Slugs: Which is more important - Diameter or Mass?

[belwar]

Im almost near the end of my humbucker design, and im currently working with the allparts bobbins. The bobbins as many know are NOT made for 3/16" (0.1875") slugs that everyone sells, they are just a HAIR under 5mm (0.195 ish). You put the slugs in there and they fit loose at the top, but snug in with the help of the plastic at the bottom of the bobbin. If you were wax potting it wouldnt be so bad because the void will be filled up with wax. However I am not wax potting, and im not crazy about that void.

So I got to thinking today. Maybe I can alter the shape of the slug, but retain the weight of a proper size slug. I bought myself a 0.01 gram accurate scale and measured several different slugs including PAF slugs, Seth Lover Slugs, Guitar Jones/Mojo Slugs, and Stewmac Slugs. The PAF and Seth lover were within 0.01 grams. However the others were 0.1 grams larger! I decided that I would make slugs that were the right size for the bobbin, but make them to the correct weight by hollowing out the center (from the bottom, and not all the way through.

I decided on using a 5mm slug so I bought a 5mm reamer and some 5mm rod for testing. I loaded the reamer in the drill press and reamed out one of the allparts slug bobbins. Then I lathed the slugs.

The tricky part was figuring out the ammount to remove from the slug. I first figured out the volume in cubic inches of a PAF slug, then did the same for the 5mm slug (at the same length as a PAF slug). I then subtracted the volume of the PAF slug from the 5mm one which gave me the difference. Since I knew the target that I wanted to remove, and the diameter of the hole I was going to make I was able to reverse engineer the depth from the calulation.. (V = Pi*R^2*H with R being the radius of the drill). What really helped is I made an excell spreadsheet where I could just type in different diameter drills untill I found the smallest diameter that I could could use without popping through the top of the slug. I settled on a 0.073 drill. Then I lathed 6 slugs with holes drilled in the bottom and low and behold .. Every one read within 0.01g of each other and they were perfectly in line with PAF weight. The slugs fit snugly in the bobbin, exactly the way it feels to push a slug in my PAF bobbins. I havent had time to wind it or to hear it yet, hopefully tommorow.

My question is this; what theoretical difference is this going to have on tone? There will be a slightly larger surface under the string, but less magnetic field because of the mass of the slug.

Thoughts / comments appriciated.

b.

[Mike Sulzer]

So I got to thinking today. Maybe I can alter the shape of the slug, but retain the weight of a proper size slug.

I think a small change in the diameter would have some subtle effects on the shape of the magnetic field reaching the string. Probably it would take some careful modeling (FEMM, maybe) to see what the effect is, but I bet it would be really small given the small change in diameter.

The mass of material certainly affects the inductance of the coil. So when you keep it the same, you probably have an inductance very similar to the original. This is important for the tone since the inductance is one important element in determining the resonant frequency, and thus the amount of highs.

[Possum]

...

The only way to tell is to listen to it, the fact that you drilled out some may change the magnetic field somewhat, you can't theorize how its going to sound, use your ears. I use the thinner slugs all the time and even if they are real loose they don't cause any problems.....

[David Schwab]

So I got to thinking today. Maybe I can alter the shape of the slug, but retain the weight of a proper size slug. I bought myself a 0.01 gram accurate scale and measured several different slugs including PAF slugs, Seth Lover Slugs, Guitar Jones/Mojo Slugs, and Stewmac Slugs. The PAF and Seth lover were within 0.01 grams. However the others were 0.1 grams larger! I decided that I would make slugs that were the right size for the bobbin, but make them to the correct weight by hollowing out the center (from the bottom, and not all the way through.

That's a neat way to do it!

I examine every pickup I come in contact with (I do guitar repair full time now), and I've been seeing a lot of DiMarzios lately. On several of their PAF looking pickups (i.e., screws & slugs) the slugs are really shaped just like the screws with a thicker cap and a thinner shaft. So they give the appearance of a slug, but are more like non adjustable screws.

I'm assuming they did this to keep the mass of the slugs the same as the screws so that both coils have the same load. The screws are short also, they don't poke put the bottom very far.

I thought it was an interesting approach to keeping a vintage look while being able to use balanced coils.

[Joe Gwinn]

So I got to thinking today. Maybe I can alter the shape of the slug, but retain the weight of a proper size slug. I bought myself a 0.01 gram accurate scale and measured several different slugs including PAF slugs, Seth Lover Slugs, Guitar Jones/Mojo Slugs, and Stewmac Slugs. The PAF and Seth lover were within 0.01 grams. However the others were 0.1 grams larger! I decided that I would make slugs that were the right size for the bobbin, but make them to the correct weight by hollowing out the center (from the bottom, and not all the way through.

I decided on using a 5mm slug so I bought a 5mm reamer and some 5mm rod for testing. I loaded the reamer in the drill press and reamed out one of the allparts slug bobbins. Then I lathed the slugs.

The tricky part was figuring out the amount to remove from the slug. I first figured out the volume in cubic inches of a PAF slug, then did the same for the 5mm slug (at the same length as a PAF slug). I then subtracted the volume of the PAF slug from the 5mm one which gave me the difference. ....

My question is this; what theoretical difference is this going to have on tone? There will be a slightly larger surface under the string, but less magnetic field because of the mass of the slug.

