How to grade vintage Alnico humbucker magnets.

[JGundry]

I stumbled upon a way to "accurately" grade vintage Alnico bar magnets to determine whether or not they are A2, A3, A4 or A5. Pehaps some of you have already tried this but here is what to do. Each different grade of bar magnet has it's own specific alloy. These different alloys have their own specific inductance signature in your test pickup at both 120hz and 1000hz. So what you do is record the inductance readings for the known magnets at both 120hz and 1000hz in your test pickup. I can't give you a baseline measurement as the baseline will depend upon the inductance of your test pickup. But once you have the reference magnets measurements done it be comes obvious as to what alnico grade an unknown magnet is from it's reading in your test pickup. The only catch to this test is that both A2 and A4 have very similar readings. They are so close that I can't with confidence yet say that I can tell the two apart with this method. But there is a clear difference between A5 and A2/4 and A3. And interestingly short magnets do not read much differently that long magnets. Some things to pay attention to is to make sure that you only handle the baseplate legs on your test pickup as any change in temperature will effect the inductance reading. Also the test pickup I used had perfectly matched coils that I wound on my computer guided winder. I do think you should be able to make this method work with a pickup with mismatched coils though. Also the numbers you get for each grade will vary a bit but they will fall into a clear range for that specific Alnico grade.

So far I have tested 5 vintage magnets. One vintage long PAF magnet was A5. Two early Patent sticker short magnets were A5 as well. An early T-Top short magnet was also A5. A fourth short magnet I have was A2 or A4 but I can't with confidence say what pickup it came from. I think it is from a T-Top but not sure. My personal favorite sonically is the long A5 magnet. Knowing the grade of Alnico of a vintage magnet is particular useful because when matched with the gauss reading of the vintage magnet you can nail the sound of that magnet by knocking down the gauss of the equivalent grade new magnet.

[kevinT]

So far I have tested 5 vintage magnets. One vintage long PAF magnet was A5. Two early Patent sticker short magnets were A5 as well. An early T-Top short magnet was also A5. A fourth short magnet I have was A2 or A4 but I can't with confidence say what pickup it came from. I think it is from a T-Top but not sure. My personal favorite sonically is the long A5 magnet. Knowing the grade of Alnico of a vintage magnet is particular useful because when matched with the gauss reading of the vintage magnet you can nail the sound of that magnet by knocking down the gauss of the equivalent grade new magnet.

What were the gauss levels of the A5s?

[Joe Gwinn]

I stumbled upon a way to "accurately" grade vintage Alnico bar magnets to determine whether or not they are A2, A3, A4 or A5. Pehaps some of you have already tried this but here is what to do. Each different grade of bar magnet has it's own specific alloy. These different alloys have their own specific inductance signature in your test pickup at both 120hz and 1000hz. So what you do is record the inductance readings for the known magnets at both 120hz and 1000hz in your test pickup. I can't give you a baseline measurement as the baseline will depend upon the inductance of your test pickup. But once you have the reference magnets measurements done it be comes obvious as to what alnico grade an unknown magnet is from it's reading in your test pickup. The only catch to this test is that both A2 and A4 have very similar readings. They are so close that I can't with confidence yet say that I can tell the two apart with this method. But there is a clear difference between A5 and A2/4 and A3. And interestingly short magnets do not read much differently that long magnets. Some things to pay attention to is to make sure that you only handle the baseplate legs on your test pickup as any change in temperature will effect the inductance reading. Also the test pickup I used had perfectly matched coils that I wound on my computer guided winder. I do think you should be able to make this method work with a pickup with mismatched coils though. Also the numbers you get for each grade will vary a bit but they will fall into a clear range for that specific Alnico grade.

I posted a related method a year ago or more, on the old AMPAGE forum. The added trick was to plot 1 KHz inductance on one axis and excess resistance (Rac-Rdc) on the other axis. With any luck, the point clouds for the various magnet materials would be well enough separated to permit unambiguous identification.

What is being measured with 1 KHz inductance and excess resistance is some combination of recoil permeability and material resistivity. Using inductance at two frequencies is measuring some other combination of the same two things. It's a matter of experimentation to see which approach best distinguishes the materials of interest.

[JGundry]

Damn, I knew I wasn't smart enought to be the only one to think of this.

The A5 gauss is between 550 and 600 after I swipe it on the Carlsen Melton meter I have.

[madialex]

Damn, I knew I wasn't smart enought to be the only one to think of this.

The A5 gauss is between 550 and 600 after I swipe it on the Carlsen Melton meter I have.

HA, dont worry, Joe is a real smart cookie, not much gets by him. He is pretty darn smart when it comes to these things, as is a lot of other people here. I have yet to come across so many really smart people in one place anywhere but here. And I might add I do really appreciate their input.

[Possum]

sheesh.....

I posted that exact method here about two years ago and no one paid any attention to it. Its not real accurate, the magnets have to be precisely the exact same dimensions (same mass) and FYI they should be each FULLY charged. If they aren't your results will be skewed. You also have to have the pickup temperature stabilized, humbuckers are so sensitive to breathing on them or handling and the readings change, even putting them under a desk lamp while you're doing the experiment changes it. Add to this that the Extech isn't always perfectly reliable. You need to take multiple measurements of the same magnet and coil, turn the meter on and off a couple times between readings. Yes, you get real blurry results between similar magnet alloys, I forget which but they were A2/A3/A4 if I remember right were kinda close. The most reliable thing you can really use out of it is distinguishing A5 from the others.

