Aluminum plate (shielding) weirdness

[bigez]

Hi all, 

I have two strats, and in my quest to kill single-coil noise, I bought two aluminum plates that sit behind the pickguard. See here:

reverb.com/item/4104762-kluson-universal-aluminum-ground-shield-for-most-american-strat-pickguards

Both strats have all single coils, vintage-y output. After installation:

On Strat #1: bridge and neck pups got very quiet, but middle got quite a bit noisier. 
On Strat #2: middle pup got a lot quieter, but bridge and neck got quite a bit noisier.

Can anyone think of a logical reason this would happen? The pre- and post-installation difference is pretty large (like, definitely noticeable). 

Thanks in advance!

[gckelloch]

In that thickness, Aluminum only slightly reduce the upper hum harmonics, but it will reduce buzz caused by poorly shielded rheostats. The eddy currents it creates in the coils also noticeably reduces the hasher upper-mids for a more syrupy tone. I have one in a guitar with a set of Wilde Micro-Coils, and it's fine. It's touching the Al tape I put in the control cavity, and grounded via the volume pot washer. Perhaps you have wired the pickups so the pickup pole ground in the noisy pickups is connected to the plate, and it's not grounded?

[ms]

If you introduce hum differences between the middle on the one hand and the neck and bridge on the other, you should suspect that the pickups are wired with the middle out of phase in order to cancel (magnetic) hum when the middle and one of the others are used at once.

The larger question is why are you doing this?  In most situations, the dominant hum with SC pickups is magnetic, not electric, and shielding can only get rid of electric, while cancellation (two coils) reduces magnetic.

If putting in a shield makes a big difference, I would suspect that maybe something is wrong with how the guitar is wired.  But who knows? It is hard to tell remotely.

[gckelloch]

Ah, yes ms! If the middle pickup is RWRP and the pickup pole/chassis ground wire is connected to the hot lead, the middle pickup poles will amplify EMI. He may have the same issue on the other guitar, but the output leads might be switched at the jack. That would explain why the two outer pickups are the noisy ones, and the middle pickup is quiet because the pole ground wire would then be going to ground. The guitar hardware, pots and the Aluminum shield would then be contributing to EMI noise in the outer pickups because they would all be in the hot signal path. Make sense?

Different shielding materials have varying effect on the upper harmonics of AC hum, but an 0.014” Aluminum pickguard shield has very little effect, and less so when not surrounding enough of the coil to disturb the higher freq EMI waves. Multiple layers of Copper tape in the pickup cavities might make a noticeable difference, and without significantly affecting the pickup response. Check out this pickup cover test:

alexkenis.wordpress.com/2016/04/13/guitar-pickup-theory-7-effect-of-pickup-cover-material-on-tone-and-noise/

[ms]

I think we are primarily concerned with hum (lower harmonics of 60 Hz, and buzz (somewhat higher harmonics).  AK's test show difference mostly in the high frequencies.  At high frequencies, metal shields can affect interference from magnetic sources as well as electric since the law of magnetic induction depends on the rate of change of flux.  But that is not what the shielding needs to stop; rather it is the lower and medium frequencies where a metal shield is only effective against electric fields, what AK calls capacitive.  I think the results of his tests are are not as useful as one would like.  For example, for electric shielding, the material, as long as reasonably conductive, should not make much difference.  Also, if he is measuring the effect of electric fields, there should be a big difference between shield and no shield at the lower frequencies.  There is not.    Therefore, I think he is seeing mostly magnetic interference , and at high frequencies.  This is not so helpful because we know that the magnetic interference that is most a problem is lower frequencies, and that requires cancelation with two coils.

[bigez]

Thanks for the replies, and sorry for this delayed reply (I was on vac.).

I’m fairly certain the pups are wired correctly but I will double check. I added the aluminum shields mainly because I have some dimmer switches the same room where I practice, and I read that this kind of shielding would help with that.

My original question was almost an academic one. Before I added the plates to the guitars, they had classic single coil noise, nothing out of the ordinary and all 3 pups on both guitars had about the same level of noise.

Adding the plates did a weird thing where they made the outer pups quieter on one guitar (and the middle one noisier), and exactly the opposite in the other guitar. This of course also screwed up the noise cancelling in positions 2 and 4, because there is now this imbalance in noise between the outer and middle pickups. (The middle is RWRP on both.) I did check magnetic polarity and I saw that the pickups that got noisier are all N facing, while the ones that got quieter are S. Not sure if this is useful or just a red herring…