Dimples on base plate of P90

[rakers]

Hey guys,
Does anyone know the reason for the two small dimples on the base plate of a Gibson P90?
Thanks,
Richard

[Spence]

They raise the magnets off the baseplate. These are only found on soap bars which had a wooden spacer between the baseplate and the magnets. The dimples also stop the spacers/magnets from sliding around.

[rakers]

Hey Spence, thanks for the reply. However I'm certain I have seen the dimple on older dog ear P90s also. And on P90s without any wooden spacers.

Maybe it's as simple as keeping things from moving around as you mention.

I'm curious if there is any noticable electrical effect on pickup performance with or without this dimple. Perhaps due to the fact that most of these base plates were brass lifting the magnets off the plate has some marginal effect on magnetic field.

thanks again

[Spence]

Actually, mostly they are nickel silver base plates although some of the really early ones from the 40's had brass.

[JGundry]

I have had old Gibson lap steel pickups come in for repair that had Nickel baseplates that looked hand cut and had wooden spacers but do dimples.

I have some Les Paul soap bars from 1952 and 1956 that have brass baseplates with either either a zinc or tin coating that have the dimples but no spacers. So I guess it varies.

[rakers]

Thanks guys for all the input. My hands on experience with P90s has been mainly with early 60s soap bar and dog ear pickups. The ones I have seen have had plated brass base plates with dimples and no wooden spacers. They also had a mustard colored rubber grommet on the wire access hole and a solder lug for ground.

I'm assuming at this point that the dimples have little effect on the electrical properties of the pickup and were inserted for mechanical reasons to raise the coil and magnets. Do you guys agree with this? If they do have an effect I imagine it to be so small as to be beyond audible perception. Be interesting to do some testing to confirm but I just don't have the time to devote to it at the moment. I'd also like to see some scientific testing on the differences in frequency response between using a brass or nickel silver base plate.

Anyway, thanks again for taking the time to respond.

[David Schwab]

I'm assuming at this point that the dimples have little effect on the electrical properties of the pickup and were inserted for mechanical reasons to raise the coil and magnets. Do you guys agree with this?

Well how would a dimple have an electrical property?

I'd also like to see some scientific testing on the differences in frequency response between using a brass or nickel silver base plate.

That's an easy one. Brass is more conductive than nickel silver, so it supports more eddy currents. This was why Seth Lover specified nickel silver for covers on the humbucker, though he really wanted stainless steel.

[rakers]

Here was my thinking concerning the dimples.(probably too much thinking or maybe not enough;-) By lifting the magents off the brass unit base there is a slight change in the distance from the magnet to the base plate and hence a different relationship is set up between the two parts. If base material truly has an audible effect on tone, which I think most agree it does to some extent, then it would stand to reason that by changing the distance between these two parts a different result would occur as the magentic flux density in the plate would be different due to exposure to a weaker magnetic field. The eddy currents in the plate would therefore be different depending on their relationship with the magnetic field.

Do you feel the material used for a cover is more important than a base plate to audilbe changes since it would be closer to the string and changing magnetic field? I would tend to think so. I can hear the difference in cover material. Not sure about base plates.(never tested this)

One more quick question, were older Gibson P90s wound counterclockwise with south polarity towards the screws? And start of coil to hot?

thanks!

[Bill M]

Older p-90's were wound ccw, coil start was hot, but I've seen both north and south polaritys in the 50's

[rakers]

thanks Bill. nothing much ever consistent when it comes to Gibson, huh. :-)