Using Femm To Model Pickups

[Mike Sulzer]

FEMM is a program intended to model magnetic systems. It is actually not powerful enough to model exactly pickups and strings because it can only do two dimensional systems or three dimensional systems that have symmetry like a cylinder (used in the plot below). A string over a cylindrical pole piece does not meet this condition. However, FEMM can model a small disk magnet over a pole piece, and this is good enough to get an idea of the effect of the magnetized portion of the string. Remember, the permanent field from the pole piece magnetizes the string (only significantly when the pickup is present) over the pole piece. Various FEMM plots have shown that significant magnetization occurs only near the pole peice. We want to look at the pattern of this field to learn how the pickup works. Actually we need the fluctuation of this field as the string moves, but this fluctuation has a spatial variation similar to the field itself, that is strong where the field is strong, and weak where it is weak.

The plot shows these two features that were discussed earlier:

1. The pole piece stops the field from falling off as fast as it does without the pole piece. This increases the strength of the field through the coil.

2. The field falls off over the length of the pole piece, so windings near the top contribute more than those below.

3. Only the field through the pole piece contributes significantly. The field outside the core, but inside a winding, contributes very little because it is weak and almost horizontal.

Remember this about the plot:

1. The field strength is indicated two ways: contours and color.

2. The closer the contours, the stronger the field.

3. The color scale saturates at the top and the bottom; so it only represents the field strength inside the pole piece.

4. The plot shows only the right half of the system. To get the 3D plot, think of rotating the plot about its left (vertical) side.

5. The magnet is a small neo.

6. The pole piece is 1018 soft steel.

http://www.naic.edu/~sulzer/neoOverSteel.png

[JoeAArthur]

However, FEMM can model a small disk magnet over a pole piece, and this is good enough to get an idea of the effect of the magnetized portion of the string. Remember, the permanent field from the pole piece magnetizes the string (only significantly when the pickup is present) over the pole piece. Various FEMM plots have shown that significant magnetization occurs only near the pole peice. We want to look at the pattern of this field to learn how the pickup works. Actually we need the fluctuation of this field as the string moves, but this fluctuation has a spatial variation similar to the field itself, that is strong where the field is strong, and weak where it is weak.

Yeah, yeah, yeah - a computer model that ignores the magnetism of the pole piece pretending that it magnetizes the string and a "magnetized string" takes over from there.

Hey, how about the strings and the coil forming a capacitor? Bet I could model that. Doesn't mean it has any basis in reality - like this model.

Make the pole piece a magnet like it's meant to be... and then see what happens. You might surprise even yourself.

[Mike Sulzer]

Yeah, yeah, yeah - a computer model that ignores the magnetism of the pole piece pretending that it magnetizes the string and a "magnetized string" takes over from there.

Hey, how about the strings and the coil forming a capacitor? Bet I could model that. Doesn't mean it has any basis in reality - like this model.

Make the pole piece a magnet like it's meant to be... and then see what happens. You might surprise even yourself.

Then you agree that the results presented in the first post in this thread are correct if I can convince you that the string is magnetized. I am not sure that I can do this using FEMM, but I will try. In the meantime, you should review how I showed this before. If you have any intellectual honesty at all, you should be trying to show why the string is not magnetized when it is placed in a magnetic field. Magnetizaton is what the physics predicts. If you think otherwise, support your beliefs.

[Mike Sulzer]

One more thing---

"Make the pole piece a magnet like it's meant to be... "

There are two ways to do this. First, make it a permanent magnet. Second, make it out of steel. Then the permanent magnet below magnetizes the steel pole piece which magnetizes the string. Or are you also claiming that the permanent magnet does not magnetize the pole piece? But it does, as you can easily show by putting a small screw driver tip near a pickup pole piece. If the permanent magnet magnetizes the pole piece, why would the pole piece not magnetize the string?

[SwedishLuthier]

Let try to keep this discussion on a nice and easy going level, will we? We might learn something from it.

I find a few things that need to be further considered to this analysis

You have used a disk shaped neodymium mag with the poles going in the up-down direction. Is this really a good substitution for a string? A neo mag is among the most powerful magnetic material there is. Stronger than the (in pickup building) ordinary used Alnico or Ceramic magnets. Will the string magnetized by the pickup really be that strong? You also need to insert a magnet instead of the steel rod to better see what’s happening.

What’s the ratio between the field strength of “string” and the magnet in your plot (if an A5 rod were to be inserted) compared to the real life ratio between the string and the A5 rod? I’m guessing that in you plot the neo mag will be significantly stronger than the magnet. I’m also guessing that that will not be the result if real life material could be used in the plot.

