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Thread: Dropping permeability of (1018?) fillister head 'bucker type pole screws

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    Dropping permeability of (1018?) fillister head 'bucker type pole screws

    Folks -

    Anyone have a fun way to make this happen?

    Bob Palmieri

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    The AC permeability of steel drops with increasing DC (permanent) flux.

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    Quote Originally Posted by Helmholtz View Post
    The AC permeability of steel drops with increasing DC (permanent) flux.
    Indeed! I like your take on "fun." I mean... a battery & pot on the axe, cap to block the DC from the signal path, we could have a New Control here!

    To be more specific, I'd like to drop the permeability of some conventional plated 'bucker screws to cool out a customer who thinks a couple of strings have too much output even with the screws recessed as much as possible with this particular design. I suppose I could maybe find some higher carbon screws but they might not look the same...

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    You may cut the screws. A shorter screw that doesn't touch the keeper will be less sensitive.

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    Quote Originally Posted by Helmholtz View Post
    You may cut the screws. A shorter screw that doesn't touch the keeper will be less sensitive.
    Well... in "this particular design" there's no keeper, and in fact the screws don't even contact a magnet. Reduced permeability is actually a good idea in this case.

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    A shorter screw means less effective permeability.

    I would help to know the PU construction.

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    Quote Originally Posted by Helmholtz View Post
    A shorter screw means less effective permeability.

    I would help to know the PU construction.
    Man... I hate being in this position. I did this design for a client, and I have investors overseeing many aspects of my actions these days; can't tell you more about this application. This is especially wrenching for me, since I feel like you're one of the most knowledgeable people on this forum, and I've benefitted greatly from your posts.
    I'll do what I can to resume posting info that I consider valuable, but for the moment... just wanna know how to knock down the permeability of existing common fillister head screws if practical. Otherwise, I think I'll just find a way to make higher carbon screws look similar enough... hope they're already plated...

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    Bob,
    Could you drill out the screw from the bottom on a lathe? (I'd be happy to do it for you). What about raising the output of the surrounding screws somehow?

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    IIRC, Mojotone offers filister screws in 1022, 1018 and 1010 alloys, both long and short.

    Sorry if it's captain obvious talking here...

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    Pepe aka Lt. Kojak
    Milano, Italy

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    just wanna know how to knock down the permeability of existing common fillister head screws if practical.
    I mentioned the (admittedly unpractical) DC bias method for the only reason that I can't think of a way to reduce the permeability by any practical treatment. It is sometimes possible to increase permeability by heat treatment (annealing), meaning removing internal stresses. But AFAIK it doesn't work the other way round.

    Otherwise, I think I'll just find a way to make higher carbon screws look similar enough... hope they're already plated...
    Have you verified that higher carbon screws significantly reduce sensitivity?
    I compared 1008, 1010, 1018 and 1022 screws in the same PU side by side. While there were changes in attack, feel and perceived treble content, the relative loudness did not change noticeably.

    I also measured the inductance values @100Hz of a P-90 design with different sets of screws and found the difference between 1010 and 1022 steel to be around 2%. Of course there is this huge air gap.

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    I know of no practical way to change the permeability of a material in the way you’re asking about other than changing some physical characteristics. One sure way to reduce the effective permeability in to introduce an air gap. You could accoplish this by splitting those particular screws into two parts. Cut each lower section with a slotted screw slot. That way you can keep your original length and aesthetic.

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    Don't forget that PUs are no closed magnetic circuits. They typically have an effective air gap (being the average flux path through air) of several cm. For this reason the influence of the permeability of the polescrews on inductance is relatively low.

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    Folks -

    Thanks for real for this. Dave % SoulFetish - the idea of removing material from the screw sounds very appealing.

    Pepe - thanks for reminding me about the various screws available from Mojotone - this will have much relevance to a project in the near future.

    Helmholtz - that is some very valuable research, and I look forward to experiencing this effect in a test setup after I get an assortment of screws.

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    Quote Originally Posted by Helmholtz View Post
    Don't forget that PUs are no closed magnetic circuits. They typically have an effective air gap (being the average flux path through air) of several cm. For this reason the influence of the permeability of the polescrews on inductance is relatively low.
    Good point! I wonder if it would have any audible affect (effect?...whatever)

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    I wonder if it would have any audible affect (effect?...whatever)
    Depending on PU construction a shorter or "interrupted" polescrew will lose some ability to focus the magnetic flux and the permanent field at the string will be lower. So it might reduce the string signal output.

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    I was playing around with pole pieces recently, originally trying to use High-tensile steel screws. one thing I noticed is that a steel pole piece with lower magnetic permeability will have more of a low cut effect with harsh highs, more than a lower output overall. So if it's the treble strings that you're trying to reduce then that might not work as desired.

    How about trying to de-gauss the magnet a little around the problem area? don't know if it'd work, but could be worth a try.

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    Quote Originally Posted by jop120 View Post
    I was playing around with pole pieces recently, originally trying to use High-tensile steel screws. one thing I noticed is that a steel pole piece with lower magnetic permeability will have more of a low cut effect with harsh highs, more than a lower output overall. So if it's the treble strings that you're trying to reduce then that might not work as desired.

    How about trying to de-gauss the magnet a little around the problem area? don't know if it'd work, but could be worth a try.
    Do you have a keeper bar around the polepieces? If so try cutting the keeper bar shorter off of the 2 polepeieces that are the problem, leaving it around the others. Then fill the gap between the magnet and those poles with a non-ferrous material like plastic or wood.

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    This seems like a strange objective, because reducing the permeability reduces the effectiveness of the screw pole piece in general. Suppose you're doing this to reduce the inductance, you will also decrease the coupling between the coil and the guitar string, which reduces the voltage output, so you could have just as soon put less turns of wire on the coil.

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    I understood that the OP wants to reduce the signal of a couple of individual strings only.

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    I'd make a few more identical coils and see if they all suffer from the same idiosyncrasies first.

    I'd look long and hard at the string choices and see how much flexibility the player has to adapt to some other choices. There are always better strings out there for the choosing but if a player is locked into a brand then perhaps contacting that brand's tech support person might unlock some further refinements. In my experience the larger the brand the more focused they are on maximizing profits but the more resources they have available to fix problems.

    From the description Fieldwrangler has given I'm imagining something akin to an Erno Zwaan Q-tuner pickup.

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    Folks -

    This business of string outputs is indeed hairy. With flatwounds, especially, there seem to be numerous ways to skin numerous cats in terms of core diameters, wrap thicknesses and even double wraps.

    One thing I'm glad about regarding this thread is this timbral difference that Helmholtz has brought up. This is quite interesting, I think, and I look forward to doing some listening around here within the next week or so.

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    You mentioned acquiring higher carbon screws and hoping they look the same, but supposing you can get some identical electrical steel screws to experiment with, you could use a dremel with a grinding stone, and then gouge sections out of the side of the screw shaft. So long as the bobbin (or whatever) these screw into is longer than the gouges, it should still work like a normal screw, with portions of its own threading missing. I think you just have to be careful not to remove so much steel that it causes the screw to easily bend in half.

    Click image for larger version. 

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