# Thread: Self-demagnetizing & Jazzmaster pickups

1. ## Self-demagnetizing & Jazzmaster pickups

Folks -

So, those Alnico rods on those pickups don't appear to follow the usual recommended aspect ratio for this material. Come to think of it, I haven't actually measured any but somehow an image persists in my mind of shorter-than-supposedly-ideal little rods.

If so, do we think there's been a significant drop in gauss level over the decades in some of the older units (or has someone measured such things?)

In any case, I'd like to start getting an idea of how much flux seems "ideal" for given string-to-polepiece distances for given string gauges in this design.

I also have a vague recollection that one of you wiser guys once said that Leo never lost any sleep over stuff like this, which is doubtless true.

Bob Palmieri

2. I'm pretty sure that JM pups use 0.471" * 0.187" A5 rods.

3. as far as typical alnico rods go the shorter the rod the weaker the magnet, once you get sown to 3/16 by 1/4 long you have a magnet that will hardly lift its own weight, dont know if the length makes it more likely to discharge over time being in proximity to like poles

4. Originally Posted by Lollar Jason
as far as typical alnico rods go the shorter the rod the weaker the magnet, once you get sown to 3/16 by 1/4 long you have a magnet that will hardly lift its own weight, dont know if the length makes it more likely to discharge over time being in proximity to like poles
Jason -

Yup, thanks for this. Just poked around to find the very general rule of thumb for Alnico (I recall this ratio being dependant on the grade of Alnico) but here we have it:

Alnico is better at resisting self demagnetisation by being longer in the direction of magnetisation than in the dimensions of the pole face (usually termed Length to Diameter, L/D, ratio). A higher L/D ratio increases the working point on the Intrinsic curve. This is why Alnico magnets are very rarely short in length (they are often seen in bar, rod or horseshoe shapes) and a L/D ration of at least 4:1 is ideally required to get good performance from an Alnico magnet or an Alnico magnetic assembly (having Alnico is a ferromagnetic assembly and magnetising in-situ boosts the L/D ratio of Alnico creating a higher magnetic output). If the design is correct with Alnico, it can outperform many magnets at ambient temperature and all other magnets at high temperatures. To get the most out of an Alnico magnet, the entire magnetic circuit (magnet, ferrous parts nearby, external magnetic fields, etc) needs to be considered.

The last part definitely drives home your point about being close to those other rods.

Bob Palmieri

5. Folks -

I'm thinking that one way to get some traction with this issue would be for those of you who have Jazzmaster pickups lying around to post gauss readings at the mag tops along with year of origin if known.

I'll report back on any data that I gather from other sources.

Bob Palmieri

6. I had a '59 Jazzmaster. The pickups were dead when I got it, but I'm pretty sure the magnets still had a charge. P bass pickups are pretty shallow too.

7. Yes, those P-bass mags are quite shallow, too.
Indeed they are...

8. It depends on what era the pickups were made, the earlier the alnico, the less gauss the stuff will hold. Plus the material wasn't consistent, by today's standards most of the vintage alnico would be considered dumpster material. To get a sense of vintage alnico, with these pickups, go BUY one, there's no other way to get reliable information. Then measure the gauss, then RECHARGE it in an actual magnet charger, take a reading, then let it sit a couple days and read it again. There were also defective magnets that ended up in everything too, I've found a fair amount of these in Gibson's vintage pickups.

9. Originally Posted by Possum
It depends on what era the pickups were made, the earlier the alnico, the less gauss the stuff will hold. Plus the material wasn't consistent, by today's standards most of the vintage alnico would be considered dumpster material. To get a sense of vintage alnico, with these pickups, go BUY one, there's no other way to get reliable information. Then measure the gauss, then RECHARGE it in an actual magnet charger, take a reading, then let it sit a couple days and read it again. There were also defective magnets that ended up in everything too, I've found a fair amount of these in Gibson's vintage pickups.
I'm barking up a similar tree. Basically I'm preparing to measure a number of vintage pickups to determine a range, then I'll get ahold of one that seems within that range to charge up as far as I can get it.

I'm considering doing this with P-bass pickups as well.

10. FWIW, I just tested the pickups in my '66 Jazzmaster - it's a mongrel I assembled in the early '90s, with pickups from '63 and '64. Most of the magnets are in the high 600 - low 700s range, but the magnets under the D and G strings on the neck pickup are down in the 350-400 range.

11. Originally Posted by MWaldorf
FWIW, I just tested the pickups in my '66 Jazzmaster - it's a mongrel I assembled in the early '90s, with pickups from '63 and '64. Most of the magnets are in the high 600 - low 700s range, but the magnets under the D and G strings on the neck pickup are down in the 350-400 range.
Thanks for this! I remember once doing a rewind on a '57 Tele neck pickup; one of the things I did was to move the rods around so that their various strengths made sense for their string positions.

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