Cryogenics

[Spence]

I grew up in Northern Alberta, and I remember many a winter going to school in -55 Celsius. Frostbite in 15-20 seconds...

I was born in Edmonton. Very cold indeed. I love the place and my wife wants to go back there but she hasn't done a winter there.

On a side note, I once walked into a blast freezer ( I was trying to give up smoking and was chewing a nicotine gum ) and keeled over in the chill. I came round stuck to the floor. ( Not quite as bad as freezing your lips to the window in Canada ).
Anyway, that episode of cryo didn't make me any better!
Frankly, I just don't believe the whole cryogenic piffle.
Perhaps someone with more physics and metallurgy than me can explain how this makes anything better.

[Rick Turner]

Do you believe in heat treating and annealing metals? Why is it so hard to believe that there might be effects from metals cooling as well as heating?

The effects of deep (and slow) cryogenics are pretty well documented for ferrous metals, mainly steel. For non-ferrous metals it's a bit more difficult to explain other than it seems to close up surface porosity thereby at least making the metals less subject to corrosion as there is literally less surface area.

With steel, competition target shooters, motorcycle racers, and machine tool operators all take advantage of the increased toughness and stability of cryo treated parts.

[Spence]

Well I know from first hand experience what annealing does. So I can see exactly what cryogenics would do to a metal providing the the metal is treated from a temperature where the crystalline growth can be effected. But hardness of steel isn't something that works well in pickups. So, why would anyone do that?

[Rick Turner]

Read up on it all. It's more than the hardness issue. Cryo treating steel, for instance, can finish the phase change from austenite to martensite, thereby changing the magnetic properties as the carbon atoms are more perfectly distributed into the crystal matrix with iron and other added alloying substances. Thereby the grain structure of the metal is changed. It's pretty interesting stuff if you read deeply enough and keep your mind open. I'm not saying whether I know what the effects would be on pickups...or not...but if we don't investigate, we'll certainly never know.

I could certainly see cryogenics having an effect on Alnico. Not sure about ceramic magnets. But certainly on steel slug and screw polepieces...

[Joe Gwinn]

For testing DCR, I think you might be better off simply letting a batch of coils sit overnight and then measure them. I do this with my humbucker coils so I can match them to within + or - 2% DCR with an exact match on turns, and when I can I get them to within 1% + -. This yields very, very good hum canceling performance. Generally the coils are all within about + or - 4% DCR, and so it's easy to match up coils in batches of 10 per polarity.

If the wire is very uniform and the winding process always causes the same degree of stretch, DC resistance measurements will approximate turns count pretty well, but counting turns is generally simpler and more accurate to match coils for hum cancellation.

[Rick Turner]

Joe, I've got a nice old Coweco "Laboratory" automatic coil winder, so I don't even think about turns count... When the brake slams on, it always overshoots by two turns, and when that's in over 8,000 turns, it's just not an issue. So I can then get to the next level and work to get DCR really, really close. Really, it's not any harder to get them close than not, given that I usually am making at least ten pickups at a time. Tension from one batch to the next may vary a little, so I try not to mix and match much from one day's winds to another, and I always wind in sequence...north up coil, south up coil, north up coil, south up...so if there is any change in the tensioning, I'll have coils of each polarity up on either side of the change. I am amazed at how consistent magnet wire is. Can you believe that they can draw that stuff mile after mile through dies without breaking it? It's really amazing. Now that is some kind of process control...

[Mike Sulzer]

For testing DCR, I think you might be better off simply letting a batch of coils sit overnight and then measure them. I do this with my humbucker coils so I can match them to within + or - 2% DCR with an exact match on turns, and when I can I get them to within 1% + -. This yields very, very good hum canceling performance. Generally the coils are all within about + or - 4% DCR, and so it's easy to match up coils in batches of 10 per polarity.

The only effect I can think of that chilling might do is to make the coils contract...shrink a bit, and then expand back. Not sure what the coefficient of thermal expansion is for copper, but it must be moving a bit.

I know a lot of those here do not think that good hum canceling is particularly important, and wind the coils with different numbers of turns in an attempt to get a unique sound, but I like to get magnetic sensitivity of the two coils as close as possible. So it is important to get the number of turns the same. But getting the DCR the same is not so important. The magnetic sensitivities of the two coils are only very weakly dependent on the differences between the resistances.

The influence of the cores on the sensitivity is more important. If you use the normal screws/slugs set up, then the screws and slugs must be chosen to increase the sensitivity the same amount if really close matching is important to you. I prefer to use the same kind of core in all coils so that this is not an issue.

[Mike Sulzer]

Here is an interesting article on cryo treatment of steel:
:: Cryogenic Treatment History and Overview ::.

