Stainless poles in a PAF

[salvarsan]

LOW field BH plot

With apologies for my ignorance in excel charting, here is a plot of remanent magnetism (B) vs applied field (H) for several soft magnetic materials.

For guitar pickups, we are interested in an alloy's magnetic conductivity at LOW applied fields, so the maximum permeability doesn't matter as much as the initial permeability.

The difficulty is in finding BH data for applied field H below 100 Amp-turns/meter.


Vertical axis is B (Teslas), the remanent field
Hortizonal axis is H (Amps/meter), the applied field

Simply, "H" goes in, "B" comes out.

Down in these low fields, type 400 stainlesses are only slightly worse than 1010 soft steel.

Supermalloy aka HyMu-80 is also interesting since it saturates so quickly, makes you wonder if a hybrid pole piece like M-27+Supermalloy would work with a weak magnet.

It makes me wonder if M-27 transformer steel would be any good. Can you get M27 in slugs or .062" thick blades?

[Mike Sulzer]

With apologies for my ignorance in excel charting, here is a plot of remanent magnetism (B) vs applied field (H) for several soft magnetic materials.

For guitar pickups, we are interested in an alloy's magnetic conductivity at LOW applied fields, so the maximum permeability doesn't matter as much as the initial permeability.

The difficulty is in finding BH data for applied field H below 100 Amp-turns/meter.

[ATTACH=CONFIG]13328[/ATTACH]

Vertical axis is B (Teslas), the remanent field
Hortizonal axis is H (Amps/meter), the applied field

Simply, "H" goes in, "B" comes out.

Down in these low fields, type 400 stainlesses are only slightly worse than 1010 soft steel.

Supermalloy aka HyMu-80 is also interesting since it saturates so quickly, makes you wonder if a hybrid pole piece like M-27+Supermalloy would work with a weak magnet.

It makes me wonder if M-27 transformer steel would be any good. Can you get M27 in slugs or .062" thick blades?

Interesting. There is a lot to explore here. By the way, the materials library in FEMM contains a lot of this data. Is that where you are getting it?

One thing to remember about the effect of permeability when using rod (open) cores: the impact of increasing permeability is large when the permeability is small, but really large permeability has only a small additional effect on inductance.

[salvarsan]

Interesting. There is a lot to explore here. By the way, the materials library in FEMM contains a lot of this data. Is that where you are getting it?

Yup. You can cut and paste from matlib.dat into most spreadsheets.

These data are stated to have come from the 8th Ed. ASM Metals Handbook, Vol 1.
Finite Element Method Magnetics: DC Magnetization Curves of Soft Magnetic Materials.

I can't find raw BH data anywhere else, so we are at the mercy of FEMM's author here.

One thing to remember about the effect of permeability when using rod (open) cores: the impact of increasing permeability is large when the permeability is small, but really large permeability has only a small additional effect on inductance.

Other concerns are the these curves are DC magnetization curves. How valid are they when applied to an AC phenomenon? For a given pickup, where along the curve is the hysteresis loop of a vibrating string? What is the static magnetic bias in a pole when the guitar strings are at rest? ...and so on.

[Mike Sulzer]

Other concerns are the these curves are DC magnetization curves. How valid are they when applied to an AC phenomenon? For a given pickup, where along the curve is the hysteresis loop of a vibrating string? What is the static magnetic bias in a pole when the guitar strings are at rest? ...and so on.

The vibrating string does not move you very far along the curve, and so all you should need is the permeability (slope of the curve) at the static point. This is kind of like operating at low level in the linear portion of a vacuum tube characteristic. (Of course, a tube, even in the linear range, does have a "sound", and so maybe this does, too?)

[salvarsan]

The vibrating string does not move you very far along the curve, and so all you should need is the permeability (slope of the curve) at the static point. This is kind of like operating at low level in the linear portion of a vacuum tube characteristic. (Of course, a tube, even in the linear range, does have a "sound", and so maybe this does, too?)

Hey, it's Rock&Roll -- it's all about exploiting non-linearities in the transduction chain, from beginning to end.

[salvarsan]

Corrections for high field BH chart

The axes are flipped on the early BH curve chart.
The corrected chart follows.

The take-away from this is that in guitar pickups, all these alloys are serviceable.

Their effect on the pickup sound becomes a matter of experimentation, taste, and judgment.

Why would anyone be interested in this?

When Seymour Duncan & Frank Falbo would go so far as to make 416 stainless jacketed nickel core slugs for their flagship PAF-form pickup ...well, it got my attention.

[Joe Gwinn]

One thing that varies widely is the electrical resistivity. The higher the resistivity the lower the eddy current loading. Stainless steels typically have far higher resistivity than mild steels. The resistivities are listed at the bottom of the FEMM wiki on DC magnetization plots.

[salvarsan]

Numeric data from jpegs

Interesting.By the way, the materials library in FEMM contains a lot of this data. Is that where you are getting it?

Mike,

There are BH curve jpegs online but seldom are there numeric data sets.

Integrated Software has an online Magnetization Curve calculator that generates a 20 point BH pair given the initial permeability and Bsaturation.

The FEMM author used WinDig to extract numeric data from JPEG BH plots back when Win98 was current.

It is known to run on WinXP and may even install and run on Win7 if you fiddle with the executable compatibility settings.

I am attempting to install an equivalent Linux application (because it feels good when I stop) called Engauge, so we'll see.

If you search on "plot digitizer" or "graph digitizer", you'll get quite a few hits.

[copperheadroads]

I thought maybe if he had all those figures, He might have the exact wire.
42 PE Black!
43?
Terry

Gotcha ......man