Brass and eddy currrents

[EFK]

With all the dangers of brass and eddy currents, could there be a problem with making a pickup ring out of brass? I've seen brass pickguards but they're quite thin. A thicker mounting ring if made out of say 1/8" brass - potential problems? Or is it not enclosing enough of the pickup to matter?

[David Schwab]

They've been on the market for years... probably too far from the coil to matter.

There's always some eddy currents in your metal parts... unless you don't use any metal parts.

[Mike Sulzer]

I think that David is right, but there is more to it if the pickup is a humbucker. The ring goes around both coils. But the magnets have opposite polarity. Changing flux from the part of the string over one pole piece makes a positive voltage around the loop. From the other, negative. They tend to cancel, much reducing the eddy current.

Why does a humbucker cover matter? Currents in the top of the cover do not cancel.

[David Schwab]

Thanks Mike... good info there.

[SK66]

Why does a humbucker cover matter? Currents in the top of the cover do not cancel.

Why not? The cover is soldered to the baseplate...the coils have opposite polarity at the top.......
Just doesn't make sense.

[Mike Sulzer]

Why not? The cover is soldered to the baseplate...the coils have opposite polarity at the top.......
Just doesn't make sense.

The voltage induced around a loop depends on the rate of change of the magnetic flux through the area enclosed by the loop. A loop that runs around both coils encloses the changing flux from both coils. This should be zero, at least approximately, since the coils are opposite in magnetic polarity. A loop that runs in the cover top over one coil encloses flux primarily from that coil, and so there is only a very partial cancelation.

"The cover is soldered to the baseplate": The baseplate could change the amount of current that flows.

The voltage induced around a loop depends on the rate of change of the magnetic flux through the area enclosed by the loop. A loop that runs around both coils encloses the changing flux from both coils. This should cancel, at least approximately. A loop that runs around the cover over one coil encloses flux primarily from that coil, and so there is only a very partial cancelation.

[Joe Gwinn]

Why not? The cover is soldered to the baseplate...the coils have opposite polarity at the top.......
Just doesn't make sense.

I would start out by ignoring the pickup coils. Imagine the magnets sitting in the hole in the pickup cover ring. As the strings move, they will cause the magnetic field to vary. Eddy currents flowing around the inner edge of the cover ring will tend to oppose that part of the varying field that would otherwise go through the hole in the ring, thus reducing what reaches the pickup coils. It matters little if the pickup is a singlecoil or a humbucker.

[RedHouse]

At the risk of drawing fire, I'm of the opinion that too much is made out of the whole eddy current thing.

Seems to me a decent cover sounds better than a crappy one, nuff said, IMHO there's no need (for me) to measure just how crappy the crappy one sucks, sorry if this oversimplifies (or annoy's anybody)

[David Schwab]

Eddy currents flowing around the inner edge of the cover ring will tend to oppose that part of the varying field that would otherwise go through the hole in the ring, thus reducing what reaches the pickup coils.

If you broke up the loop, as in cutting the ring in one spot, that should reduce the eddy currents, right?

(then take both open ends and attach them to a transformer... hey! A transensor or alumitone pickup! )

[Mike Sulzer]

I would start out by ignoring the pickup coils. Imagine the magnets sitting in the hole in the pickup cover ring. As the strings move, they will cause the magnetic field to vary. Eddy currents flowing around the inner edge of the cover ring will tend to oppose that part of the varying field that would otherwise go through the hole in the ring, thus reducing what reaches the pickup coils. It matters little if the pickup is a singlecoil or a humbucker.

I agree that ignoring the coils is good. But the magnetic fields from the cores or magnets located in the two coil positions point in opposite directions, producing oppositely pointed flux changes when the string vibrates. A loop corresponding to a pickup ring encloses flux changes of both signs The sum tends to zero. A loop drawn around either the up or down directed magnets encloses mostly flux changes of just one sign so you get a voltage.

[David Schwab]

At the risk of drawing fire, I'm of the opinion that too much is made out of the whole eddy current thing.

Seems to me a decent cover sounds better than a crappy one, nuff said, IMHO there's no need (for me) to measure just how crappy the crappy one sucks, sorry if this oversimplifies (or annoy's anybody)

But that's the reason the crappy cover sounds crappy.

Depending on what you are trying to accomplish, the presence of eddy currents could ruin the tone you are looking for.

That doesn't apply to PAF's, as long as you use the right materials, since how they are made is part of the tone, but it seems to be a common consensus that brass baseplates and covers don't sound good for that tone. And that was part of Seth Lover's consideration when he wanted stainless steel covers and baseplates.

Now the question is does DiMarzio and Carvin use brass on purpose, or they don't hear any difference with their designs.

Then you look at Bill Lawrence pickups, and he doesn't have metal baseplates or covers at all. EMG as well.

[Joe Gwinn]

If you broke up the loop, as in cutting the ring in one spot, that should reduce the eddy currents, right?

Correct, if the cut is radial.

(then take both open ends and attach them to a transformer... hey! A transensor or alumitone pickup! )

The lawyers will be coming for you...

[Joe Gwinn]

I agree that ignoring the coils is good. But the magnetic fields from the cores or magnets located in the two coil positions point in opposite directions, producing oppositely pointed flux changes when the string vibrates. A loop corresponding to a pickup ring encloses flux changes of both signs The sum tends to zero. A loop drawn around either the up or down directed magnets encloses mostly flux changes of just one sign so you get a voltage.

The key idea is that this is a linear system in the full engineering sense of the term. What that means is that you can pull the system apart in any way you like, solve each piece as if it were alone in the universe, and then sum the solutions, and get the correct answer.

Given that how to pull the system apart is arbitrary, one chooses for simplicity of analysis.

One classic way to detect analysis errors is to pull the system apart in two different ways, solve and sum both, and see that the two final solutions are the same. If not, there is at least one error.

[RedHouse]

...But that's the reason the crappy cover sounds crappy...

Yes, I'm purposfully oversimplifying the eddy current thing.

As it is for me, I just put a cover on a known good pickup,
take a listen, if it's crappy it goes into the crappy parts bin
under my bench and I don't buy those ones anymore.

I like to triage things that take up my time, IMHO some things
should just be done, like when testing if one likes strings, just
put 'em on, play 'em, love 'em or leave 'em, but don't worry
about the metalurgy or tensile strength, percentage of nickel
composition yadda yadda etc.

Probably way too simple, minimal thought required, but I like to save
my remaining brain cells for more important things...dang,
where'd those darn wire cutters go...again....

BTW, the crappy parts bin does get used, see when my nephew's
friends come around wanting free'bies and new pickups I tell them
the facts of life (no free lunch, parts, labor etc.) and then offer to
make them something out of the crappy parts bin... if they wash
my car or mow my lawn!. It's a fair deal because they do a crappy
job washing the car and miss lot's of spots mowing the lawn.

[Mike Sulzer]

On the thread about the effect of pickup covers, I have posted a measurement that is relevant to this topic. It shows that when you send ac current through a humbucker, the flux pretty much cancels around both coils because the coils are wound oppositely. This is similar to what happens when a string vibrates over two oppositely directed magnets.