Keeperbars - why.

[StarryNight]

I was putting together a p94 the other night when I realized I forgot to insert the keeperbar. Before I started disassembly I took a gauss reading at the screw heads. When I installed the keeperbar and put it all back together (using the same magnets) I took another reading and found it to be exactly the same as without the keeper. So the question that came to me was why have a keeperbar on a p90? On a humbucker it makes sense as to fill the space between slugs and screws, but then I thought why does it have to be on the screw side? Why couldn't the magnet touch the screws and a simple steel spacer inserted between the magnet and the slugs? No holes to drill and easy to make. Anyone have experience with this?

[jack briggs]

Lollar makes his P90s without a keeper. He said he feels that there is more magnetism available at the polepiece tops without it.



cheers,

[the great waldo]

Hi Starrynight

I think maybe the keeper on the screw side of a humbucker is to keep the bar magnet in the middle of the 2 bobbins for better magnetic balance. It might be an idea to try a solid keeper on the other side as suggested to get a sort of asymetrical effect which winders seem to be doing by mismatching coils.

Cheers

Andrew

I was putting together a p94 the other night when I realized I forgot to insert the keeperbar. Before I started disassembly I took a gauss reading at the screw heads. When I installed the keeperbar and put it all back together (using the same magnets) I took another reading and found it to be exactly the same as without the keeper. So the question that came to me was why have a keeperbar on a p90? On a humbucker it makes sense as to fill the space between slugs and screws, but then I thought why does it have to be on the screw side? Why couldn't the magnet touch the screws and a simple steel spacer inserted between the magnet and the slugs? No holes to drill and easy to make. Anyone have experience with this?

[JGundry]

This is why.

A magnet keeper, or armature is a ferromagnetic bar made from soft iron or steel which is placed across the poles of a permanent magnet to help preserve the strength of the magnet by completing the magnetic circuit; it is particularly useful for magnets which have a low magnetic coercivity, such as AlNiCo magnets.

[StarryNight]

I agree that in a humbucker the magnetic circuit must be complete by using a keeper to provide ferromagnetic contact to both screw a slug sides but I am questioning the design choice to have the keeper on the screw side as it would be no different to the gauss strength measured at the screws (based on my one little test), less costly and more time effective to have a steel bar between the magnet and the slugs and have the magnet touch the screws. Or for that matter keep the keeper on the screw side but instead of the inconvenience of drilling 6 precise holes in a 1/8" piece of steel just have a steel shim wide enough to make contact between the magnet and the screws. I can only think that the eddy currents would be altered slightly if at all. The gauss strength certainly doesn't seem to be effected.

[JGundry]

I agree that in a humbucker the magnetic circuit must be complete by using a keeper to provide ferromagnetic contact to both screw a slug sides but I am questioning the design choice to have the keeper on the screw side as it would be no different to the gauss strength measured at the screws (based on my one little test), less costly and more time effective to have a steel bar between the magnet and the slugs and have the magnet touch the screws. Or for that matter keep the keeper on the screw side but instead of the inconvenience of drilling 6 precise holes in a 1/8" piece of steel just have a steel shim wide enough to make contact between the magnet and the screws. I can only think that the eddy currents would be altered slightly if at all. The gauss strength certainly doesn't seem to be effected.

I may be wrong in my thinking with this but with a vintage P-90 you have both south poles of the bar magnet facing each other. In a P-90 assembly I think of the keeper bar as a sort of magnetic sink that would preserve the charge better than no keeper bar at all. But again maybe that is wrong thinking. Personally I prefer the tone of the vintage P-90 design that includes a keeper bar.

[big_teee]

I've made pickups with 2 slug coils, They work great.
Don't see any difference in two slug coils, and one slug, and one screw coil, if the screws fit tight up against the magnet.
I thought the reason for the keeper is just to shim up the difference so a 1/2 inch wide magnet will fit snug, and continue the magnetic field.
T

[JGundry]

You have to have a keeper bar for a humbucker, because that is how Gibson designed them.
I've made pickups with 2 slug coils, They work great.
Don't see any difference in two slug coils, and one slug, and one screw coil, if the screws fit tight up against the magnet.
I thought the reason for the keeper is just to shim up the difference so a 1/2 inch wide magnet will fit snug.
T

The prototype P.A.F. had two slug coils made from trimmed down P-90 bobbin drilled for slugs. Gibson lap steels had a version of a P-90 with steel slugs and no keeper bar. I think the keeper bar was part of the design of an adjustable screw P-90 intentionally. When you consider it would have saved a buck to leave it out I have to believe some though went into including it.

[Mike Sulzer]

This is why.

A magnet keeper, or armature is a ferromagnetic bar made from soft iron or steel which is placed across the poles of a permanent magnet to help preserve the strength of the magnet by completing the magnetic circuit; it is particularly useful for magnets which have a low magnetic coercivity, such as AlNiCo magnets.