[JGundry]

Props to you Possum. I missed your post.

I also tried an almost completely degaussed A2 bar magnet and the the results I was getting were almost identical to a fully charged A2. I also tried it with several other progressively degaussed A2 magnets with no significant difference in the reading. The results were certainly within the range for A2. Also the shorter magnets I have do not seem to fall outside of the range for their specific grade either from my tests. I have done them several times on different days and the results are easily repeatable. The short ones vary a bit but they is still no mistaking the grade of them. A5 is a pretty obvious reading. A3 also seems to have it's own profile. A2 and A4 are very close. I think you could probably come up with some parameters that would distinguish A2 from A4 but it would be a bit hit or miss for those.

[madialex]

Props to you Possum. I missed your post.

I also tried an almost completely degaussed A2 bar magnet and the the results I was getting were almost identical to a fully charged A2. I also tried it with several other progressively degaussed A2 magnets with no significant difference in the reading. The results were certainly within the range for A2. Also the shorter magnets I have do not seem to fall outside of the range for their specific grade either from my tests. I have done them several times on different days and the results are easily repeatable. The short ones vary a bit but they is still no mistaking the grade of them. A5 is a pretty obvious reading. A3 also seems to have it's own profile. A2 and A4 are very close. I think you could probably come up with some parameters that would distinguish A2 from A4 but it would be a bit hit or miss for those.

Props to possum from me as well, I include you in that mix of people Possum.
Possum is the hardest working guy in the biz, he never stops trying new stuff. He is what I call innovative.

[Possum]

psychic test....

I guess the thing to do would be to completely degauss each grade of magnet, make sure they are all the same dimensions (alot of vintage magnets weren't the same, PAF magnets are typically about .133" thick, and the short magnets are typically less thick than 1/8" if I remember right....), write the alnico grade on the bottom then scramble them up somehow so you can't tell which is which, then see if you can idenify them by the method.

I got kinda fried messing around with trying to figure out which was which, the most useful thing I found out was some mid-50s P90 magnets were most likely A5.

I'm not the hardest working guy in the business, Lollar is, his work hours would kill me :-) I am just one curious s.o.b. is all. I do probably have the most unique pickups among most of us, some of them I quit making because they aren't profitable at all despite sounding really good. when it takes 6-7 hours to make one pickup and you get $100 for it , do the math, duh.

[Joe Gwinn]

I posted that exact method here about two years ago and no one paid any attention to it. It's not real accurate, the magnets have to be precisely the exact same dimensions (same mass) and ...

Hmm. The ratio of inductance to excess resistance should be largely independent of mass. On an inductance by excess-resistance plot, the effect of this is that the points for each magnet material will fall on a line that also passes through the origin (zero ohms, zero henrys).

If one also weighs the magnets (or measures their physical size and computes the weight), one will be able to normalize for weight, and the lines will coalesce into a dot for each material type.

FYI they should be each FULLY charged. If they aren't your results will be skewed. You also have to have the pickup temperature stabilized, humbuckers are so sensitive to breathing on them or handling and the readings change, even putting them under a desk lamp while you're doing the experiment changes it.

I wouldn't think that charging would have much effect, but I suppose that the recoil permeability will vary with where in the magnetization curve one is. In any event, one can standardize is it turns out to be important.

As for temperature, it will be the DC resistance of the coil and perhaps the resistivity of the magnet material that vary with temperature. Excess resistance is independent of coil temperature, leaving bulk resistivity. I wouldn't think that the variation in excess resistance would be that strong a function of temperature, but it's something to test. If it is important, then we must use a standard temperature.

Add to this that the Extech isn't always perfectly reliable. You need to take multiple measurements of the same magnet and coil, turn the meter on and off a couple times between readings.

I have not had this problem. How old is the battery? If you rap on the case while it is measuring, does the indicated value jump?

[JGundry]

As a matter of course I always assume that temperature variations may have some effect on any measurements I am taking. So I always make sure that the pickup and parts I am measuring have been in the same environment for some time and I handle them carefully to avoid temperature changes. This is one reason I find testing resistance while winding to be useless. I think the wire heats up while winding and throws off the resistance reading. Counting turns with known wire is the way to go.

While I was taking the magnet readings I experimented with having a gap between the magnet and the slugs or keeper bar and I could not reliably tell if there was a change in inductance with or without a gap. I frankly think that one effect a gap or lack of a gap may have is that it would effect the transmission of sound waves to the slug coil by way of the magnet. If we are talking about an unpotted coil I think that this may have a real effect on the tone of the pickup.

[Possum]

charge...

Andy C. is the one that reported inductance readings change depending on magnet charge, I never checked it out myself, duh. It does make sense in some way, since you're putting an electrical pulse through a coil, the higher the magnet charge would affect the strength of feedback reading which might change things.
I noticed that the Extech tech can sometimes come up with minor differences in its readings , turning it off then back on seems to get more stable repetitive readings. If you're using a humbucker for testing just having the magnet in and it slightly moves a little changes the AC resistance sometimes, or how tight the bobbins are screwed down. I've even seen the readings change while its sitting there. I keep a good battery in it but also have noticed if the battery gets weak it gives inconsistent readings. Anyway, overall its a fun experiment but was only useful one time and never used it again.