Switchable additional winds

[Helmholtz]

<My explanation is the following:

When using the tap as output, the disengaged part of the winding is still there and remains to be inductively coupled with the rest of the coil. It becomes the secondary of a transformer. This secondary appears to be open but is actually terminated by its own self-capacitance. The latter gets transformed/reflected to the primary side (the active part of the coil). And thus the reflected capacitance of the secondary adds to the capacitance of the primary - as long as you don't physically remove the disengaged part of the winding from the pickup.

This effect does not depend on electrical connection between the coil parts.>

Here
http://www.datatronics.com/pdf/distr...ance_paper.pdf
is a good description of the summing-up of distributed capacitances of coupled windings.

[Antigua]

Here
http://www.datatronics.com/pdf/distr...ance_paper.pdf
is a good description of the summing-up of distributed capacitances of coupled windings.

Thanks for the link, I'll have to look over that carefully to see if it's equations might apply to transformers that have coils with a common axis and plane.

[Helmholtz]

Thanks for the link, I'll have to look over that carefully to see if it's equations might apply to transformers that have coils with a common axis and plane.

The effect only requires sufficient magnetic coupling. Good coupling means that the windings share most of the same magnetic flux. Concentric arrangement of the windings on a common ferromagnetic core provides good preconditions for strong magnetic coupling. The coupling factor can be determined from the ratio of primary inductance with secondary shorted (Lps) to primary inductance with secondary open (Lpo). Perfect coupling (100%) means Lps/Lpo = 0 and no coupling means Lps/Lpo = 1.
In case of weaker coupling, the transformed secondary self-capacitance will appear on the primary side in series with the transformed secondary leakage inductance, constituting an additional series resonance circuit. As a consequence the pickup's frequency response will show additional peaks and dips above the main primary resonance frequency.

[okabass]

HI
I tried the wiring you suggested on a 51 type P bass single coil. Coils are 6,7 kΩ / 2,6 H (tapped) 7,8 kΩ / 3,4 H (full). It works very nicely. The sound difference is subtle but useful. It could be more so I'll try something like 5,7...6,5 kΩ and 7,8 kΩ.

[Jason Rodgers]

HI
I tried the wiring you suggested on a 51 type P bass single coil. Coils are 6,7 kΩ / 2,6 H (tapped) 7,8 kΩ / 3,4 H (full). It works very nicely. The sound difference is subtle but useful. It could be more so I'll try something like 5,7...6,5 kΩ and 7,8 kΩ.

To which wiring diagram do you refer? The one in the original post, or Antigua’s of Feb 28?

[okabass]

To Antigua's.
As a matter of fact I just wind a 5,7 kΩ + 1,9 kΩ (7,8 kΩ) coils to an old Duncan quarter pounder frame. But there's some short in coils (reads 50Ω when it should be ∞Ω). Didn't have time to check more closely. Must perhaps rewind that.

[Antigua]

I'm glad to hear that worked out. I'll have to make a four lead tapped single coil and try it myself. I think the difference between two pickups tends to be pretty subtle, but a big deal is made of it, because it's the sound you have to live with. Put a switch in there and you might demand a bigger shift. I figured that was why tapped single coils presently on the market had such radical tap points, until I found out that the capacitance, and possibly inductive coupling, made it somewhat necessary, too.

[okabass]

Wind 5,7 kΩ + 1,9 kΩ (7,8 kΩ) coils to normal 51 PU frame (ca. 5 mm magnets). I found that Quarter pounder magnets made it too thick sounding. There's volume difference, but it ain't bad. I like the most that you get a 50's and a modern fuller sound without character change.

[Joe Gwinn]

In a sense, you are all correct - all these effects do happen, independently of one another, so it's a matter of relative magnitude, and experiment is the only way to sort it out.

The floating open-circuited coils gets both electrostatic and magnetic coupling. The floating coil increases the self-capacitance of the nearby connected coil. This happens even if the connected coil is only connected at one end, so no metallic currents flow. What does flow through the capacitances are displacement currents due to changing electric fields. Currents through the connected coil will induce voltages in the floating coil, which is capable of resonance with its own self-capacitance, and this effect will be reflected back into the connected coil. It can get pretty complicated.

How to reduce the effects? Two main remedies. First, lay down a few layers of mylar or waxed paper tape on top of the inner winding, before starting the outer winding. This will decrease the capacitance between coils, but will not have much effect on the transformer coupling between the coils. One can suppress a lot of the effect by loading the floating coil with a fixed resistor, thus reducing the effects of resonance. The best way to find the correct resistor value is a pot and a golden ear.

[okabass]

Thank you for the post.
Mylar plastic. Do you mean clear mylar only or with metal foil (like space sheet)? How about this? https://www.ballisticproducts.com/Th...ctinfo/MYTHIN/

Fixed resistor? Parallel to the floating coil? What value pot would be a good starting point if the floating coil is around 2 kΩ ? The other coil is 5,8 kΩ, total 7,8 kΩ.

