Consistent gauss across all rod magnets/slugs

[Gonzo id]

Hello to all,

I've recently decided to make myself a set of single coil kits from StewMac. (A5, 52mm, 42AWG)
After assembly, the pickups were put through a 'sled' that I made from an old vice and two long neodymium bar magnets.
With a gaussmeter I borrowed from a technician friend, I measured across all 6 poles of my single coils.
The longer magnets D & G string show readings around 150mT on this meter, but the shortest magnet, the B string shows about 70-80mT.
The gauss is consistent with the distance from the charging magnets in the sled.
We measured a pickup from a manufacturer and found the gauss to be spot on 99.98mT averaged across all 6 magnets, very consistent.
This leads me to my question (if I could please kindly tap you more knowledgeable fellas on the shoulder here).

How would one achieve a consistent gauss reading across all 6 magnets?

Any input greatly appreciated!

Cheers, Gonz

[jop120]

Yeh, you should be getting fairly close gauss readings from your magnet, magnet length can effect the charge they hold, but with alnico rods, once the length goes past x2 diameter they won't get any stronger.

So if you've passed your mags through your neo's a couple of times, they should be holding a full charge, unless the rod was previously charged with the opposite polarity, you can't change the polarity of an alnico rod and bring up to full charge with Neo's, for that you need a proper magnet charger.

Another possibility is that you've got an A2 rod mixed in with your A5's

As for taking measurements, you'll probably find that you get the strongest reading from the top edge of the magnet, rather than in the centre.

Hope that's of some help.

[Helmholtz]

After assembly, the pickups were put through a 'sled' that I made from an old vice and two long neodymium bar magnets.

I never got consistent results with the neo/vice method. The problem is that moving the PU out of the vice/field distorts the magnetic patterns and often partially demagnetizes the poles.

Here is a simple and effective method:



Each rod consists of 25 neos measuring 8mm diameter x 3mm thickness.
It is essential to approach and withdraw the neo rods exactly on the magnetic axis of the individual pole to be magnetized. Never tilt or move them sideways when within a distance of 1 inch (or even more depending on neo strength) from the poles.

From magnet theory, a longer magnet can hold a somewhat higher charge. I typically find up to 25% difference between the longer and the shortest magnets in a vintage staggering and fully charged magnets.

BTW, the best way to find out if your magnetizing field is strong enough and the magnets are full charged is to reverse the magnetic polarity and see if you get the same Gauss values as before.

[Antigua]

A lot of advice on the internet shows people passing the pickup or AlNiCo in between neodymiums, but as Helmholtz says, the return path is strong enough to demagnetize the AlNiCo, which means two things, this first being that the best way to magnetize with a neodymium is by placing the neo and AlNiCo together, and then pulling them perfectly apart, so that the two magnets remain on a common axis until they are far apart. One way to do it is to hold the magnet with folded cardboard, and then used the cardboard as a handle to pull the AlNiCo and neo apart while maintaining their orientation. Some suggest having neo magnets on both sides of the AlNiCo at the same time, but IME it works well enough to use one neo, charging both sized of the AlNiCo one face at a time. In the case of AlNiCo pole piece pickups, I don't think it's even necessary to charge the bottom size of the pickup, and so a Strat or Tele pickup can be magnetically charged without even having to remove the pickup from the guitar.

The other thing is that the neo has to be as wide, or wider, than the AlNiCo, because if the neo is smaller, some portion of the AlNiCo will be in the return path of the neodymium. When two wide magnets are face to face, it can be nearly impossible to separate them without using some trick, like sliding them apart, and so the cardboard trick is a must, in order to pull them cleanly apart.

[Helmholtz]

once the length goes past x2 diameter they won't get any stronger.

As can be read from the demagnetization curve of A5, the charge continuously increases at least up to a L/D ratio of 4 to 5 (corresponding to a B/H ratio or permeance of around 20). Can be tested by stacking 2 fully charged alnico rods and checking for an increase of pole surface B.

you can't change the polarity of an alnico rod and bring up to full charge with Neo's, for that you need a proper magnet charger.

A5 requires a magnetizing field of up to 3000 Oe (corresponding to 3000 Gauss in air) along its magnetic length for full saturation. The neo method I showed easily allows full charge polarity reversal. I verified with my strong electric charger.

[Antigua]

As can be read from the demagnetization curve of A5, the charge continuously increases at least up to a L/D ratio of 4 to 5. Can be tested by stacking 2 fully charged alnico rods and checking for an increase of pole surface B.

An interesting thing about that is that while pole pieces in Strat and Tele pickups, and PAF style AlNiCo bars have a face width to length ratio of 1/4 to 1/5, there are a number of pickups with AlNiCo magnets with a short length relative to their width, such as Jazzmaster pickup pole pieces, Quarter Pound pole pieces and Filter'tron AlNiCo bars that are not as strong as one would expect, due to their less ideal dimensions. With both Jazzmaster and Quarter Pound pickups, I've measured about 750G at the tops of the AlNiCo 5 poles, where as Strat or Tele pickups usually measure over 1000G.

