Talk to me about inductance and output. . .

[kayakerca]

OK, here goes. . .

Well gang, it’s time to teach the “curious, recently retired ‘ex-accountant’ layman newbie winder” more. Hopefully, just enough to make him a danger to himself!

How is inductance related to resistance and magnet strength in a pickup in terms of output of the pickup (in layman speak as much as possible)? In the context of guitar pickups, my layman understanding of inductance is the ability of a ferrous metallic object in motion (plucked electric guitar string) to induce current in a conductive material (p/u wire coil) when inside a magnetic field (AlNiCo magnets).

Let’s us one of my recent single coil winds as an example.

Strat style single coil p/u:
Bobbin height: .4375”
Magnets: AlNiCo 5
Wire: 42AWG SPN
Turns: 8,600
Resistance: 6,350 ohms (I am assuming my cheapo multi meter is measuring DCR, but probably not)
Inductance: 2.30 henries (before charging the magnets)
Inductance: 2.60 henries (after charging the magnets)

Back to the question(s). Can there be a valid inductance measurement without the magnets being charged? I’m guessing yes, but that the pickup would not work with the 2.3 henries (from above) without the magnets being charged, even though many spec’s you’ll see on the internet (Fender, et al) will have pickups with inductance values below 2.3 henries. Would it be fair to say that “most” of the inductance reading in henries is attributable to the coil characteristics versus the magnet strength/charge?

In the above case, if the magnetic strength was backed off from the current full charge levels (~ 1,100mT – 1,300mT) to , say half, would the output (measured by the ear listening to the p/u through an amp) be about half, even though the henries may still be in the 2.4 – 2.5 range?
I’m guessing my measurement tools are at least so-so OK as I measured a couple of Fender p/u’s to get some baseline type comparisons and they seem reasonably similar (I think). They measured as follows:

Fender “Texas Specials” in a SRV Signature Series Strat (middle p/u):
Resistance: 6,180 ohms
Inductance: 2.51 henries
Magnet charge: +/- 1200mT (depending on the specific magnet)

Fender Custom Shop “Fat 50’s (middle p/u):
Resistance: 6,150 ohms
Inductance: 2.35 henries
Magnet charge: +/- 1100mT (depending on the specific magnet)

My measurements do not appear to be miles off the published values when you take into consideration that they have been in use for number of years, environmental variables and lower quality test equipment.

With the inductance, what is it in the coil wind (more tension/less tension, more tpl/less tpl, higher/lower bobbin height) that would produce an inductance measurement for a given resistance that is above the manufacturer spec detail even before the p/u is magnetized? How far could you realistically back off a full charge on the magnets and still have an acceptable output level, and how would that affect tone? Not looking for secrets here, just some more understanding.

Look forward to your thoughts. . .

[big_teee]

No help here.
I just try to wind, what sounds good.
Big wire and Open sound on the neck.
Same as the neck on the middle, but a little hotter.
Short and fat bobbins on the Bridge, is what sounds best to me.
The neck and middle are easiest.
As with all pickups, the bridge pickup is the hardest, and to each player, can be great or terrible!
T

[kayakerca]

. . .

I just try to wind, what sounds good. . .

T

Unfortunately it's like a disease with me Terry. I just can't help but wonder about the physics of everything I see in life. I was going to spend the morning potting a couple of sets of p/u's, but then I took some readings, looked at some published spec's and well, out came my questions.

How I ever ended up as CA, I'll never know.

[big_teee]

Unfortunately it's like a disease with me Terry. I just can't help but wonder about the physics of everything I see in life. I was going to spend the morning potting a couple of sets of p/u's, but then I took some readings, looked at some published spec's and well, out came my questions.

How I ever ended up as CA, I'll never know.

I hear you, We're on opposite ends of the Spectrum.
I worked High Tech for Years.
Now I do the KISS method (Keep it Simple Stupid).
I like real basic, and getting my hands a hold of everything,
It's all about the sound and end result to me.
However, I may break down and buy a Extech.
That may ruin the Urge of Doing everything the hard way Scenario, that I dig so much!
That's why it's so great to have all kinds of Winding folks here.
IMHO, there is no right or wrong to make pickups.
T

[kayakerca]

I hear you, We're on opposite ends of the Spectrum.
I worked High Tech for Years.
Now I do the KISS method (Keep it Simple Stupid).
I like real basic, and getting my hands a hold of everything,
It's all about the sound and end result to me.
However, I may break down and buy a Extech.
That may ruin the Urge of Doing everything the hard way Scenario, that I dig so much!
That's why it's so great to have all kinds of Winding folks here.
IMHO, there is no right or wrong to make pickups.
T

Ya, it's weird how that works, what we did to what we do and how we do it now.

BTW, I did read a review somewhere on the Extech 380193 and the review was saying it did not measure DCR, but rather AC. If that is important to the decision, you might want to verify either way with Extech themselves. Darn, there I go again, doing the "layman" detailed look at everything that passes in front of me.

