PAF pole screws - length and material

[larsenpeople]

Hello,
This is my first post. I'm a 39yo hobbyist that have been fascinated by pickups since I started playing in the early 90's. I registered here after reading various Antigua's posts and analysis here and there. My knowledge in electronics from college is very limited. I have experimented a little with pickups, so I have a few tone references.

Here is my first question to the community :

What would be the effect or shortening the pole screws of a Humbucker ? (This is for replacing a Dimarzio with short screws in a shallow cavity). I would assume that it would lower inductance and brighten it. But then I stumbled on this drawing that confused me, unless it's meant to compare form factors for the same volume of core.


I have removed screws entirely in a set of PAF before, leaving the slugs alone. I noticed a slight drop in output, and mostly a lot of highs coming back (A great tone actually). I can't say what is the main factor at play there. Could be the lower global inductance. Could also be the narrower shape of the magnetic field.
In this case the steel screws were replaced with plastic screws to plug the holes, most likely a permeability of 1. It made me wonder, what material are humbucker screws really made of, and would it be interesting to play with different screws to obtain different tones. I noticed permeability values vary greatly between different materials.

So this is my second question : Has anyone experimented with different pole screw material ?

Good day to you all

[ms]

Pickup pole pieces are short and part of an open magnetic circuit. That is, the magnetic circuit is mostly air gap.  This means that changing the permeability of the pole pieces only has a small effect when the permeability is large.  So changing from a carbon steel with a permeability of 100 to one with 200 would not make a lot of difference.  In both cases, the coil inductance would be about three times that with air pole pieces.  So when you took the pole pieces out of one of the coils of a humbucker, you lowered the inductance a noticeable amount, but also ruined the hum cancelation.  (If you are in a "hummy" environment, you can hear it increase as you remove each pole piece.)

Edit:  That figure is BS for large mu (even though it is from a "reputable" site).  The equation would be almost correct for large mu if the core is closed, such as a toroid or the more usual closed transformer core type.  With the core shown, it would be OK for mu = 2 or 3, maybe.

[larsenpeople]

Thanks for your answer.

I don’t understand the hum cancellation part of you answer. The two coils are still connected in series, why would the cancellation stop ? I don’t remember hearing any hum since I removed them.

[ms]

Sept 23, 2023 17:49:52 GMT -5 larsenpeople said:

Thanks for your answer.

I don’t understand the hum cancellation part of you answer. The two coils are still connected in series, why would the cancellation stop ? I don’t remember hearing any hum since I removed them.

Because the cores increase the sensitivity to magnetic fields.  If you remove them from one coil, then you have made one coil less sensitive to hum than the other.

[larsenpeople]

I didn’t know that. I believed the magnet could be anywhere as long as the string is sufficiently magnetized near the coil.

[gckelloch]

Sept 24, 2023 0:33:50 GMT -5 larsenpeople said:

I didn’t know that. I believed the magnet could be anywhere as long as the string is sufficiently magnetized near the coil.

That's true, but the type of alloy in the core of the coil affects the inductance.

[ms]

Sept 24, 2023 0:47:57 GMT -5 gckelloch said:

Sept 24, 2023 0:33:50 GMT -5 larsenpeople said:

I didn’t know that. I believed the magnet could be anywhere as long as the string is sufficiently magnetized near the coil.

That's true, but the type of alloy in the core of the coil affects the inductance.

An ac magnetic field generated by current in one loop of the coil generates a voltage around other loops of the coil.  A permeable pole piece increases this voltage, and so increases the inductance.  Also, it increases the voltage generated by a hum magnetic field.