Another thing: Will the magnetized sting have the same direction as the disk in the plot? Remember that the string vibrates up and down, sideways and in a circular motion. Do we actually know that the string doesn’t rotate? If I remember correctly that is what the string really does.

I think that you need to simulate a *weak* magnet over a strong A5 rod to get a relevant picture.

You will also have to simulate different shaped magnet field (from the pickup that is) to better understand what’s happening in the coil. The Fender way of winding a pickup is not the only way…

1. The pole piece stops the field from falling off as fast as it does without the pole piece. This increases the strength of the field through the coil.

You cannot say that for sure before you have actually used a real magnet in the plot. I suggest this: Insert a magnet instead of the steel. You might also want to have a look at the plot if you make the poles so that the disk-magnet-substitution-for-a-string and the magnetic rod have opposite polarities facing each other. That plot would be interesting to see. How will that magnetic field look?

2. The field falls off over the length of the pole piece, so windings near the top contribute more than those below.

See above. #2 is only valid for if we *know* that the string gets magnetized in the way you have simulated it, and doesn’t spin, twist of whatever.

3. Only the field through the pole piece contributes significantly. The field outside the core, but inside a winding, contributes very little because it is weak and almost horizontal.

Please expand a bit on this part. I’m really curious what you mean (and my English might not be good enough to get what you mean). Do you mean that the field that is actually usable to generate current in a coil is only a tiny bit of the field (energy) available? Following that statement we should wind the pickup with the wire inside the magnetic pole. Of cause un-doable, but very interesting.

I think that you need to expand things a bit to understand what’s going on in a system, not isolate one single factor.

[erikbojerik]

EDIT: Misconceptions removed.

Edited October 9, 2007 by erikbojerik

[SwedishLuthier]

You could test this easily by winding a pickup with stacked coils on a single bobbin (same number of turns), and measure the outputs on the top coil and the bottom coil and compare. The coil nearest the strings will have more output (higher current).

Wouldn't that mean that a stacked single coil sized humbucker wouldn’t be humbuckin…

For a pickup to be hum cancelling you need the two signals from the coils to be as close to each other as possible regarding the output. The idea behind a humbucker is to induce the same amount of hum in the both coils, but out of phase with each other. If one coil had a higher output than the other the hum would not be cancelled.

All of the stacked Hummers I have dissected have had equally sized upper/lower coils

Edited October 4, 2007 by SwedishLuthier

[Mike Sulzer]

Both Peter and Erik raised are concerned about the strength of the neo magnet is used to represent the magnetized string. It is the direction and relative strength of the magnetic field at different spatial locations that is being modeled. You can use any strength magnet to show this as long as it does not saturate the steel, and a small neo magnet does not, in my experience. (The test involves bringing a magnet near a coil with such a steel core and measuring the change in inductance, if any. There is no change.) I can easily modify the field strength in the FEMM model and see if this changes the pattern of the field.

"If you want to model a string over a pole piece, make your neo disc unmagnetized steel, leave the pole piece as it is, and add a disc with a B field of ~1000 G touching the bottom of the pole piece."

The problem with that is that it is harder to see the effect of the "string" because the field induced in the string is weak and that in the pole piece is strong. FEMM is not set up to look at small differences easily, but it might be possible to finagle the scales on the plot enough to show the effect in this way. Otherwise, I can probably find a way to write the fields to a file, first without the "string", then with it, and subtract the two in another program. This would then show the field due to the magnetization induced in the "string". But this will take some time. In this simple model, I used linearity, because it is the easiest way. There should be no problem with it, either.

"Conclusion #2 is the opposite,..."

No, you are looking at my model upside down. My conclusion is your conclusion, and both are right.

"You could test this easily by winding a pickup with stacked coils on a single bobbin (same number of turns), and measure the outputs on the top coil and the bottom coil and compare. The coil nearest the strings will have more output (higher current)."

Yes, this is exactly the point I made in the earlier discussion on this topic. The stacked humbucker shows that the induced field is weaker further from the string. However, that conclusion did not satisfy everyone, so I have done the modeling to show it.

"Conclusion #3...I dunno...I don't know what "the core" refers to."

By the core I menan the pole piece. Sorry about the confusing terminology. Do you see why it is only the field in the pole piece that really matters, not throughout the coil?

Spatial scale: the neo is .1 inches from the pole piece. But it is relative dimensions that matter here, and the conclusions are not really very sensitive to them either.

Peter said "Another thing: Will the magnetized sting have the same direction as the disk in the plot? Remember that the string vibrates up and down, sideways and in a circular motion. Do we actually know that the string doesn’t rotate? If I remember correctly that is what the string really does."