[Helmholtz]

Using the charts in Moskowitz's "Permanent Magnet and Application Handbook", a PAF style bar magnet has an equivalent length/diameter ratio of only 2.5. This explains why bar magnets charge about 30% less than the longer strat cylinder magnets, measured at the center of the pole faces.
The B values always increase towards the edges of the pole face caused by decreasing self-demagnetization and edge effects.

[okabass]

I never got consistent results with the neo/vice method. The problem is that moving the PU out of the vice/field distorts the magnetic patterns and often partially demagnetizes the poles.

Here is a simple and effective method:

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Each rod consists of 25 neos measuring 8mm diameter x 3mm thickness.
It is essential to approach and withdraw the neo rods exactly on the magnetic axis of the individual pole to be magnetized. Never tilt or move them sideways when within a distance of 1 inch (or even more depending on neo strength) from the poles.

Thank you for the tip. It works.

[Helmholtz]

Thank you for the tip. It works.

Thanks for trying and confirmation.

It's also the only method I know of that allows recharging (and discharging) of individual magnets in a single coil.

[David King]

I also use Helmholtz method to demag individual poles if one string is too hot.

[okabass]

I also use Helmholtz method to demag individual poles if one string is too hot.

Ok. Just wondered if the method works for that. I haven't need to correct the string balance before, but now I have a '68 Tele bass and the A-string is hotter than E-string. How big a difference that method cures?

[Helmholtz]

Ok. Just wondered if the method works for that. I haven't need to correct the string balance before, but now I have a '68 Tele bass and the A-string is hotter than E-string. How big a difference that method cures?

What are the actual Gauss values measured at the centers of the poles?

Controlled demagnetizing is tricky. Too easy to do too much and different alnicos may give different results.
It would be much safer to remagnetize the other magnets - if possible.

If you need to demagnetize, I would do it in steps. Use shims between the neo rod and the pole piece of decreasing thicknesses. And never get close to the other pole pieces with the neos. Remember demagnetizing is an instant process. This means that the result does not depend on exposure time. But repetition may give a stronger effect.

[okabass]

What are the actual Gauss values measured at the centers of the poles?

Controlled demagnetizing is tricky. Too easy to do too much and different alnicos may give different results.
It would be much safer to remagnetize the other magnets - if possible.

If you need to demagnetize, I would do it in steps. Use shims between the neo rod and the pole piece of decreasing thicknesses. And never get close to the other pole pieces with the neos. Remember demagnetizing is an instant process. This means that the result does not depend on exposure time. But repetition may give a stronger effect.

HI
Thank you.
The A magnet is 1400 Gs and the E magnet is 1100 Gs. And the A mag. is longer and ca. 2 mm higher than the E magnet. The D mag is 1100 and the G mag is 900 Gs. The Magnets are south up.
I'm using a cheap Gauss/Teslameter (WT10A), but I think the measurements are close enough and its the relative strength that matters. I have several other pickups to practice.

[Helmholtz]

The A magnet is 1400 Gs and the E magnet is 1100 Gs. And the A mag. is longer and ca. 2 mm higher than the E magnet. D mag is 1100 and G mag is 900 Gs.

Wow, 1400G, that's a strong magnet indeed (what's the actual length of the magnet?) The highest B I measured at A5 rod magnets so far was around 1300G. I always try to center the probe sensor in the middle of the pole face, where B typically has a minimum. This gives me the most consistent and repeatable results for comparisons. Off-center values are often higher, especially towards the edge.

You still may try to strengthen the other magnets.

[okabass]

Wow, 1400G, that's a strong magnet indeed (what's the actual length of the magnet?) The highest B I measured at A5 rod magnets so far was around 1300G. I always try to center the probe sensor in the middle of the pole face, where B typically has a minimum. This gives me the most consistent and repeatable results for comparisons. Off-center values are often higher, especially towards the edge.

You still may try to strengthen the other magnets.

Hi
I was a bit imprecise plus I measured when the PU was on the bass. Now, when the PU taken out I get A 1340 Gs. It's 18 mm long. The E mag is 17 mm. The length measurements are not so precise because the mags are not in level with the Forbon plate.
However, I practiced with another PU and now remagnetize the Tele Bass mags. Now they are E 1350, A 1340 Gs, D 1180 Gs, G 1050 Gs. I try that, it's quite probably better than before.
Thank you for the good tip and your advice.

Ps. I made a ground plate on the bottom of the PU cavity from Mu-metal. It seems to lessen the noise abit. What do you think has it any pros or cons compared to a brass plate (what 7ender uses)?