Eddy currents?

amstratnut · March 13th, 2010, 01:27 PM

All my pups work now, but I still have a set of GFS alnico IIs laying around that I'm suspicious of. What are some definitive tests I can do to see if they're good or not? Symptom is occasional loss of volume. I never tracked down the source. I asked a similar question in another thread , but maybe someone could explain if eddy currents can cause that, and tell me definitive tests for pups. For instance what tests with a multimeter and possibly other equipment can I do?

Can there be such a thing as an intermittent short? Whatever I'm totally out of my depth here.

I already emailed GFS and since I've had them a couple years I'm on my own.

Thanks.

sjtalon · March 14th, 2010, 09:22 AM

>intermittent short<

that could happen with temperature change or vibrations, yes............if the coil wire has a bad (shorted) insulation area.

multi meter set on 20K range, test leads on positive and negative wires.

The resistance should be from 5-8 Ohms.

If it' real low, (2-3) then you could have a bad connection where the leads attach to the coil wires, or a bad coil ( insulation short)

Stratplayer · March 14th, 2010, 09:49 AM

amstratnut,

You mentioned that it was a set of pickups that does this. Assuming this is a set of 3 for a strat, I doubt that all 3 of them would have gone bad in a way that would cause them all to have the same problem. I would be looking at your other electronic components and connections--stuff that all 3 pickups actually have in common.

amstratnut · March 14th, 2010, 03:30 PM

Thanks you're probably right about the set thing. I've had my rig cut out twice at a gig when I was using these pups in differnt guitars, so thats why I'm nervous to put em in anything. I'll test em with a mutimeter and maybe put em in a non-gig strat till further notice. At least I'll have an excuse to do a cheap project strat, I mean I already have the pups right? In fact I already have a beat up charvell charvette body routed for a SC and a humbucker. All I gotta do is route a space for the middle pup and viola! Thanks guys I dunno what I'd do with out you.

Donovan · March 14th, 2010, 05:34 PM

Quote:

Originally Posted by sjtalon

>intermittent short<

that could happen with temperature change or vibrations, yes............if the coil wire has a bad (shorted) insulation area.

multi meter set on 20K range, test leads on positive and negative wires.

The resistance should be from 5-8 ohms.

If it' real low, (2-3) then you could have a bad connection where the leads attach to the coil wires, or a bad coil ( insulation short)

kOhms, not Ohms.

sjtalon · March 14th, 2010, 10:34 PM

Aaaaaah, what's a few Ohms here or there among friends !

Good catch Donovan !

gonzo · March 15th, 2010, 02:52 PM

Eddy currents alter sound and output of a pickup and play an important role in pickup design.

Eddy currents are induced in metals in the vicinity of an AC magnetic field, creating a secondary magnetic field which opposes the inducing magnetic field of the coil. The dimensions, conductivity and permeability of the metal, along with the frequency of the current in the coil, determine the magnitude and phase relation of the eddy currents. An internal short in a pickup coil forms a conductive loop which, also, becomes the source for internal eddy current interference.

Bill Lawrence Website

amstratnut · March 15th, 2010, 06:39 PM

Quote:

Originally Posted by gonzo

Eddy currents alter sound and output of a pickup and play an important role in pickup design.

Eddy currents are induced in metals in the vicinity of an AC magnetic field, creating a secondary magnetic field which opposes the inducing magnetic field of the coil. The dimensions, conductivity and permeability of the metal, along with the frequency of the current in the coil, determine the magnitude and phase relation of the eddy currents. An internal short in a pickup coil forms a conductive loop which, also, becomes the source for internal eddy current interference.

Bill Lawrence Website

Thats where my question came from. does he mean that if there's a short there'll be eddy currents? Because, duh, if there's a short its obviously not good. His web site is the only place I've seen any discussion of this. And its vague at best.

gonzo · March 16th, 2010, 09:53 AM

i'll tell you, Bill is a fountain of information.

there's more, on all of these issues there, at that site, but here is something directly connected with your question:

Windings

Scattered vs Layered

"Scatter-wound" is just another sales slogan like "aged magnets". A more accurate term would be "random-wound," and any multi-layer winding of wire, finer than 38 AWG, is more or less random-wound. Electricity doesn't distinguish between layered and random-wound coils -- what matters is the count of turns-per-square and the relation between the length and cross-section of a coil.

Tensioning

Even the most advanced coil winders need modification to wind elongated pickup bobbins. Oblong bobbins pull different lengths of wire during the phases of each rotation. This requires synchronized changes in tension during each rotation to avoid pressure points at the narrow ends of the bobbin. These pressure points can cause shorts in the coil which create eddy currents.

Hand vs Machine

Do you really believe someone hand-guides 8000 turns of copper wire that is as thin as a human hair on a bobbin to complete, maybe, 30 coils per day and can guaranty any consistency? A well-wound coil is a well-wound coil regardless if it's wound with professional equipment, or if somebody's great-grandmother winds it to an old French recipe with Napoleon's modified coffee grinder and chops off the wire after a mile with an antique guillotine!

Bill Lawrence: All About Tone