The pole piece magnetizes the string in the vertical direction over the pole piece. I have chosen that direction for the neo magnet. The string moves in a complicated way. I chose to discuss vertical motion because that makes the largest fluctuations. Horizontal motion changes the field through the core less. But in any case, the magnetization is almost vertical no matter how the string moves.

" Do you mean that the field that is actually usable to generate current in a coil is only a tiny bit of the field (energy) available?"

No, I mean pretty much the opposite. The field that generates the voltage around a particular winding is potentially the field anywhere within that winding. But only the field in the pole piece has significant strength and the right direction.

[Mike Sulzer]

You could test this easily by winding a pickup with stacked coils on a single bobbin (same number of turns), and measure the outputs on the top coil and the bottom coil and compare. The coil nearest the strings will have more output (higher current).

Wouldn't that mean that a stacked single coil sized humbucker wouldn’t be humbuckin…

For a pickup to be hum cancelling you need the two signals from the coils to be as close to each other as possible regarding the output. The idea behind a humbucker is to induce the same amount of hum in the both coils, but out of phase with each other. If one coil had a higher output than the other the hum would not be cancelled.

All of the stacked Hummers I have dissected have had equally sized upper/lower coils

The stacked humbucker rejects hum because the magnetic field causing the hum is far away, and so its relative change over the distance of the coil separation is very small. Thus the two coils pick up the same signal and the out of phase connection cancels them. The signal made by the magnetized string is significantly smaller in the lower coil as the modeling here shows, and as one would expect because the difference in the distances to the two coils is relatively large. Thus the out of phase connection cancels only part of the signal.

[Mike Sulzer]

On the issue of the neo field strength and linearity: I ran the model again after lowering the coercivity of the small magnet by a factor of 1000. Note from the new plot that the resulting field strengths are lowered by a factor of 1000, but that the spatial pattern is essentially identical. Although I would be happy to discuss this further, it is clear that the assumption of linearity is good, and it is a valid way to analyze the pickup problem.

http://www.naic.edu/~sulzer/lowField.png

[erikbojerik]

This example is set up fine to show that a piece of steel near a permanent magnet will affect the surrounding field (i.e. strings will have a small induced field associated with the nearby pole...as well baseplates, mounting screws, etc all at some level). It is also sufficient to show how horizontal the field is between the ends of a single pole piece near a single disc magnet.

EDIT: Misconceptions removed.

I've never used FEMM, but couldn't you (for example) have the program draw contours only within a selected field range? Or display the field on a log-scale?

Edited October 9, 2007 by erikbojerik

[Mike Sulzer]

The problem I have with the way this example is set up is in its adequacy for modelling a pickup polepiece. The output of a pickup is dependent on the perturbation to its magnetic field by the string; the stronger the magnetic potential gradients around the string (i.e. the closer the contour lines), the greater the output (and induced fields). My problem is that here, the "string" is represented by a permanent magnet, and so by definition the gradients are stronger around the "string" than at the end of the pole piece, when in reality, the opposite is true (the gradients are stronger at the end of the pole piece than at the string).

I've never used FEMM, but couldn't you (for example) have the program draw contours only within a selected field range? Or display the field on a log-scale?

I think I have not been very clear, so here is another attempt. Please be patient, this is not so simple! First, the pole piece magnetizes the string. Now we look at the effect of this magnetization on the pole piece. To do this, we assume linearity. This just means that we can subtract off the part due to the permanent magnet leaving us with the part from the string. We want to do this to make the problem simpler; it is OK because we know that only the string part fluctuates when the string vibrates. So we assume some level of magnetization in the string; it does not matter exactly what the level is because we just want to look at how this field varies in space. So we represent this string magnetization by a small magnet. Why not? The magnetization of the string does not change much when the string vibrates, so a good model is a permanent magnet. The field due to this magnet is stronger at the string than at the pole piece, since the string is its source. So the reality is that the model shows exactly what it should: the field from the string magnet gets weaker with increasing distance, and we see that we lose somewhat more than a factor of 10 from one end to the other of the pole piece.

FEMM does indeed allow you to draw contours within certain field bounds. I have set these pretty wide here so that one can see the field strength in the pole piece and the space around it. It is hard to see the field strength near the magnet on this plot because the lines get too close. You can print out sample points, and these show that the field strength increases about a factor of ten from the top of the pole to the string magnet. There might be a log option on either the contours or the color but I have not found it yet. I will check again later after work.

