oh!)It was taking off uncontrolably at times with undesirable feedback. I used copper foil to shield the pickup cavity and a portion of the pickguard (strat style). When I play it live it helps with squealing while leaving a reasonably resonant and open sound (jury's out for me on how potting a pickup affects the tone).
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Without doing before and after tests you’ll never know exactly how potting affects the tone of a particular pickup. A lot will depend on just how the pickup was wound as to the degree of microphony a pickup has to begin with. I have seen some videos of before and after tests on humbuckers, and there was a difference, but very slight and far less than the difference you could get by changing the speakers in your amp.
If you assume that the coil wires are held tight at the ends of the coils so they can’t vibrate there, then the maximum single length of wire free to vibrate is in the order of 50mm or so, which will be less in most instances due to wires crossing over each other. So a very short vibration length. The wire tension won’t be anything like as much as a guitar string, but the wire is so very thin its mass per unit length is very low. It all results in a high-pitched resonant frequency. At low volumes/low gain, you’ll have a number of lengths of coil wire resonating in sympathy with higher string harmonics. As this coil wire is moving in a magnetic field, it induces a small current in itself which will add a very small amount of higher frequencies to the basic pickup sound.
Turn up the volume and/or increase the amp gain or add in a drive pedal and you can get enough acoustic feedback going for those loose turns to continue to resonate, even when you stop playing. So you get that uncontrollable high-pitched squeal that’s now unrelated to any note you are playing.
Wax pot the pickup and depending on the degree of wax potting, the coil turns are either completely stopped from moving, or just some of them are. So partial wax potting (it all depends on how long you leave the pickup in the wax bath for the wax to penetrate the windings) can give some extra high-end air to the sound and allow louder volumes and more gain before feedback occurs, whilst full wax potting allows all-out loud metal mayhem.
With a high degree of scatter winding and a high wind tension you can get unpotted pickups that remain squeal-free until you use very high gain levels. The more the wires cross each other, the shorter the lengths of wire left free to vibrate. The higher the tension, the higher the pitch and the smaller the amplitude of the vibration will be. So you can push the resonant frequencies up to areas where the guitar speaker is falling off in frequency output, and the resonant signal is less anyway, and you can generally make them squeal free up to loud volumes and/or gain levels.
The trouble with high-tension winding is that the wire is far more likely to break in the process, so it's not something you'll get that often from large-scale pickup manufacturers. So they will generally pot their pickups just to be sure.
It is fairly easy to wax pot or re-pot your own pickups. You just need a temperature controlled heating bath and a ideally a mixture of 90% paraffin wax and 10% beeswax (or so I've read). There are plenty of cheap body waxing heating baths available on the internet, you just need one big enough to drop a pickup into. Temperature control is important as you just want the max above its melting point. Too much hotter and you can damage winding insulation and the plastic on the bobbins.
You'll need to carefully remove any tape wound wound the coil before dipping, (so you'll need to get some more to replace it afterwards). It's then basically a question of immersing the pickup in the melted wax and waiting until all the small bubbles of displaced air stop appearing. Then pull it out and wipe off the excess wax and let it cool. Always wear protective water/wax-proof gloves as the wax is hot and will burn!
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Just finished and played this guitar, enormously pleased with it. Haven't yet checked intonation but like playing it.
I ain't pretending that its as good as the v expensive American Fender Tele I sold recently because that guitar was a pointless extravagance given my lack of skill, but this simple single P90 guitar does the job well enough for me -- so far.
I'm sufficiently pleased to have just ordered the twin humbucker single cut mahogany body kit. But this time I ordered it with the Grover tuners and bone nut in anticipation that it may be worth upgrading ancilliary parts later. Tru oil brilliant on the mahogany
7 January 2023
Ive now been using this guitar for a few weeks now. Enormously pleased. Intonation v good in spite of my earlier fears
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+1 for Tru-oil looking nice on mahogany.
