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Re: Gaussmeter


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2/4/2006 12:06 AM
Dave Stephens Re: Gaussmeter
I think alnico loses its charge maybe faster than you think. Strat pickups from the 70s I've seen 20-30% lower than new charged magnets. But yeah they may not have been charged fully and who knows what kind of alnico formula is really in them. But telling alnico grade via guassmeter is nearly pointless. You can usually spot A5 but the rest you won't be able to tell. I have an A4 and an A2 set of buckers in my guitar and guass readings are nearly identical.  
 
 
 
Are you testing a bunch of vintage guitars? That oughta be some major fun, I hope you have a good tube amp there to play them all through?
 
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2/4/2006 5:29 AM
Spence

We're testing Strats from the earlist we can get hold of to a 1965 transitional. We'll be using a Fender Champ (1950's era ), a Vox AC15 and my Mesa Boogie F50.  
The article's gonna be humour driven as well as pretty in depth with techie stuff.
 
2/4/2006 8:33 AM
Joe Gwinn

On 2/4/2006 7:06 AM, Dave Stephens said:  
quote:
"I think alnico loses its charge maybe faster than you think. Strat pickups from the 70s I've seen 20-30% lower than new charged magnets. But yeah they may not have been charged fully and who knows what kind of alnico formula is really in them."
Many manufacturers claim only a few percent decline, most happening in the first few years, but there are a few caveats. First, not all magnets are of the same quality. Second, partially charged magnets and/or undersized magnets (relative to the air gap the magnet is trying to drive field through) decay faster than the few percent usually quoted in datasheets.  
 
One can tell between material properties and age by measuring the strength of an old magnet, recharging it, then measuring magnet strength again.  
 
quote:
"But telling alnico grade via guassmeter is nearly pointless. You can usually spot A5 but the rest you won't be able to tell. I have an A4 and an A2 set of buckers in my guitar and guass readings are nearly identical."
Basically true, especially if one is measuring magnets as found in a pickup, without recharging them.  
 
I have not tried this, but I bet that putting the magnet inside a tight-fitting single-layer coil and measuring the inductance and AC resistance (compared to the same coil with only air inside) would be able to tell many kinds of magnet alloy apart. One would calibrate the setup my measuring magnets of known heritage. Magnet size and shape will have some effect, and there may need to be a calibration per size/shape. It won't matter if the magnet is charged (magnetized) or not.  
 
The resulting data would be analyzed and presented as follows:  
 
For each magnet tested, a dot will be added to a plot.  
 
One coordinate would be a combination of the material permeability and resistivity, computed as the excess of AC resistance (at 1 KHz) over DC resistance, yielding a value in ohms.  
 
The other coordinate would be the incremental permeability, computed as the inductance with magnet in place divided by the inductance with only air inside, yielding a dimensionless ratio.  
 
With any luck, the dots will naturally fall into neat little neighborhoods, and one can tell what an unknown magnet is simply by noting which neighborhood it falls into.  

 
There are better methods, but they are a lot more work. For example, one can directly measure the entire B-H curve, plus the resistivity, which together will probably always be able to tell magnet types apart in guitar practice, but is a lot of work to collect all the needed data.

 
2/4/2006 11:08 AM
Jimi
Joe, I was thinking about making your gauss meter. The Allegro device seems difficult to get hold of in the UK. I can get one that covers + - 500 gauss (Honeywell) is that enough range? What do guitar pickups measure usually?
 
2/4/2006 8:19 PM
Dave Stephens
Spence are you going to take the pickguards off and switch the selector so you can measure each pickup by itself with the Extech? Where is this going to be published?
 
2/5/2006 2:32 AM
Spence

I'll be taking the pickguards off and depending on the willingness of the owners, taking the pickups out. I may end up just taking measurements from the pickups at the eyelets.  
How many of these guitars have original windings on I'm not sure. I know that I've rewound at least three pickups on these guitars. One pickup on a '59 and two on a late '62. I'm pretty sure that one guitar has a full complement of Kent rewinds and one has a rewound neck pickup by a Welshman called Dai Williams ( a good pal of mine who was the only guy doing rewinds in Wales for years. Oh and you got whatever he had to hand! ).  
The finished article is looking like a six page spread which will be front page stuff. We're doing the shootout very soon and will be published whenever Guitar & Bass fancy. I did an article for them once on pickup winding which took six months to go in. That was a four page spread which they wanted to fit in, in conjunction with other articles about building a telecaster.  
Guitar & Bass goes all over the world I'm told. It's a thinking man's read without a CD on the front. It's published by IPC media.
 
2/5/2006 10:19 AM
Joe Gwinn

On 2/4/2006 6:08 PM, Jimi said:  
quote:
"Joe, I was thinking about making your gauss meter. The Allegro device seems difficult to get hold of in the UK."
Mouser et al will sell to people in the UYK, and these devices are pretty small, so shipping won't be awful. Also, Allegro may have a rep in the UK or at least in Europe. Go to the Allegro website and sniff around.  
 
quote:
"I can get one that covers + - 500 gauss (Honeywell) is that enough range? What do guitar pickups measure usually?"
It sounds a bit small to me. Dave Stephens - what do you think?
 

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