E-Piano's Pickups Demagnetization Thoughts

[sarakisof]

Should i worry about stacking my Hohner Pianet M on top of a Philicorda organ, in terms of pickups demagnetization?

Philicorda has a large power transformer (as you can see in the second pic - green rectangle) inside without a metal screening can housing and as we know pickups could be affected by magnetic interference a big iron like this could cause.

On the other hand Pianet M itself has its own power transformer and speakers inside in a distance closer than Philicorda's trafo.

Am i overthinking it?

The topic could go for any e-piano using small electromagnetic pickups or for any guitar/bass but the last should have been covered many times in the past i guess.

[Rob Strand]

It would take some effort to put numbers down for it but I doubt you should be concerned.
- The field coming off transformers is pretty low.
- The field off most things drops off very quickly with distance.
- Magnets don't demagnetize that easy anyway, especially from low level fields.

[sarakisof]

As i expected. Reading nowadays all these scary things about (guitar) pickups demagnetization and the likes i wonder what the guys in all those old studios and labs were doing for this in the past. Back in the days there were irons everywhere. In every old studio I've been or seen from pics I've met stacked gear and hard cases all over the place, speakers, cabinets laying around, guitars sitting in front of them and so on. You know, many years means gear overload means lack of space and dust everywhere.
I guess those old guys knew well that in such conditions pickups could get weakened a bit and let well happen for good reason, my granny used to call it "mojo". 

[anotherjim]

Rickenbacker used a magnetised horseshoe pickup on the basses through to the 1960s until stocks ran out. These did demagnetise and by the 80s they would be pretty much dead. They are identified by the air gap in the middle of what looks like a handrest over the strings which is really the sides of the horseshoe *. The modern "vintage" style they make now is faked with the air gap for show and a conventional bar magnet under the coil. Most of them that you will see have an ungapped chromed plastic handrest cover to mimic the style of the horseshoe and a conventional pickup below.
I don't think the Pianet/Clavinet designs are old enough to have stooped to magnetised iron which may only be found now in toy/education horseshoe magnets.
* I do wonder if when not being played, a "keeper" should have been placed across the airgap of those pickups. I also wonder why when pickups became too weak, people didn't just stick a permanent magnet on them somewhere.

BTW, nice insides of the Philicorda. I wouldn't want to tune it.

[Mark Hammer]

I never fully understood how horseshoe magnets were polarized;that is, what the magnetic orientation was.  And, given that a horseshoe pickup had two horseshoe magnets, were they each polarized the same, or in some complementary fashion?

[anotherjim]

The original patent by Beauchamp is out there somewhere. It's ancient now. 1930s or so.
Ah, the facing ends of the horseshoe's top (and bottom) side are the same polarity so a keeper won't work.
Magnetizing is probably by methods 2 or 3 in this...
https://studyelectrical.com/2019/12/methods-of-magnetization-steel-bar.html

[sarakisof]

BTW, nice insides of the Philicorda. I wouldn't want to tune it.

More insides. Full of ZA1001 Philips neon gas mini bulbs for sawtooth oscillations.











On topic, i now read there's another field i should probably care about. It has to do with reeds the Pianet uses. Those seems to be prone to demagnetization and it's a common to re-magnetise them when you have a dead note. So apart from pups, should i worry about them mostly?

[anotherjim]

So it does depend on magnetised steel! How do they fix them? Stroking with a permanent magnet will probably do it.

[sarakisof]

So it does depend on magnetised steel! How do they fix them? Stroking with a permanent magnet will probably do it.

Yeap pretty much.
You can find many info if you search for "Hohner Pianet reeds magnetization" or similar online.
https://www.tapatalk.com/groups/clavinetdotcommessageboard39332/viewtopic.php?p=3813#p3813

So, i should worry about stacking it on top of Philicorda, close to amp cabinets and the likes?

[Rob Strand]

I never fully understood how horseshoe magnets were polarized;that is, what the magnetic orientation was.  And, given that a horseshoe pickup had two horseshoe magnets, were they each polarized the same, or in some complementary fashion?

Think of a horseshoe magnet as a bar magnet which has been bent into shape and the poles are at the open ends, like the bar magnet.

With a bar magnet the field must go from one end, through a long path through the air, back to the other end.   Whereas with a horseshoe magnet the field is concentrated at the gap between the ends.

A bar magnet is very poor at magnetizing stuff because you only get the local field at the end of magnet.  When you place a magnetic object between the claws of a horseshoe magnet the air gap effectively reduces and this actually increases the field.   Horseshoe itself "conducts" the magnetic field around the loop and concentrates it at the small air gap.  That makes it very effective for magnetizing things.   If you can imagine the claws are designed match the size of the object you place in there a the object will be subject to a high field.

You might notice how horseshoe magnets are much stronger at lifting things than a bar magnet.  The smaller air gap and closed magnetic path are the key.  Magnetic cranes work on the same idea.   You have to be careful about apples to apples comparisons when using magnets.   The comparisons only work like the basic theory for alnico magnets because the magnetic material itself has a high permeabilty ("iron like") and conducts the magnetic field through the magnet which forms a loop in the horseshoe case.

When you have ceramic and rare-earth magnets the magnet itself looks like air so you don't really get the closed loop.   When ceramic magnets are used in speakers iron is used to conduct the magnetic field from the magnet and form the loop.   Even though the physical airgap is small the magnet itself looks like an air-gap and sets a limit on the strength of the field.   That's also why ceramic magnets in speakers tend to be flat.  (For a speaker the desired field is across the gap, so the field passes across the conductors.)