Mike

[SwedishLuthier]

The pole piece magnetizes the string in the vertical direction over the pole piece. I have chosen that direction for the neo magnet. The string moves in a complicated way. I chose to discuss vertical motion because that makes the largest fluctuations. Horizontal motion changes the field through the core less. But in any case, the magnetization is almost vertical no matter how the string moves.

If I remember my vibration classes from university that is a much to simple approximation of a vibrating string. I think that the picture need to be expanded to understand what happens in a pickup.

t only the field in the pole piece has significant strength and the right direction.

So how should we use this knowledge when designing pickups?

[Mike Sulzer]

If I remember my vibration classes from university that is a much to simple approximation of a vibrating string. I think that the picture need to be expanded to understand what happens in a pickup.

So how should we use this knowledge when designing pickups?

Well, you need two modes such as horizontal or vertical. For each harmonic you need an amplitude and phase for each mode. This gives you an ellipse for each harmonic. It gets more complicated when you consider coupling between the modes.

One way to use the knowledge is to avoid pickup myths. Such as, a wide coil samples the string over its full width.

[JoeAArthur]

One way to use the knowledge is to avoid pickup myths.

Smartest thing I've heard so far... And it applies to this instant string magnet myth as well.

[SwedishLuthier]

Please Joe, let Mike continue. If this thread gets out of hand I will leave this discussion like I have done with all of the earlier discussions. I will not take part of a discussion were we are getting personal. If you have a problem with the things Mike says, please argue and ask him to expand. If he is wrong it will easily show in his answers. Up to today I have had some problems with parts of what Mike is saying. Most particular the part that the top windings are the most contributing part of the coil and that a wide coil is not sensing a wider part of the string. The reason for this is not that I can say that Mike is wrong. Scientifically he is probably right in most things (if not all) he is saying. My problem is twofold: First he is using a much too simplified model to describe a complex system. Second most of what he is saying contradicts what I have experienced when winding pickups.

Regarding the top winds are the most contributional part I think that I have it all figured out. But before I get into that I would like to get peoples thoughts on this:

Play a standard Tele bridge pickup with the steel base plate. Listen to the output and sound. Now remover the steel base plate and play the exact same pickup in the same amp. What is the difference in sound? Output? Now let’s also compare this experience to the pictures here:

http://www.ampge.com/SKGS/sk/Images/pickup...f/Magnetics.htm

Look at #1 and #3 from the top. That is actually the magnetic structures of a Tele bridge pickup without and with the base plate. The drastic difference in the magnetic field between those two plots appears were the bottom part of the coil is. If “windings near the top contribute more than those below”, then why are the difference so significant? I think I know the answer, but I would like to hear some thoughts first.

Secondly I have personally made Strat sized SCs and R90s (P90 shaped pickups with rod magnets). The magnetic structure is exactly the same, as I use Strat pole pieces (A5 mags). For one customer I under wound the pickup to be were close to an over wound Strat (he wanted a cleaner, thinner sound) meaning roughly the same turn count. Do you think that the sound was similar? No way! So I would really like to hear someone expand on why those two pickups (the over wound Strat and the under wound R90 with identical magnets) were so different. My personal, first hand experience contradicts this:

One way to use the knowledge is to avoid pickup myths. Such as, a wide coil samples the string over its full width.

To take this further I would suggest to compare two different plots. Both with a disk magnet as a substitution for the string (as we have nothing better to use). We will also have to accept that we look only on the up/down part of the movement as I guess FEMM is only designed for static simulations (hey, I must say a big Thanks! to Mike that puts up with us and constantly takes on our challenges like this, kudos to you). Now plot a real magnet and a weaker disk magnet hovering above it. Then make a new plot with a larger distance between the two magnets. Putting both plots side by side should give us an idea about how the *resulting* magnetic field (from the magnet and string) really changes during the vibrations of the strings. THAT would really be interesting to see.

And BTW Joe. If I understand Mike correctly, he is not talking about the string being permanently magnetized by the pickup. Its much more like when you have a magnet, take a nail, hold it against the magnet, and then you can pick up more nails with that *magnetized* nail.

[erikbojerik]

I think I have not been very clear, so here is another attempt. Please be patient, this is not so simple! First, the pole piece magnetizes the string. Now we look at the effect of this magnetization on the pole piece. To do this, we assume linearity. This just means that we can subtract off the part due to the permanent magnet leaving us with the part from the string. We want to do this to make the problem simpler; it is OK because we know that only the string part fluctuates when the string vibrates. So we assume some level of magnetization in the string; it does not matter exactly what the level is because we just want to look at how this field varies in space. So we represent this string magnetization by a small magnet. Why not? The magnetization of the string does not change much when the string vibrates, so a good model is a permanent magnet.

