Tapped coils (not split coils)

[Mick Bailey]

Here is something to keep in mind to form a good mental model of pickup loading. Typical high impedance pickups use what I call the “rule of 40” to match the pot load to the pickup resistance. Pot resistance loads are about 40 times higher than the DCR of the pickup with single coils having 250K volume pot values and humbuckers having 500K pot values.

Often guitars with mixed pickup types have a single pot value which is either too large or too small for one or more pickups. At least Suhr recognizes this and uses a Superswitch to effectively lower the pot resistance from 500k to 250k when selecting a single coil in a HSS guitar. I have a Tyler 'Strat' player where I've changed many of his guitars from two tone controls to a single master tone control and two separate volumes - 500K for bridge humbucker and 250k for middle/neck. He prefers this setup and tonally is much better than running a 500k pot with single coils. Interestingly he also uses a higher than usual capacitance lead and patch cables (Belden 9778 - 178 pf/M).

Are there any guitars with where a tapped pickup switch also switches the pot value? Not a single one has come my way. Maybe that's part of the reason (amongst other things) why a regular tapped humbucker doesn't sound right to me as a single coil.

[Mike Sulzer]

Here is something to keep in mind to form a good mental model of pickup loading. Typical high impedance pickups use what I call the “rule of 40” to match the pot load to the pickup resistance. Pot resistance loads are about 40 times higher than the DCR of the pickup with single coils having 250K volume pot values and humbuckers having 500K pot values.

......
Joseph J. Rogowski

Typical guitar cable has about 30pf per foot. I doubt that you can find any much below about 15pf/ft. Thus the cable capacitance adds up, and it is more important than the pickup capacitance for setting the resonant frequency.

The load on a pickup is most important at the resonant frequency because that is where its impedance is highest. The load has little effect at low frequencies. Therefore, the resistance of the tone control, even on 10, is just about as important as the volume control on 10 (since the impedance of the tone capacitor is low at the resonance, like a coupling capacitor), and therefore if you are using 250K pots, you should consider the effective load from the controls to be 125K.

A current transformer is designed to sense the current flowing in the circuit the primary is connected to. Therefore, it is very low impedance so that it has negligible effect on the current in the circuit it is in. When you use a current transformer connected to your very low impedance pickup, it appears that you are using it as a voltage transformer, just one designed to be used in a very low impedance circuit. That is, the term "current transformer" is a description of its intended use; it is very much like any other transformer, but designed for a very low impedence. Thus, it is perfect for your application even though it is not what it was designed for.

[Mike Sulzer]

Often guitars with mixed pickup types have a single pot value which is either too large or too small for one or more pickups. At least Suhr recognizes this and uses a Superswitch to effectively lower the pot resistance from 500k to 250k when selecting a single coil in a HSS guitar. I have a Tyler 'Strat' player where I've changed many of his guitars from two tone controls to a single master tone control and two separate volumes - 500K for bridge humbucker and 250k for middle/neck. He prefers this setup and tonally is much better than running a 500k pot with single coils. Interestingly he also uses a higher than usual capacitance lead and patch cables (Belden 9778 - 178 pf/M).

Are there any guitars with where a tapped pickup switch also switches the pot value? Not a single one has come my way. Maybe that's part of the reason (amongst other things) why a regular tapped humbucker doesn't sound right to me as a single coil.

A tapped humbucker does not sound like a single coil because the Fender type signal coil has low eddy current losses in the cores (Alnico magnets) while the humbucker has steel cores with high eddy current losses. Therefore the real single coil pickup has a brighter sound because the resonance is not so damped as with steel. Switching the pot value cannot fix this.

[Mick Bailey]

I understand the differences (that's why I said "amongst other things") but would not switching in an appropriate pot value improve matters?

[Mike Sulzer]

I understand the differences (that's why I said "amongst other things") but would not switching in an appropriate pot value improve matters?

Not enough even if you increased above 500K, because you would have to use a pot value "beyond infinity" in order to decrease the damping to a sufficiently low level. The eddy currents in steel cores give more damping than the resistances of the volume and tone pots. Also such damping is frequency dependent, and so even if you got it right in one frequency range, it would be off in another. Obviously Alnico is not the only choice for low damping poles. For example there are various ceramic materials, either the magnetically soft or hard varieties, that can do the job as well, but with traditional materials, doing it as Leo did is the right way.

[Helmholtz]

Getting back to the reflected capacitance problem with tapped coils.

