42 vs 43 questions/ winding pattern differences

[David Schwab]

...
To answer your question using the above math, a "flatter" core with the same number of turns should have a greater inductance because l ( the equation's divider ) is smaller, so the resonant peak would be lower and the Q factor higher ( the pickup would be more "focused" on the frequencies close to the resonant peak. Changing from 42 to 43 while keeping the number of turns the same would increase DC resistance and bring down the Q factor some, but it's a first-order effect, while changing the number of turns would have a second-order effect on inductance ( N^2 in the formula ).

Makes sense, and seems to match my observations.

Foolproof equipment is hard to design, because fools are VERY ingenious...

How about:

"A common mistake that people make when trying to make something completely foolproof is to underestimate the ingenuity of complete fools." -Douglas Adams

[David Schwab]

This is the lesson of the question above: if you have individual coils, the leakage from the signal of one string into another coil has the opposite sign. You can measure this; it really does work this way.

That's interesting. I'm working on a multi coil bass pickup, and also a variation with neo rods.

[Mike Sulzer]

To answer your question using the above math, a "flatter" core with the same number of turns should have a greater inductance because l ( the equation's divider ) is smaller, .....

How can you be sure of that with an iron core coil?

[David King]

Another factor to consider with same amount of turns and different wire, the thinner wire will make a smaller coil, that is more directly in the "hot zone" where the flux is the densest, so the whole coil is in that stuff, where as a larger coil of larger wire, the outer winds are out where the flux is weaker. You can't really measure this but is a very active thing that is affecting how the pickup sounds and reacts....

Good point David.

I'd heard from Ralph Novax that flux density is 0 in the middle of a rod magnet so that a Fender style pickup coil should have an hourglass wind profile to put the most turns nearest the greatest flux density. This one should be easy enough to check out.

When I wind I try to start the coil with an hourglass shape and then fill in the middle at the end of the wind. I can't say it makes any difference and I can't see why it would really.

[David Schwab]

I'd heard from Ralph Novax that flux density is 0 in the middle of a rod magnet...

That's why Leo later made the "Magnetic Field Design" pickups. He moved the magnet to the bottom so the coil is not in the null point between poles. (Patent #4220069)

[Mike Sulzer]

Good point David.

I'd heard from Ralph Novax that flux density is 0 in the middle of a rod magnet so that a Fender style pickup coil should have an hourglass wind profile to put the most turns nearest the greatest flux density.

This is wrong on two levels:
1. The flux density is not zero in the middle of a rod magnet.
2. Even it it were, that would not imply that you should wind in an hour glass shape. What matters is the changes in flux density from the vibrating string, and that is something else altogether.

Details:
1. This can be understood by looking at the properties of magnetic field lines and simple geometry. Field lines go from one pole to the other. They are continuous loops, leaving one end of the magnet, circling around and entering the other end. That is fundamental, and it means that the density of field lines (the flux) cannot go to zero in the middle of the magnet.

2. When a string vibrates there is a small component of changing flux that decreases with increasing distance from the string. A high permeability pole piece slows down this decrease. That is, the field lines tend to remain confined to the pole piece so they spread out less. The density of lines cannot go to zero and then increase again further from the string.

[Robert M. Martinelli]

Makes sense, and seems to match my observations.



How about:

"A common mistake that people make when trying to make something completely foolproof is to underestimate the ingenuity of complete fools." -Douglas Adams

Maybe that's the reason why nothing in this world is completely foolproof, or maybe it takes a complete fool to design completely foolproof equipment...

As to Mike's question:
"How can you be sure of that with an iron core coil?"

Mike,
I'm mot sure I have understood your doubts, if so bear with me, ( I'm neither English nor American after all ) but the core's variables in the Inductance formula are represented by the magnetic permeability um and the core lenght l, ( which in our case is the core height ). Being the core's lenght l at the divider, the inductance is directly proportional to the material magnetic permeability and inversely proportional to the core length ( height ). All the other factors being the same, a flatter coil ( thus having a shorter core, making for a smaller l ) should have a greater inductance.

Cheers

Bob

[Mike Sulzer]

:...represented by the magnetic permeability um and the core lenght l, ( which in our case is the core height ). Being the core's lenght l at the divider, the inductance is directly proportional to the material magnetic permeability and inversely proportional to the core length ( height ). All the other factors being the same, a flatter coil ( thus having a shorter core, making for a smaller l ) should have a greater inductance....

But that is for a closed core. There also is a useful equation for a somewhat more general case, for a core with a small air gap. It involves the length of the magnetic circuit in iron, the length in air, the cross section of the iron, and the effective cross section of the air gap. The reason that there is no simple equation for a multi-layer coil with a core partially filled with short steel pole pieces is that it is not simple. And not of much use for anything except guitar pickups.

[Bruce Johnson]

So, from all these comments, I'm hearing that it's the inductance that most affects the tone, assuming that the resistance and magnetic field are held constant. I guess the trick is in figuring out how the inductance is being changed by dimensional changes in the coil. And, how that figures into the shape of the magnetic field.

My specialty is mechanical "percussive" bass pickups, which are a different animal from normal magnetic pickups. In these, I'm using stationary hollow coils, and the magnets move within the coils. That's the reason for my questions about coil geometry. I'm playing around with different arrangements of coils and magnets and trying to understand what to expect.