Adventures in P90's, hum and dummy coils

Amp Mad Scientist said:
It's not the conductivity that blocks interference.
Some materials absorb and dissipate the noise and some reflect the noise.
Some materials don't even need to be grounded to block the magnetic fields.
The noise comes from magnetic fields.
Like the magnetic fields around wiring inside the walls. Or the magnetic field of the amp's power transformer.
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I find this interesting which I'm sure will lead to questions.
Of the materials on the list, the only one that will attract a magnet would be the tin, correct?
As I know the other ones like copper, brass, lead etc. don't
But for now I shall absorb knowledge and beer. :cheers:
 
Hackmaster said:
I find this interesting which I'm sure will lead to questions.
Of the materials on the list, the only one that will attract a magnet would be the tin, correct?
As I know stainless does not attract a magnet.
But for now I shall absorb knowledge and beer. :cheers:
Click to expand...

Pure stainless is non-magnetic, but most stainless is not 100% pure and is magnetic / does rust.
Tin is paramagnetic—it is very weakly attracted to a magnet. Diamagnetic materials are weakly repelled. Ferromagnetic materials are what most people think of as “magnetic”—only iron, cobalt, nickel, their alloys (such as the many kinds of steel) probably a few others are ferromagnetic.

You would notice that the best pickup covers are nickel, which blocks external noises.
Gibson put metal cans inside the Les Paul / SG on some guitars, as shielding. But that metal is basically Mu Metal or some close variation of Mu Metal.

But the reason you don't see much Mu metal used as shielding - is that it's really expensive. Nickel costs a lot.
But you will see it used on high end equipment when the price is high.

Like the Switchcraft shielded 1/4" jack. The shielding is Mu Metal.
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Hackmaster said:
I've been tinkering again with a bag of P90's and an old guitar.
Some of you have seen my double cut triple pickup perversion posted elsewhere on this fine forum.
Well now I'll address an issue that I've been dealing with, hum.

The pickups that are in the guitar were stock Gibson P90s.
I found another P90 that when added in with the stock pickups, squelched most of the hum.
I added that one to the middle position, and wired it up with the neck & bridge pickups going thru the 3 way switch to one volume & tone pot, and the middle pickup to the other volume & tone pot.

Now with all three pickups on, everything was pretty quiet. The problem surfaced when I would turn down the middle or the neck & bridge set, the hum would increase as the pickup was turned down.
This is fine if you want all three pickups on all the way all the time, or combinations of neck & middle or bridge & middle on all the time.
If you turned both volumes down the same amount, the hum would also be squelched.
Gettin' complicated now.

This would not do...

So I did some research and armed my self with just enough information to make me dangerous and reckless.
The article that I referred to...
Dummy Pickup Coils for Single Coil Powered Guitars

So utilizing one of the pickups of a Precision Bass set,
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I removed the plastic cover, ripped off the magnet and checked to make sure that the
wind direction was correct to achieve the hum canceling. Bingo! Quiet as a church mouse.
So I took this hapless little pickup and ground the square edges off one end and covered both sides with epoxy.
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Then put a shrink wrap sleeve over the whole thing.
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I hooked it up to engage with a push pull pot so that I would reap the most benefit from the addition.
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It adds the coil between the output going to the jack and ground.
Everything fits nicely in the corner of the control cavity.
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Now your probably thinking how is that idiot going to keep that coil from flopping around in that control
cavity? The cavity was getting pretty crowded now, I thought about a piece of stiff foam, but that would have
to be cut to the proper shape & thickness to work well.
So I searched for my answer from above... No I'm not getting religious on everyone.
Remember the plastic cover originally on the donor pickup?
It makes a good source of stock to make a nifty little angle bracket out of
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that got epoxied to the underside of the cavity cover.
DSC06479_zpsvrblukto.jpg

The cover fits on with no interference and no one sees anything from the outside of the guitar.
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Now for a proper daylight picture of the conversion...
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I just may add it direct so that the dummy coil is on all the time, but I thought the push pull would give me
a chance to compare the tone with and without the addition of the dummy coil.

If any of our more knowledgeable wiring and tone gurus would like to comment pro or con on this little
addition, I'd love to know your thoughts. Feel free to let me have it with both barrels.:D
Click to expand...
You didn't shield the insides no wonder it hums....
I would recommend shielding paint and copper foil, then use shielded wire inside the cavities...

The dummy coil needs to be a match to the pickup coils. The dummy coil is in series with the single coil.
If the dummy coil is hooked up backwards it will hum.
The dummy needs to be out of phase to the pickup, for it to work right.
 
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