So the "keeper bar" under discussion here is misnamed because it does not connect the poles. Guitar pickups are not a closed magnetic circuit; the circuit is dominated by open space, rather than a high permeability material around a complete loop. Does anyone know how this spacer bar was named a "keeper bar"?

[StarryNight]

So the "keeper bar" under discussion here is misnamed because it does not connect the poles. Guitar pickups are not a closed magnetic circuit; the circuit is dominated by open space, rather than a high permeability material around a complete loop. Does anyone know how this spacer bar was named a "keeper bar"?

I don't think that is quite correct. I believe I'm stating the obvious by saying the magnetic circuit needs to be complete (I.e. Connected by ferrous metal) at the bottom of the pickup to transfer maximum magnetic strength to the top, much like a horseshoe shape. The poles need to be physically connected at the bottom. At least that's my understanding.

[StarryNight]

The prototype P.A.F. had two slug coils made from trimmed down P-90 bobbin drilled for slugs. Gibson lap steels had a version of a P-90 with steel slugs and no keeper bar. I think the keeper bar was part of the design of an adjustable screw P-90 intentionally. When you consider it would have saved a buck to leave it out I have to believe some though went into including it.

It would be fascinating to know the history of the keeper. It's probably very true Gibson made design choices based on cost without sacrificing tone, but practical choices are relative learned experiences from the past and Gibson in the 50s didn't have that luxury. I've seen lap steel guitars with crazy steel holding brackets for the pickup that were way overkill. Humbuckers with threaded baseplates for the screws is another example of a design that we would not consider practical today. The keeper obviously kept the screw bobbin at the same height as the magnet while providing physical contact to it. Perhaps mass production of parts as they were doing (or ordering) meant drilling 6 holes in a piece of steel was no big deal. They already had many other steel parts made to order that were formed drilled and tapped. Maybe it was a package deal? I won't deny I love the tone of many humbucker and p90 pickups as they are with keeperbars, but sometime I just like questioning convention, especially when it can save me money and time.

[big_teee]

If it is like John G says, and the original was layed out for 2 bobbins with slugs.
Then when they decided that the pickup had to have adjustable poles on one bobbin?
They Probably had to come up with the spacer to make the screws reach the magnet that was already made to be 1/2 inch wide.
Instead of reinventing the wheel they probably just redrilled one bobbin with screws, and made a steel spacer bar.
That sounds like logic to me.
I see no other reason to need the bar, the magnetism seems just fine with or without the spacer bar.

[LtKojak]

If it is like John G says, and the original was layed out for 2 bobbins with slugs.
Then when they decided that the pickup had to have adjustable poles on one bobbin?
They Probably had to come up with the spacer to make the screws reach the magnet that was already made to be 1/2 inch wide.
Instead of reinventing the wheel they probably just redrilled one bobbin with screws, and made a steel spacer bar.
That sounds like logic to me.
I see no other reason to need the bar, the magnetism seems just fine with or without the spacer bar.

The keeper bar was already invented. It was part of the... P-13, the predecessor of the P-90!

The Japanese were making p'ups in the 80s with a row of threaded slugs, so you don't need a keeper bar anymore, but unfortunately didn't catch on... the kind of thing Seth Lover would've used, if the keeper bar didn't exist.

[David King]

I knew it! The keeper bar came down with the 10 commandments. Question not the wisdom of the ancients or face the wrath of the of the musical illiterati.

[Jim Darr]

I don’t see the need for the keeper bar on a P90 style pickup. It doesn’t serve the same function as a “Magnet Keeper” which is used on both the north and south poles of a magnet since it is only touching the south sides of both magnets.

In a HB it makes sense to eliminate the air gap, but why not just use a wider magnet???? I recall a very old interview with Seth Lover (perhaps a Seymour Duncan conversation transcription from the late 70's) where Seth said they went with adjustable poles on one bobbin due to the demands of the sales staff. The original prototype, that I’ve seen in pictures, show slugs on both coils. If I am not mistaken, the HB patent application drawing showed no keeper bar. Maybe Gibson had a large quantity of magnets of this width on hand, or more likely they standardized on that width in the 50’s. I don’t know and we can only speculate.

Now wait a minute, DiMarzio uses that air gap to their tonal advantage…they even have a patent on it. And a lot of people really like the sound of it. Another data point to consider in tone coloring.

Anyway, I have a Guild style P90 from the 60’s where the coil is encapsulated in epoxy and there was no keeper bar!!! Both magnets just touched the threads of the screws. It sounds really good and has a “raw” vibe to it.

Somebody said earlier in this thread that the Lollar P90 styles don’t use a keeper bar…I’ll have to check the one I have to confirm, as I’ve never had it apart. The Lollar screw head is also a different profile than a standard HB screw, at least on the one that I have. The Lollar P90 sounds very good to my ears.