Well, some first impressions:

Eddy currents in a cylindrical slug are proportional to the square of the diameter, but depend very little on the length (so long as it's longer than the coil is high). This is another way of saying that the eddy currents of interest largely flow under metal-air surfaces nearby the coil.

Drilling the center out is a lot of work, probably for minimal effect, although I do recall a patent that sported hollow-tube slugs (the intent being to shape the field). First of all, the square-of-diameter dependence means that the center contributes little to eddy currents. Second, the permeability of iron and steel is something like 1000 times that of air, so the flux pattern and intensity is largely set by the air path, and hollowing the slug out a bit won't remove enough metal to cause much change.

The mass of a permanent magnet is important to determining how much field it can generate, but is not really the issue for iron objects that conduct the field.

[jonson]

hollow pole patent

You Probably rememeber this one Joe.

[belwar]

Well what are the odds! My winder is a modified design from an original Glen McDougal winder from 1985. Glen is a tinkerer. Spends countless hours making things up in his shop..


I wonder if you could get around the double cream trademark using those slugs...

[Joe Gwinn]

You Probably remember this one Joe.

Yes, that's the one.

[Mike Sulzer]

Here are some FEMM modeling results of this question. The model consisted of a .5 inch long slug (1018 steel) either .2 or .1 inches in diameter. On the back was a small alnico magnet, same in both cases. The idea is to determine the field along the path a string would follow over a pickup. (The dimensions used here only approximate reality, but are good enough to give an idea of what the effect of changing the diameter is.

The results are here: http://www.naic.edu/~sulzer/effectOfSlugDia.png
The field is larger over the narrower slug, but it extends over less distance. The integrated effect is 20% larger for the larger diameter.

Another question is how the diameter of the slug affects the voltage picked up by the coil. This is a bit more complicated to model.

Edit: I just discovered that I ran the contour to close to the top of the slug in FEMM. I will have to redo the plot when I get time.

Another edit: This file: http://www.naic.edu/~sulzer/effectOfSlugDiap1.png has the correct distance of the plotted contour above the slug (close to a string height). The peak field values are nearly the same on axis, but the slug with the greater diameter gives a somewhat wider field.

[belwar]

I have to learn the FEMM modeling software that I keep seeing.

Well I tested the pickup today with the slugs. The pickup was 8.03k with slightly offset coils. I put it in the neck position with A5 partially degaussed. I was too impatient to install a cover, so I went topless. WOW. Nice crisp high end and ZERO mud. Once I get a digital recorder I'll post some sound samples.

b.

[Joe Gwinn]

Well I tested the pickup today with the slugs. The pickup was 8.03k with slightly offset coils. I put it in the neck position with A5 partially degaussed. I was too impatient to install a cover, so I went topless. WOW. Nice crisp high end and ZERO mud. Once I get a digital recorder I'll post some sound samples.

Did you do an A-B test of solid versus center-drilled slugs?

[Mike Sulzer]

I added the results for an even smaller diameter to the plot. The field must decrease as the diameter gets really small.

Results here: http://www.naic.edu/~sulzer/effectOfSlugDiap3.png

[Diablo]

Well, some first impressions:

Eddy currents in a cylindrical slug are proportional to the square of the diameter, but depend very little on the length (so long as it's longer than the coil is high). This is another way of saying that the eddy currents of interest largely flow under metal-air surfaces nearby the coil.

Drilling the center out is a lot of work, probably for minimal effect, although I do recall a patent that sported hollow-tube slugs (the intent being to shape the field). First of all, the square-of-diameter dependence means that the center contributes little to eddy currents. Second, the permeability of iron and steel is something like 1000 times that of air, so the flux pattern and intensity is largely set by the air path, and hollowing the slug out a bit won't remove enough metal to cause much change.

The mass of a permanent magnet is important to determining how much field it can generate, but is not really the issue for iron objects that conduct the field.

Are eddy currents a bad thing or a good thing? Correct me if I'm wrong, but aren't the eddy currents generated by the changing field of the vibrating strings? Isn't the magnetic flux of the strings relatively weak? So, isn't the flux density in either the slugs or screw poles relatively weak? And wouldn't the eddy currents also be very weak? If eddy currents are a bad thing, couldn't you reduce them by making laminated poles in the same way they make motors and generators out of lamination stacks?

[bbsailor]

Hollow Core Speculation Questions

Can anyone speculate about using a 0.25" diameter ring magnets with a 0.125" hole rather than hollow core pole pieces?

The main thesis of the above referenced patent is that the hollow core presents less direct downward pull on the strings and thus allows the string to vibrate longer.

Since it is only the up and down motion of a string that contributes to the voltage induced into the pickup coil, it would seem that if seven 0.25" ring magnets were used on a six string guitar, positioned between and closer to the strings and outside the end strings, there would be less direct downward pull. With this magnet arrangement, would there be a possibility to recover some of the horizontal string movement between the magnet pairs into coil voltage?

Joseph Rogowski

[David Schwab]

Since it is only the up and down motion of a string that contributes to the voltage induced into the pickup coil...

That depends on the shape of the field. In the two Bartolini patents, he points out that rod magnets produce a that that picks up the up/down and side-to-side motion about equally and his planar tipped pole pieces were designed to pick up more of the up/down motion. If you have lines of flux perpendicular to the top of the guitar, the string's sideways motion will cut through those lines. (well of course the string moves in a circular pattern)

You can certainly hear the difference between rod shaped poles and blades. The rods have a more "plucky" tone.