[jason lollar]

its simple enough to just go ahead and wind some stuff up to the same turn count and vary some of the specs to see what it does to the inductance reading.
More TPL- higher inductance- taller narrower coil less, shorter fatter coil more, higher charge on a magnet of the same material more inductance. The odd thing is its not just the strength of the magnet its the type- compare al2 and al5. see what a different wire gauge does etc.

[kayakerca]

its simple enough to just go ahead and wind some stuff up to the same turn count and vary some of the specs to see what it does to the inductance reading.
More TPL- higher inductance- taller narrower coil less, shorter fatter coil more, higher charge on a magnet of the same material more inductance. The odd thing is its not just the strength of the magnet its the type- compare al2 and al5. see what a different wire gauge does etc.

On the tpl parameter, I've been working at +/- 100 tpl (accounting for minor winder vs. traverser speed fluctionations) on a 7/16" Strat style single coil bobbin to this point. On this type of bobbin, what might be a reasonable min/max tpl range that would still produce tonal results that a range of players would like? 35 - 150 sort of thing where 156 would be an unattainable perfect side by side layering?

[SonnyW]

Ya, it's weird how that works, what we did to what we do and how we do it now.

BTW, I did read a review somewhere on the Extech 380193 and the review was saying it did not measure DCR, but rather AC. If that is important to the decision, you might want to verify either way with Extech themselves. Darn, there I go again, doing the "layman" detailed look at everything that passes in front of me.

Yes the Extech 380193 measures ACR, not DCR. It needs to use ACR in its computations of other things it is measuring. But that doesn't mean you have to use it to measure the DCR. Just use a regular meter for that. Knowing the ACR has some uses too.

On the "layman" thing here are a couple of simplified ways of looking at pickup specs that I use with my customers. First, I tell them DCR is not that useful of a measurement by itself. Unfortunately, you will hear too often some statement like this. "Man that pickup is hot, it puts out like 16k ohms." That shows that many laymen don't understand the physics like I think you probably already do. For one thing, it isn't "putting out" ohms. DCR is really only useful as a way of keeping up with how much wire is on the coil. It is like counting turns. It is the number of turns that relate to the inductance value. If the pickups to be compared have the same design and the same sized wire, DCR can give an idea of output, but inductance is a much better way of comparing because it is measuring the principle on which the pickup works. Think of inductance like a kind of inertia. I use the example of the weight of a bell to illustrate this. A large heavy bell is going to be louder sounding and darker than a smaller lighter bell would be. So a large inductance will be louder and darker. The same number of turns of different sized wire will give you similar inductance but greatly different resistances. There are many details that determine the character of a pickup but inductance is probably the biggest factor. Also note that the inductance measurement is affected by the frequency so that has to be taken into account too.

Yes there can be a valid inductance reading without any magnet. That happens all the time in chokes for example in an amp. But without some magnetic strength a pickup can't work. That is because the moving magnetized strings are what is creating the current. I suppose if you could somehow get permanently magnetized strings you could in theory make a pickup with no magnet in it. But that probably isn't happening anytime soon. You wanted to know how much the inductance is related to the coil structure vs the magnet etc. It is mostly the turns count that determines the inductance. And you can vary the magnetic strength quite a bit and still have output. A lot of strat pickups for example have up to four times enough gauss reading at the magnets than the minimum needed to work. Some read over 1000 gauss, and some read 250 gauss. Both of them will work. The ones that read 1000 will have more output, but will be also slightly darker and at some point too much magnetism leads to "Stratitis" - loss of sustain. And the magnet alloy itself as Jason mentioned has an effect of its own. Anyway just my thoughts on your questions. Hope I didn't oversimplify it too much.

[David Schwab]

My measurements do not appear to be miles off the published values...

I'd bet you if you have those pickups to measure they would not match the published specs.

If your pickup sounds good, that's all you need.

[Mike Sulzer]

If it is the physics that you want to understand, you are not asking quite the right questions. Let's start with a special set of circumstances, impossible to achieve in practice, but useful for understanding.

Let the pickup coil have no capacitance, and instead of being connected to a guitar cable, connect it to a voltage measurement device with an infinite input impedance. The output voltage from a single turn of the coil at any time is determined by the rate of change of magnetic flux through that turn. The voltage from all the turns add because they are connected in series.

The magnetic flux is determined by the degree of magnetization of the vibrating string, its position with respect to the coil, and how much the flux is modified by the material of the core. For now ignore the electrical conductivity of the core material, but consider its magnetic properties. The magnetic property in question is permeability. A vacuum has a relative permeability of one. Ferromagnetic materials can have permeabilities as high as about 100,000. A permeability greater than one increases the flux through the turn in question and so raises the output.

However, very high permeabilities have little benefit over moderate permeabilities because the cores are short and open, unlike a transformer. There is little increase in output as the permeability increases beyond about 5 or 10. Neodymium magnets have a permeability of about 1.02. They are very strong, and nearly all the magnetic domains are already lined up. Alnico might be somewhere between 5 and 10. Steel up to about 1000.

The pickup is used in an electric circuit. The circuit properties of the coil and the other elements in the circuit modify the output described above. But that is another question.