AH! OK I see what you're doing now, thanks for clarifying that. To put it another way....the plot is intending to model CHANGES to the static magnetic field that occur in the presence of a VIBRATING string that has an induced magnetic field from the nearby pole. In effect...subtracting the permanent field (string not moving) from the time-dependent field (string moving) and looking at what's left. Got it. I too think this is probably the best way to model that particular phenomenon.

Thanks Mike for bearing with me...I was not fully immersed in the details of previous threads on this subject, so I may have been missing some implicit assumptions.

[erikbojerik]

My problem is twofold: First he is using a much too simplified model to describe a complex system.

Peter that is the very nature of any computer model of a complex system (and some models are better than others), I do a fair bit of this modelling trying to understand the flight of charged ions in electric and magnetic fields (mass spectrometry) and geological motions of planetary interiors (fluid dynamics). That's the day job that pays the lutherie bills.

The goal of such models is to understand the physics of what's happening, and (hopefully) to use that understanding to design a better mousetrap. The problem with starting off with a full-blown perfect model description of a full-sized pickup is that if you tweek something in the model to test an idea or assumption, then it becomes really really difficult to understand the result because of the feedback between different parts of a complex system. So you have to start simple (like one pole piece), perturb the hell out of it, and when you understand that, gradually increase the complexity.

The other way to approach this is the "experimental" approach, where you build a lot of (let's say) pickups with incremental differences in various ways, measure them to death, and see if you can pull out any relationships between what you've measured and what you've changed....build intuition, and use that to build a better mousetrap. Sort of like the Alan Carruth school of lutherie.

Both approaches are valid and indeed, one always wants experiments done to test the theory. As a theorist, you're dead if you refuse to let data stand in the way of a good model.

Play a standard Tele bridge pickup with the steel base plate. Listen to the output and sound. Now remover the steel base plate and play the exact same pickup in the same amp. What is the difference in sound? Output?

I've never done this (care to describe the difference?), but I will make an educated guess...you'll have the same output, maybe an ever so slight change in the resonant frequency (due to magnetization of the baseplate), and the sound will be not as bright.

But in doing this, you're doing a lot more than just altering the environment around the pickup. If you screw it to the wood, you've made it less microphonic (will have an effect on sound), but more importantly you've also removed a certain amount of steel support for the saddles, and this will effect tone and attack (many many many people mistake a fast attack for bright tone....they are not the same). Much the same phenomenon as replacing your cast aluminum Strat trem block with a brass or stainless steel one.

Now let’s also compare this experience to the pictures here:

http://www.ampge.com/SKGS/sk/Images/pickup...f/Magnetics.htm

Look at #1 and #3 from the top. That is actually the magnetic structures of a Tele bridge pickup without and with the base plate. The drastic difference in the magnetic field between those two plots appears were the bottom part of the coil is. If “windings near the top contribute more than those below”, then why are the difference so significant? I think I know the answer, but I would like to hear some thoughts first.

Clearly the field at the base of the pickup is way different with the baseplate attached. But that's not where the vibrating string intersects the field lines. What you want to look at are the plots of magnetic field across the pole 3/8" above it....that's where the string is intersecting the field and generating current in the coil (and that's why the author chose to look at the field right at that exact spot).

An overlay of these two curves shows that the shape of the curves is identical (the small wiggles are FEMM resolution noise), but the magnitude of the field is about 10% higher with the baseplate attached.

Is this due to the magnetic field of the baseplate? Or the elevation of the rod in the vertical direction in FEMM by ~1/16" with the baseplate underneath? We don't know because we're missing the information about HOW the author added the baseplate (i.e. did the rod move in the Y direction between these models or not?). Details....assumptions....tiny things (like 1/16" in the Y direction) can make a difference both in theory and in actual pickup making.

Secondly I have personally made Strat sized SCs and R90s (P90 shaped pickups with rod magnets). The magnetic structure is exactly the same, as I use Strat pole pieces (A5 mags). For one customer I under wound the pickup to be were close to an over wound Strat (he wanted a cleaner, thinner sound) meaning roughly the same turn count. Do you think that the sound was similar? No way! So I would really like to hear someone expand on why those two pickups (the over wound Strat and the under wound R90 with identical magnets) were so different. My personal, first hand experience contradicts this:

One way to use the knowledge is to avoid pickup myths. Such as, a wide coil samples the string over its full width.