It says among other things that you have to do a good heat treatment first. Not sure how any of this applies to pickups!

[salvarsan]

Not sure how any of this applies to pickups!

Copper conductivity changes slightly for the better, but probably not significantly after cryogenic treatment.

The amorphous Alnico alloys (2,3,4) are partly bicrystalline where a higher-remanence crystal couples to a high-coercivity one (see "alnico domain-pinning". The cryogenic treatment will change the crystal sizes, so the magnetic properties will change with it, but I am unable to say in what way.

-drh

[Joe Gwinn]

Joe, I've got a nice old Coweco "Laboratory" automatic coil winder, so I don't even think about turns count... When the brake slams on, it always overshoots by two turns, and when that's in over 8,000 turns, it's just not an issue. So I can then get to the next level and work to get DCR really, really close. Really, it's not any harder to get them close than not, given that I usually am making at least ten pickups at a time. Tension from one batch to the next may vary a little, so I try not to mix and match much from one day's winds to another, and I always wind in sequence...north up coil, south up coil, north up coil, south up...so if there is any change in the tensioning, I'll have coils of each polarity up on either side of the change.

If you are really getting to within two turns in 8,000 turns, you probably don't need to bother with the DCR matching.

In a uniform field, voltage generated is proportional to turns; resistance does not matter. There are some long forum threads on this, but to summarize, cancellation is proportional to (N1-N2)/(N1+N2), where N1 and N2 are the turns counts of the two coils of the humbucker.

So, if you have two coils each being 8,000 turns with a maximum error of two turns for each coil, the worst case would be (4)/(16000)= 0.0003, which is 20*Log10[0.0003]= 72 dB of cancellation, which far exceeds need.

Nor will 72 dB be achieved in practice, because practical hum fields are not sufficiently uniform. And, I'm ignoring the effects of nearby magnetic materials in the pickup itself.

In practice, 20 dB of cancellation is generally sufficient. How much turns error does this allow? About 10% per coil: (10+10)/(100+100)= 0.1 = 20*Log10[0.1]= 20 dB.

And for 30 dB of cancellation, the allowed error is about 3%.

This error tolerance is why one can mismatch coils (usually in search of better tone) and yet achieve adequate humbucking.

[Mike Sulzer]

Nor will 72 dB be achieved in practice, because practical hum fields are not sufficiently uniform.

Exactly. This is why if you are really interested in good hum rejection, multiple coil pickups can be useful. Field gradient effects are most important in the long dimension of the pickup. So if you keep one dimension small by making single coil size pickups and then use multiple coils in the other dimension, you can do better. Six coil pickups with alternating sensitivity can be good, as well pickups using six pairs of coils. Coils can be connected together, effectively making a gradient canceling pickup, or they can be used to amplify each string individually.

[David Schwab]

This error tolerance is why one can mismatch coils (usually in search of better tone) and yet achieve adequate humbucking.

I have an old Hi-A 2B "Mini Beastie" pickup that reads 4.62K and 3.4K. The 4.62 coil is closer to the bridge. It's a very quiet pickup.

[Mike Sulzer]

I have an old Hi-A 2B "Mini Beastie" pickup that reads 4.62K and 3.4K. The 4.62 coil is closer to the bridge. It's a very quiet pickup.

Maybe there are other compensating factors. Are the pole pieces or core of the two coils very different?

[David Schwab]

Maybe there are other compensating factors. Are the pole pieces or core of the two coils very different?

Well I'd assume it's based on the patent at the time, so the cores and poles are the same.

The web site lists the standard humbuckers as:

1C our first humbucker ~1975 - same wire, unequal windings for a clear, clean tone that tends toward single coil - less warmth in the mids than equal winding designs

1CTA very near the 1C tone (part way to single coil tone) but equal windings for best noise rejection - at Tuck Andress's request ~ 1980 - used only in neck position

Bartolini Guitar Pickups

I've offset humbuckers by 1,000 turns, and they do remain quiet.

[Spence]

Read up on it all. It's more than the hardness issue. Cryo treating steel, for instance, can finish the phase change from austenite to martensite, thereby changing the magnetic properties as the carbon atoms are more perfectly distributed into the crystal matrix with iron and other added alloying substances. Thereby the grain structure of the metal is changed. It's pretty interesting stuff if you read deeply enough and keep your mind open. I'm not saying whether I know what the effects would be on pickups...or not...but if we don't investigate, we'll certainly never know.

I could certainly see cryogenics having an effect on Alnico. Not sure about ceramic magnets. But certainly on steel slug and screw polepieces...

Well, I'm not a complete moron Mr Turner. I know that just taking and treating it cyogenically and then returning it to ambient will have absolutely no effect other than to bugger the pickup. Do you seriously think no one's tried this?
Marketing bullshit is all it is.