[Helmholtz]

In a sense, you are all correct - all these effects do happen, independently of one another, so it's a matter of relative magnitude, and experiment is the only way to sort it out.

The floating open-circuited coils gets both electrostatic and magnetic coupling. The floating coil increases the self-capacitance of the nearby connected coil. This happens even if the connected coil is only connected at one end, so no metallic currents flow. What does flow through the capacitances are displacement currents due to changing electric fields. Currents through the connected coil will induce voltages in the floating coil, which is capable of resonance with its own self-capacitance, and this effect will be reflected back into the connected coil. It can get pretty complicated.

How to reduce the effects? Two main remedies. First, lay down a few layers of mylar or waxed paper tape on top of the inner winding, before starting the outer winding. This will decrease the capacitance between coils, but will not have much effect on the transformer coupling between the coils. One can suppress a lot of the effect by loading the floating coil with a fixed resistor, thus reducing the effects of resonance. The best way to find the correct resistor value is a pot and a golden ear.

While the mylar foil might help to better separate the windings (even though its epsilon is around 3) and thus reduce interwinding capacitance, this will have neglegible effect on the active coil's resonance. The reason is that interwinding capacitance is not in the circuit as long as the outer coil is floating (and not shielded by a grounded outer copper foil). The primary (active) coil resonance only changes if its effective capacitance to ground changes.

Loading the floating coil with a resistor will as well load the active coil with the reflected resistance (and - depending on coupling - a leakage inductance), thus reducing Q and resonance peak. This might not be desirable.

[Joe Gwinn]

Thank you for the post.
Mylar plastic. Do you mean clear mylar only or with metal foil (like space sheet)? How about this? https://www.ballisticproducts.com/Th...ctinfo/MYTHIN/

Yes, clear mylar. Actually, kapton would work just as well. Or scotch tape.

Fixed resistor? Parallel to the floating coil? What value pot would be a good starting point if the floating coil is around 2 kΩ ? The other coil is 5,8 kΩ, total 7,8 kΩ.

Connecting the ends of the floating coil, to compete the circuit. I have no idea what value is best. It may be that shorting the ends together is best, given the large DC resistance of the floating coil. Start with a 10 K pot and fiddle.

[Joe Gwinn]

While the mylar foil might help to better separate the windings (even though its epsilon is around 3) and thus reduce interwinding capacitance, this will have neglegible effect on the active coil's resonance.

Agree.

The reason is that interwinding capacitance is not in the circuit as long as the outer coil is floating (and not shielded by a grounded outer copper foil). The primary (active) coil resonance only changes if its effective capacitance to ground changes.

Not true. There is always some effect, and adding the interwinding layer reduces that effect. Interwinding layers are standard practice in transformer building. As are interwinding shields, for which one would can use metallized mylar. The shield metal must be grounded.

Loading the floating coil with a resistor will as well load the active coil with the reflected resistance (and - depending on coupling - a leakage inductance), thus reducing Q and resonance peak. This might not be desirable.

It's true that the loss of the connected coil will increase. The intent is to make it flatter, to reduce coloration. This must be assessed by ear - instruments are not much help here.

[Helmholtz]

>Not true. There is always some effect, and adding the interwinding layer reduces that effect. Interwinding layers are standard practice in transformer building. As are interwinding shields, for which one would can use metallized mylar. The shield metal must be grounded.<

Not convincing. In high frequency transformer applications open (non-terminated) floating windings are typically avoided.
A grounded interwinding shield effectively reduces interwinding capacitance (capacitive coupling between windings) but inevitably increases ground capacitance for both windings, which seems counterproductive in PUs.
Can you show measurements and/or an equivalent circuit?
BTW: I generally assume the inner start of the active pickup winding to be grounded.


>It's true that the loss of the connected coil will increase. The intent is to make it flatter, to reduce coloration. This must be assessed by ear - instruments are not much help here.<

In my experience most guitar players prefer some coloration i.e. resonance peak. Otherwise they could as well back the the tone control a bit to get a similar result.

[Mark Hammer]

Interesting thread, but here's something I never usually see discussed.

We tend to think of coil-tapping in terms of "additional winds". That is, wind enough to make a viable coil, then add some more turns to make a hotter coil, but retain the selectable connection to the point where the added turns begin. Because the additional turns are on the outside, the added circumference makes for greater resistance per turn.

But how does that compare to making the additional turns on the inside? For example, put 1500 turns snuggled up against the polepieces, run a lead out to a solder terminal, then add another, say, 6500 turns. I won't pretend to understand Eddy currents or magnetism in general, nearly as well as my colleagues here. But at the very least, the scenario I describe moves the "start" of the default coil outwards, and obviously changes the circumference of the turns.

So what I'm wondering is: how does the relocation of the additional turns to the "inside" change things with respect to inductance, resonant peaks, etc., and which would be the preferred arrangement - added coils on the outside, or the inside?