No doubt these sounded different, assuming you tested both on an actual instrument(s); the question is why, right? Were the pole pieces the same height? Were the poles staggered the same (or all flat)? Were the pickups the same distance from the strings on the same guitar? Were the strings just as new/old for both tests? Did they have the same DC and/or AC resistance? Same inductance? Same frequency response curve? Did all the poles on both pickups all measure out to the same gauss levels? Etc etc etc...some of those questions are stupid questions, but some are not. When you're conducting a real-world experiment like this, it is often not easy at all to isolate one specific effect when there's so much feedback between the components.

When its done right, that's the beauty of a good computer model...you can switch things on and off that would be very difficult to do in a real-world situation.

Edited October 5, 2007 by erikbojerik

[Mike Sulzer]

OK, I think we have some major areas of agreement now; thank you both. The model I would like to do, as you would expect, has a magnetized pole piece, achieved various ways, and steel over the pole piece to model the string. You do it twice with the string at slightly different heights and compare the differences. The problem is the comparison. I have not attempted this yet, but the major problem appears to be that the differences are very small, and so it probably is necessary to subtract the two fields numericallly and plot the differences. Remember, the static field is strong and spatially varying. The difference between the two cases is very small and has a different spatial variation. As far as I can tell, FEMM does not allow you to subtract the fields from two different model results. It does contain a scripting language (lua), and so it might be possible to write out the fields and do the subtraction externally. I am working on this.

I would like to comment more on these last posts, but no time until later. Erik, do you get to the Fall AGU?

[SwedishLuthier]

Sorry to cut up your post Erik, but it’s the easiest way for me to comment on things.

Peter that is the very nature of any computer model of a complex system (and some models are better than others

Yeah, I know. And some things doesn’t lend themselves to modeling in a simple way. Because we eliminate to many factors that is actually important. I have suggested a way to further improve the simulations.

The other way to approach this is the "experimental" approach, where you build a lot of (let's say) pickups with incremental differences in various ways, measure them to death, and see if you can pull out any relationships between what you've measured and what you've changed....build intuition, and use that to build a better mousetrap. Sort of like the Alan Carruth school of lutherie.

That is the way I have learned how pickups work. I haven’t built 100s of pickups but a quite substantial amount. I draw my conclusions from actual winds, not modeling, and when the model says that I am wrong…sorry guys, I have to listen to what the (sound of the) pickup is telling me.

But I am always willing to learn more to understand WHY the pickups behave the way they do.

I've never done this (care to describe the difference?), but I will make an educated guess...you'll have the same output, maybe an ever so slight change in the resonant frequency (due to magnetization of the baseplate), and the sound will be not as bright.

But in doing this, you're doing a lot more than just altering the environment around the pickup. If you screw it to the wood, you've made it less microphonic (will have an effect on sound), but more importantly you've also removed a certain amount of steel support for the saddles, and this will effect tone and attack (many many many people mistake a fast attack for bright tone....they are not the same). Much the same phenomenon as replacing your cast aluminum Strat trem block with a brass or stainless steel one.

Whell Erik, I don’t think that we are talking about the same thing here. I am talking about the steel base plate *under* the pickup, not a part of the bridge. No screwing that base plate to wood or anything. Look here:

http://www.stewmac.com/shop/Electronics,_p...es.html#details

That is the thing that is directing the magnetic field up against the strings in plot #3 in Steven Kerstin’s page.

The difference in sound…describing it in words…hmmm. OK, I’ll try. With the base plate: Added treble and output, or if we were to use a more commonly used word: More “twang”. A lot of the characteristic Tele sound is from using that steel base plate. Remove it and you have a sound that is mellower and less ear-piercing. But please note: the difference is NOT small. No way. Believe me. And the only difference between those two sounds is the difference in magnetic field in the bottom of the coil. Anyone care to try to explain that if the top part of the coil is most important. I have an idea, but I really would like to hear other peoples opinion first (A bit sneaky I am there…)

And be sure that the pickup is securely fastened to the bridge as the base plate is removed/added, so you can be pretty sure that the distance to the strings and so on is pretty constant.

No doubt these sounded different, assuming you tested both on an actual instrument(s); the question is why, right? Were the pole pieces the same height? Were the poles staggered the same (or all flat)? Were the pickups the same distance from the strings on the same guitar? Were the strings just as new/old for both tests? Did they have the same DC and/or AC resistance? Same inductance? Same frequency response curve? Did all the poles on both pickups all measure out to the same gauss levels? Etc etc etc...some of those questions are stupid questions, but some are not. When you're conducting a real-world experiment like this, it is often not easy at all to isolate one specific effect when there's so much feedback between the components.

