Does it matter if the nickel humbucker cover touches the fixed slugs?

[minim]

I never thought about this to be honest. If the nickel cover is pushed on firmly and theoretically contacts the tops of all fixed slugs simultaneously, how does this influence the magnetic field, and therefore the sound? It seems inevitable that something's going to touch somewhere at some stage, whether it's between cover and fixed, or cover and screws. Or all at the same time for that matter.

Is this just a part of the 'covered sound'?

Are there any downsides to leaving an air gap between pickup and cover?

[Helmholtz]

Quality nickel-silver is paramagnetic (almost non-magnetic). Otherwise it would shield the magnetic field from the strings. If the slugs are flush with the bobbin there is little chance that they contact the cover. But even if they do, they won't change the magnetic field. Some PU manufacturers damp mechanical vibrations of the cover by tape or silicone between bobbins and cover to reduce microphony.

[minim]

Okay thanks, that pretty much sums it up. The key word there for me is 'quality'. The covers I have are off my '93 Flying V and are hopefully genuine Gibbo ones. This is the problem. Distinguishing the quality of covers when obtained used. I have a new set of generics from Guitar Center that show copper when sanding the inner surface for soldering. Not sure if that's a good thing or if they are shitty. A quick search suggests that copper is not desirable in terms of tone coloration.

[Helmholtz]

The key word there for me is 'quality'.

You shouldn't be able to lift the cover with a medium sized neo magnet. Otherwise the material isn't well suited for PU covers. (Keep neos away from PUs and Alnico magnets.) This lifting test works for nickel-silver. HB covers of typical size and thickness (mass).

Nickel-silver covers with an intermediate copper layer tend to have increased Eddy current losses and thus somewhat reduced treble response. Just try your PUs with and without covers for comparison.

Some covers are made of brass instead of nickel-silver. As brass is a much better conductor than nickel-silver it produces considerably more Eddy current losses. Brass is even less magnetic than nickel-silver.

[minim]

Hmm interesting. I see Stewmac has some pretty cheap neodymium magnets. I could definitely have some fun with those

Thanks

[David King]

Chrome plated covers are almost always going to be "triple plated" (copper then nickel then chrome over German silver). Each layer will be a mil or two thick.

[Helmholtz]

Chrome plated Gibson T-Top covers had no copper layer and measure exactly like original PAF covers. Seymour Duncan Seth Lover Covers (nickel plated) have an intermediate copper layer and show noticeable Eddy current damping.

[LtKojak]

noticeable Eddy current damping.

Would be so nice to define the parameters or numbers that would be considered as "noticeable"?

Thank you.

[Helmholtz]

Would be so nice to define the parameters or numbers that would be considered as "noticeable"?

Thank you.

Volentieri

Typical eddy current effects are increased losses and reduction of inductance. Both effects increase with frequency.

My notes state that a Seth Lover cover increased the PU's dissipation factor (D = 1/Q) by 14% and decreased inductance by 2% (both measured @1kHz.)
Nickel silver covers without copper layer including a couple of original PAF, SD Antiquity and around a dozen other covers showed no decrease of inductance and the dissipation factor changed by less than 5%. Same with unplated covers.

By direct comparison in a LP, the Seth Lover cover made the PU sound duller.

[David King]

I'm pretty sure any nickel-plated cover would get attracted to a neo magnet. Not sure what that means in this context.

[Helmholtz]

I'm pretty sure any nickel-plated cover would get attracted to a neo magnet. Not sure what that means in this context.

In fact almost all plated (nickel, chrome, gold) covers I tested got slightly attracted to a neo magnet, but typically not enough to lift the cover. Most unplated covers showed essentially no attraction, but I found unplated covers that could actually be lifted. So I assume that the alloy wasn't "right". But there were also nickel plated covers that weren't noticeably attracted, so I am not sure about the influence of nickel plating .
This test certainly is no precise qualification procedure but works well enough for me.

Generally more magnetic covers can be expected to be more prone to feedback/microphony, because any vibration of a magnetic cover will be picked up and amplified.

[David King]

My experience is more with nickel conductive spray paint under the covers and I was surprised at how much the nickel paint dulled the tone, much more than aluminum or copper tape even though the layer of nickel was thinner and presumably had a higher resistance. Very audible effect.

[Helmholtz]

My experience is more with nickel conductive spray paint under the covers

Not sure if I understand. You used conductive paint under (metal?) covers?

[David King]

I used to use it under plastic and wood covers for shielding around single coil pickups.

[Helmholtz]

I used to use it under plastic and wood covers for shielding around single coil pickups.

Are you sure that the sovent of the nickel paint did not melt the wire insulation and caused a partial winding short? This would explain the dulled tone. An L(CR) meter that displays Q or D=1/Q could give information.