Usually the self-capacitance of a PU (typically 50pF to 200pF) is no real problem as total capacitance is dominated by the cable capacitance (+ amp input capacitance).

Things change with tapping.

Here's a numerical example:

Suppose a (hot) 8 Henry PU is tapped at 50% turns. In tapped mode it will have an inductance of 2H (strat ballpark).
Now if the full coil has a self-capacitance of 150pF, the tapped coil will have a self-capacitance of 600pF(!).
This means that the PU's self-resonance doesn't change with tapping.

As a a result the loaded resonance will be way lower than that of a strat PU, making it sound duller.

[Mike Sulzer]

Getting back to the reflected capacitance problem with tapped coils.

Usually the self-capacitance of a PU (typically 50pF to 200pF) is no real problem as total capacitance is dominated by the cable capacitance (+ amp input capacitance).

Things change with tapping.

Here's a numerical example:

Suppose a (hot) 8 Henry PU is tapped at 50% turns. In tapped mode it will have an inductance of 2H (strat ballpark).
Now if the full coil has a self-capacitance of 150pF, the tapped coil will have a self-capacitance of 600pF(!).
This means that the PU's self-resonance doesn't change with tapping.

As a a result the loaded resonance will be way lower than that of a strat PU, making it sound duller.

I made a humbucker sized tapped side winder. Side winders have two important advantages. First, they buck hum with only one set of poles despite having two coils, and second, the mutual inductance is negative, and so the net inductance is somewhat less than SC or normal HB, allowing more output for a given inductance. I was trying for something like 9 H, which was easy to achieve, of course, and a bit over three, like a tele bridge. I used #42 for the low H part and #44 for the rest. I got somewhat more C than I expected in the low H mode, and so I did not quite get what I wanted. However, it is actually pretty close. Switching is not trivial because you have two tapped coils, but it can be done with a single four pole switch for the pickup selector. The two modes of the tapped SW appear on the switch as separate pickups..

I like sidewinders because they take advantage of the fact that the flux from the vibrating string returns to the string, and there are two equivalent flux paths where the two coils can go. Thus, it is not so hard to confine most of the flux with high permeability ceramic pole pieces.

[bbsailor]

Typical guitar cable has about 30pf per foot. I doubt that you can find any much below about 15pf/ft. Thus the cable capacitance adds up, and it is more important than the pickup capacitance for setting the resonant frequency.

The load on a pickup is most important at the resonant frequency because that is where its impedance is highest. The load has little effect at low frequencies. Therefore, the resistance of the tone control, even on 10, is just about as important as the volume control on 10 (since the impedance of the tone capacitor is low at the resonance, like a coupling capacitor), and therefore if you are using 250K pots, you should consider the effective load from the controls to be 125K.

A current transformer is designed to sense the current flowing in the circuit the primary is connected to. Therefore, it is very low impedance so that it has negligible effect on the current in the circuit it is in. When you use a current transformer connected to your very low impedance pickup, it appears that you are using it as a voltage transformer, just one designed to be used in a very low impedance circuit. That is, the term "current transformer" is a description of its intended use; it is very much like any other transformer, but designed for a very low impedence. Thus, it is perfect for your application even though it is not what it was designed for.

Mike,

You are correct about the coax cable capacitance being about 30 of per foot. My error.

Building an FET buffer in the guitar end of the metal jack using point to point soldering of the few parts in the guitar end and other power supply related parts in the amp end makes a good device to make a pickup sound brighter because the high impedance pickup is isolated from the full effect of the coax capacitance. I built one of these many years ago and it had a real audible effect on all passive guitar pickups with no need to modify the guitar.

Here are some thoughts on current transformer based pickups. The skin effect of using thick solid wire for the string loop determines some tonal changes that you can hear as with thicker solid wire the higher frequencies do not fully penetrate to the core and thus generate less current at these higher frequencies beyond the listed skin effect for AWG 6, 8, 10 which I use for the string loop with the magnet or magnets in the center. Using stranded wire will overcome some of this effect. Here is another case where the ear becomes a good judge of what sounds good or better than some alternatives.

Pickup loading for an extremely bright pickup might make it sound better. Understanding electronic theory helps us define why one alternative is different than the other but in the end the ear is still the final testing tool. This is why this forum is so good. Those who tinker, build pickups from scratch, play and measure the final results share some good information on this pickup forum.

Thanks

Joseph J. Rogowski

[Helmholtz]

I made a humbucker sized tapped side winder.

Yes, I read about your design and think it is well-thought-out.