We are talking hand buildt pickups here so there are no two pickups that are “the same”. But I can tell you one thing. They were used with the same magnet types and sizes, the same coil wire and roughly the same amount of turn count (and roughly the same DC resistance). The FEMM plot of the pickups would be as identical as a pair of Strat Pickup. Not identical but almost. There are of cause differences but the main, big differences are the shape of the coil! Try to look at the big picture. With the big difference in sound (ah, a pity I don’t have any sound clips) all the factors you mentions are small once compared to the shape of the coil. And no, the frequency response curve was far from identical, but that’s my point here…

And Mike: I am very interested to see what you come up with. Just putting those two plots side by side migth give some information. Thank a lot for putting up with me "please do this and that in FEMM" as I do not have the time or interest in learning FEMM. I had a hard enough time learning CAD...

Edited October 5, 2007 by SwedishLuthier

[Mike Sulzer]

Peter,

Some ideas on the Telecaster, an interesting guitar. My untested ideas of how it works: The big steel plate that the bridge sits on appears massive enough to isolate the higher frequency vibrations from the wooden body, at least to some extent. This has an effect on the sound, and it can be detected by the pickup if the string vibration is affected by the big plate. In order to maximize the effect, it is necessary to adjust the resonant frequency of the bridge pickup to accentuate this. The small steel plate on the bottom of the pickup increases the inductance of the coil some. I do not know how much, so I am just speculating. This lowers the resonant frequency, and can make the guitar sound brighter if the initial resonance was above the peak response of the guitar. I believe it also maximizes the response the frequency range where human hearing is very sensitive. Using this plate is different from winding on more wire because the latter would also increase the resistance. Some of this can be tested, beginning with plate/no plate inductance. Also maybe some analysis of the effect of the big steel bridge plate.

[SwedishLuthier]

I hear you, but it is against what’s generally considered ”common knowledge" among pickup winders.

The steel in the Tele bridge is actually thinner than most modern flat-top stop tailpieces, so that this should significantly change the vibrations of the strings isn’t really valid. And to think that this has been taken into consideration when Leo Fender designed the Tele pickups is not right. The Tele was the first mass produced electric guitar so there were *no* adjustments done to get it to sound like an electric guitar should. Because people didn’t knew what an electric guitar should sound like. Leo just slapped some stuff together that was cheep to buy and quick and cheep to assemble.

Measure Inductance -> not easy to do. Can’t do it with a simple multi meter…Can it be simulated with an external coil? That would be an easy way to test that theory? That idea (changed inductance) would also mean that if the resonant peak of the Tele pickup with out the base is above the resonant peak of the guitar, the same thing would roughly be valid in a Strat. The Stat pickup is the closest relative to the Tele bridge (without a base plate).

I will return with my ideas why the bridge plate has such a big impact on the sound pretty soon. But what about the difference in sound between the under wound “R90” and an over wound Strat pickup (pretty similar in most parts except coil shape as mentioned above)? Any idea? My believe is still that the width of the coil do make the pickup more sensitive over a longer portion of the string and that change the way the pickup sound.

If you say that the steel bridge plate might affect the sound, acting almost as a magnetic lens, you will have to acknowledge that the magnetic field in the whole of the pickup is taking part of the tone shaping as the bridge plate is roughly at mid height of the pickup coil.

Edited October 5, 2007 by SwedishLuthier

[Mike Sulzer]

"I hear you, but it is against what’s generally considered ”common knowledge" among pickup winders. "

Peter, I think you must have some idea that I think that there are serious problems with a lot of "common knowledge".

"The steel in the Tele bridge is actually thinner than most modern flat-top stop tailpieces, so that this should significantly change the vibrations of the strings isn’t really valid."

There is no comparison to a stop tailpiece; the comparison is to the bridge, which is what terminates the string and is thus more responsible for the vibration of the string than the mass of the tail piece. By 1949 electric guitars of one type or another had been around for about fifteen years. The broadcaster (tele's original name) was designed, based on earlier ideas, but very much its own thing. I do not think Leo knew much electronics, but he certainly knew how to experiment, and I think he maximized the twang.

"But what about the difference in sound between the under wound “R90” and an over wound Strat pickup (pretty similar in most parts except coil shape as mentioned above)? Any idea?"

The inductance of a coil depends upon the dimensions of the coil as well as the number of turns.

"Measure Inductance -> not easy to do." No problem. Put a known capacitance in parallel and measure the resonant frequency, then use the simple equation.

"If you say that the steel bridge plate might affect the sound, acting almost as a magnetic lens, you will have to acknowledge that the magnetic field in the whole of the pickup is taking part of the tone shaping as the bridge plate is roughly at mid height of the pickup coil."

