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Discussion Starter #1 (Edited)
A while back, I sold my Epi DC LP with Lindy Fralin P90's to a friend.
He's always been a 'bucker player and can't get used to the hum of singles.
Told him to bring the guitar back and I'll install a dummy coil.
Figure it's easy enough, but I want to make sure.
I solder it between the input jack and center lug of the selector switch. Correct?
or, do I split the lead from the dummy to each pup and the other end to ground?
 

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Won't that involve some routing??
 

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I whipped up a dummy coil and installed it in the control cavity of a buddy's Tele 25 years ago. It worked fine. The thing to keep in mind is that in their capacity as antennae tuned to radio-station 60hz ("All hum, all the time; the hum you want to hear!"), the pickups and dummy coil need to sense the exact same amount of hum in order to provide maximum cancellation. In a dual-coil pickup, it is hard to get hum-sensing any more identical than with matched coils, side by side, in the same orientation and same distance from the hum source. A dummy coil in the control cavity may not provide identical sensitivity to the hum source, simply because of how it is positioned. It's not impossible, but not easily achieved. The upshot is that one will get noticeable hum reduction, but not necessarily anything you'd call hum "rejection" or cancelling. That said, dropping audible hum by 6db can make all the difference in the world.

BTW, Fralin makes decent units. As an SC pickup set, are his neck and bridge units reverse polarity to provide hum-cancellation/reduction when both are on? If so, that would imply that use of the dummy coil would only be useful when a single pickup is selected. It would also mean that the polarity of the dummy would need reversing for the neck vs bridge. But that's a BIG maybe.
 
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Discussion Starter #7
Thank you gentlemen.
I think some cavity shielding will also help.
 

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One alternative is to try shielding the coil of the P90. I built a '52 P bass a few years ago and with the single coil pup it had its share of hum. I shielded the cavities and ran all grounds to one point and also ran a wire from the shielding in the pup cavity to the single ground point as well. It helped but not as much as I had hoped. So, I tried this...I carefully wrapped the pickup coil in shielding tape soldered a wire to that shielding (I did the soldering first before wrapping the coil to avoid damaging the coil) and ran that wire back to the single ground point as well and the result was rather incredible. There is now virtually no hum from the pickup at all now. I had never tried this before but it worked very well. It may pay you to try lining the P90 covers in shielding tape and connect a wire to that shielding and run it to the ground point in the control cavity and see what it gets you. Its a lot less invasive and wouldn't take much time to try. And to my ears it hasn't affected the tone of the pickup at all.
 

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From what I've researched, none to very little.
You would notice it more on a guitar with pups straight to jack (after switch); the coil will load them.... but with V/T controls in there, they're loaded already - another 6-12k (+ some reactance) is nearly insignificant if it's already got 250K (or higher) pots in there.

On an actual (typical) humbucker, both coils pick up signal (in addition to loading each other) so the second coil contributes more to the tone (why tapping makes a difference). A dummy coil shouldn't even have pole pieces, or even a magnet; you litterally just want the (reverse wound vs the pickup(s)) coil as an antenna that is out of phase with the actual pickup(s). This last bit is what makes it not so easy to implement on a 2 pup guitar, as @mhammer mentioned, most 2 pup sets are already reverse wound /reverse (magnet) polarity so they buck when used together; adding a dunmmy coil in this situation would buck with one pickup, but actually increase hum with the other one (negligable effect either way with both). In such a case you'd need a different dummy coil for each pup, and it becomes a not worth it situation. Better to get different (noiseless) pups or replace one existing pickup with a similar one that isn't RW/RP to the other.

This is basically how some noisless Fender Strat/Tele pups work; stacked humbucker with the bottom coil being a dummy. Gibson also did that with the original low impedance (aka Recording) line. Alembic used a dummy coil mounted between the 2 pickups.
 
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