Les Paul P-90 dummy coil wiring

[copperheadroads]

I'm here playing around with adding a dummy coil in my les paul that has P-90's I'm really think it's worth doing as I've tested it with test clips but I cant figure how to use 1 dummy coil for both pickups without a switch like a push pull . my plan was to mount the dummy under the bridge pickup .

[Mark Hammer]

Ideally, optimal hum rejection occurs when the EMI detected by the dummy coil and pickup are equivalent. Being in a location where the two are equally susceptible to EMI helps in achieving that. But AFAIC, the objective doesn't HAVE to be hum elimination. Mere hum reduction that makes a guitar more liveable is also a good thing. At the very least, it means that any noise-control pedals/devices being used can be set in a more generous fashion, with a much lower turn-on threshold.

Many years back, I installed a dummy coil in a buddy's Telecaster in the control cavity. It certainly didn't "kill" the hum, but reduced it enough to be acceptable, without wrecking the inherent tone of the guitar. At least that's my recollection.

So, I wonder if a dummy coil might be more successful in the LP control cavity, rather than sitting under a pickup baseplate.

[John_H]

I'm here playing around with adding a dummy coil in my les paul that has P-90's...

I played around with dummy coils on P-90's when I first started winding. I didn't have a LC meter at the time, but I'm sure it would have been handy. Any reduction in noise is nice. It's what you give up in exchange that isn't. I think you are committed to using two dummy coils because your P-90's are reverse wound. The single dummy coil would be in phase with one pickup, and actually increase the noise. You could fix this with switching, but it would be easier with two dummy coils. The dummy coil doesn't need to be the same shape as the pickup coil, but it needs to be on the same plane. My crude experiments involved taping different coils to the back of the guitar, and hooking them up with test leads through the control cavity of my Junior. Use the push pulls to turn the passive coils on/off. In middle position, or anytime else you could turn them off.

[Helmholtz]

I think you are committed to using two dummy coils because your P-90's are reverse wound.

Are we sure that the OP's P-90s are reverse wound (one being RWRP)?
All the vintage 2xP-90 guitars I have played were not.

Dummy coils are typically wired in series with the PUs. In order to not kill too much highs, their inductance should be much lower than the PU's. On the other hand they ideally should produce the same hum voltage as the PU. This is best accomplished with a very wide loop coil of a only few hundred turns (see Ilitch systems).

[copperheadroads]

I don't really like the tone of rw/rp in P-90's & don't usually make p-90's with rw/rp ..I guess now it's turning into more of Pickup discussion .. I'm wondering about using smaller wire to make the dummy smaller to get it under the neck pickup .

[John_H]

I don't really like the tone of rw/rp in P-90's & don't usually make p-90's with rw/rp ..I guess now it's turning into more of Pickup discussion ..

Oops, sorry. I shouldn't have assumed.

I'm wondering about using smaller wire to make the dummy smaller to get it under the neck pickup .

It wouldn't be hard to test the effect of a smaller coil.

I like Mark's idea for placing it under the control cavity cover.

[olddawg]

I’m very curious if this is effective without too much “tone suck”? I have a Les Paul Jr I built that I love to play but is unusable in half of the venues I play. I can see putting one in the control cavity with a recessed switch in the control cavity cover to switch it in and out. Any specs, sources, or schematics?

[copperheadroads]

I think a may wind a smaller coil with 44 gauge with close to the p-90 number of turns & compare it to the traditional p-90 bobbin I'm using as a dummy .

[anchor]

This is an interesting discussion. I have been looking into pickup wiring options recently and I should get informed here. Tell me if I'm right. The dummy coils aren't connected electrically to the pickups but their proximity couples the EMI/RFI to reduce the noise?

[Mark Hammer]

Not exactly.


Normal humbuckers have two coils that both sense the string perturbations of the magnetic field AND any EMI. But because of the way they are wired and the orientation of the magnetic field, the string vibrations sensed will sum, while the EMI sensed will cancel.

Dummy coils are wired like the second coil of a humbucker, EXCEPT that, lacking polepieces or any magnetic field, they ONLY sense EMI. So there is no summing of string vibrations, but there IS cancellation of hum.

Ideally, whatever hum is picked up by both coils of an HB should be equal in amplitude, in order to achieve complete cancellation. If coil A senses a bit more hum than coil B, you'll get significant hum reduction, but not complete cancellation.

As such, dummy coils should be situated in a location where they pick up abut the same amount of hum/EMI as the regular pickups. The Suhr backplate tries (and pretty much succeeds) in tackling this challenge for the 3-pickup Strat by having the dummy coil run along the perimeter of the vibrato spring cover. This essentially surrounds the 3 pickups, or close to it, such that it doesn't sense more EMI, relative to one pickup than to any other.

Make sense?

[anchor]

I think so. It sounds like the dummy coils are electrically connected but neutered (no poles). If the dummy loads are on a humbucker bobbin, aren't they close enough to be energized by the magnets on the other coil? Is that the other reason why Suhr puts the dummy coil off the bobbin? By the way, does Suhr still sponser Guthrie Govan?

[Mark Hammer]

As long as the dummy is not technically wound around a magnet/polepiece/slug, it will not sense the string. If I'm not mistaken Chip Kinman's stacked humbucker puts the dummy coil below the proper one, but sticks a layer of mu-metal or something between them.

As near as I understand it, Suhr (who produces it but did not actually develop it) puts the coil on the backplate so that it doesn't favor any single pickup more than any other. Think of it this way. Imagine the dummy coil was between the bridge pickup and bridge. It would cancel hum more for the bridge pickup than for the neck, because it would be located in a position where it picked up as much EMI as the bridge pickup.