I am not saying the bridge plate has any significant magnetic effect, and I do not think that it directly affects the sound of the pickup very much. The pickup plate is another matter.

[SwedishLuthier]

Peter, I think you must have some idea that I think that there are serious problems with a lot of "common knowledge".

So there is no wisdom in 60 years of collective pickup winding experience? C’mon

"The steel in the Tele bridge is actually thinner than most modern flat-top stop tailpieces, so that this should significantly change the vibrations of the strings isn’t really valid."

There is no comparison to a stop tailpiece; the comparison is to the bridge, which is what terminates the string and is thus more responsible for the vibration of the string than the mass of the tail piece. By 1949 electric guitars of one type or another had been around for about fifteen years. The broadcaster (tele's original name) was designed, based on earlier ideas, but very much its own thing. I do not think Leo knew much electronics, but he certainly knew how to experiment, and I think he maximized the twang.

You brought up the effect of the steel in the bridge, not me.

Leo knew a hellova lot about electronics. He was originally a radio repair man and manufacturer. The main criticism against the first electrics were the “nasty, trebly sound”, and thus not something Leo really tried to make on purpose. All his subsequent designs have been mellower. And the first treble attenuating circuitry of the early Teles was tothere to take care of the too loud treble.

The idea behind the steel bridge plate had *nothing at all* to do with altering the sound. It was used because Leo was very concerned with reducing hum. The original steel bridge cover (mostly removed and used as an ashtray) and the steel base plate under the pickup was there for one single reason: Shielding. Period. It was originally not there for altering the sound. Than the twang grew on us…

"But what about the difference in sound between the under wound “R90” and an over wound Strat pickup (pretty similar in most parts except coil shape as mentioned above)? Any idea?"

The inductance of a coil depends upon the dimensions of the coil as well as the number of turns.

Interesting but I’m not convinced. I will have to try to alter the inductance and se what happens. You don’t happen to know the typical inductance of a pickup. It is never mentioned in data sheets and I can’t se how I could measure it…wait...I see it coming below.

"Measure Inductance -> not easy to do." No problem. Put a known capacitance in parallel and measure the resonant frequency, then use the simple equation.

AFAIK the resonant frequency isn’t that easy to measure either without specialised equipment. At least I don’t know how to.

I am not saying the bridge plate has any significant magnetic effect, and I do not think that it directly affects the sound of the pickup very much. The pickup plate is another matter.

My mistake. You didn’t. But it actually does that exact thing. But as you do not belive the bottom part contribute much to the sound then you do not care about whats happening in the lower parts of the coil. But now you really need to expand on that part. If the bottom of the coil has little to do with anything, why are we still making the coil this way. Why haven’t some one tried to wind “half a strat pickup” (only upper part that is) and discovered that they can cut manufacturing costs with 50% for the wire and 50% for the time winding the pickup(rhetoric question)?

[Mike Sulzer]

"AFAIK the resonant frequency isn’t that easy to measure either without specialised equipment. At least I don’t know how to. "

Peter, you have all the hardware you need: your computer. Just get some signal generating software and a 'scope program. This is a lot easier then the stuff you have been doing with the magnetics.

"So there is no wisdom in 60 years of collective pickup winding experience? C’mon"

I did not say none, but you have to admit that a collective wisdom that does not take much account of the fact that the pickup forms a resonant circuit with the cable capacitance could be missing something.

" All his subsequent designs have been mellower. And the first treble attenuating circuitry of the early Teles was to there to take care of the too loud treble."

Is a strat mellower than a tele? Less twangy, but thinner IMO, and no less treble. Leo's early amplifiers were pretty much out of the handbooks with one important exception. The tone stack is not stock HiFi, and it is not "flat" in the neutral settings of the knobs. It is designed to boost bass, upper midrange, and treble but suppress the middle, in typical positions, in order to make the correct sound. (http://users.chariot.net.au/~gmarts/ampbasic.htm) This was partly necessary because guitar speakers sound a bit weak at these frequencies. Too much treble is not a problem if it needs to be boosted in the amp.

"The idea behind the steel bridge plate had *nothing at all* to do with altering the sound. It was used because Leo was very concerned with reducing hum. The original steel bridge cover (mostly removed and used as an ashtray) and the steel base plate under the pickup was there for one single reason: Shielding. Period. It was originally not there for altering the sound. Than the twang grew on us…"

OK, I am no expert on the history, but you think Leo went to all that trouble on the tele to reduce the hum, and then took none of those precautions on the strat, which was intended to be his high end guitar? Surely the "ash tray" was about style, not hum reduction. A pickup sitting in the middle of metal plate is well shielded. A cover has little additional effect.