Switchable additional winds

[rjb]

I have a guitar in mind that will have one pickup and very simple controls (maybe just volume with push-pull for tap or coil cut, maybe just a toggle for full/cut/off) and this thread came up at the perfect time.

I'm considering doing the same thing with a lap steel. I was thinking a fairly hot humbucker with an on-on-on dpdt for series/cut/parallel. The tones would be selected in order from raunchy to clean. And with a "twin dual-rail" [EDIT: AKA "quad rail"] pickup (each side a humbucker), it would be hum-free.

-rb

[Steve A.]

Inner and outer halves will sound slightly different, not sure this will merit the switching complication, maybe you can hear something playing alone in a basement at low volume and *focusing* on it, but I guess onstage, loud, and with other musicians around any difference will be lost.

That could be said for a lot of the tweaks we might do to guitars, amps and pedals but there are another factors besides sound, such as the response to your playing. When I think of a guitar rig being "hot" it is more than volume, tone or gain — it is how everything responds to my playing. So even though a listener might not be able to hear an audible difference the tweaks made to my equipment might make me feel more inspired which would improve my playing. Just a thought...

Steve A.

[David King]

My experience with tapped coils is that they only sound good when the whole coil is engaged. As mentioned above the unused portions are dead weight that will have a significant capacitance. No different from having shorted turns inside a coil. I consider them a waste of time but some guys think they gotta have everything in one pickup. If you could somehow peel the extra turns away when you weren't using them that would be handy.

[Chuck H]

I gave a like because this is an excellent observation to the discussion. Still, I will mention that you CAN'T peel then away the unused coil (well, maybe you can, more in a sec). So the burden of the extra winds is offset by the versatility for a given design. At least that's the ideal I think. So... Probably more trouble than it's worth, but if you could switch open both ends of an additional wind (yes, complicates switching horribly) you shouldn't suffer any capacitance or mutual inductance (relative to the end that isn't open if one is left connected). Perhaps some bending of the EMF due to what amounts to a copper case surrounding the inner coil though.?. Just thinking out loud.

[Antigua]

I gave a like because this is an excellent observation to the discussion. Still, I will mention that you CAN'T peel then away the unused coil (well, maybe you can, more in a sec). So the burden of the extra winds is offset by the versatility for a given design. At least that's the ideal I think. So... Probably more trouble than it's worth, but if you could switch open both ends of an additional wind (yes, complicates switching horribly) you shouldn't suffer any capacitance or mutual inductance (relative to the end that isn't open if one is left connected). Perhaps some bending of the EMF due to what amounts to a copper case surrounding the inner coil though.?. Just thinking out loud.

Yes, if both ends of the tap coil are disconnected then you can truly have your cake and eat it to, and I wish a commercial pickup maker would exploit this fact. If both ends of the tap coil are disconnected, it still remains in circuit, because there is still capacitive coupling along the length of the tap coil and the primary, and you see the same issue occur when you split a humbucker when you fully disconnect the secondary as opposed to simply shunting to ground, but the effect of that capacitive coupling will be small, compared to the usual practice of keeping one end of the coil physically connected to the circuit.

[Hout]

Okay, so that means it would be most effective to use dtdp switching like the diagram in the first post? Essentially having two separate coils around the same polepieces. With some more elaborate switching, you would be able to use 3 different coil "sounds".

[Antigua]

Okay, so that means it would be most effective to use dtdp switching like the diagram in the first post? Essentially having two separate coils around the same polepieces. With some more elaborate switching, you would be able to use 3 different coil "sounds".

I don't really understand the OP diagram, but you could do it like this:



When the DPDT on-on switch is down, the inner portion of the coil is in circuit and the outer portion is disconnected at both ends. When the switch is up, the inner and outer coils are in series, as if it were just one big coil.

Some tapped coils on the market are wound with equal turns inner and outer, and presumably the reason they do this is because the high capacitance of industry standard wiring causes the tapped coil to have a very low resonance, so if you end up using this DPDT scheme, you could have the inner coil wound with a substantially higher wind count than the outer portion, and still get a good effect. If I were doing this, I'd try 7,000 turns on the inner coil, and then add another 2,000 turns to the outer coil. That should cause the inductance to switch from about 2 henries tapped, up to 3 henries in full series mode, or "vintage" to "hot".

[David Schwab]

An under appreciated aspect of coil tapping is that, suppose you wind it to 7,000 turns, then tap it, then wind it another 2,000 turns for a total of 9,000, when you use the 7,000 tap, it will not sound the same as a coil that was wound to 7,000 turns without the tap, because those extra 2,000 turns of wire remain in the circuit as a capacitive coupling. The amount of capacitance added by that extra bulk of coil is audibly significant.

Not if you leave the other end of the extra winds open. If you form a closed loop, eddy currents will change the tone. It's not really the capacitance, but inductive coupling.

[Antigua]

Not if you leave the other end of the extra winds open. If you form a closed loop, eddy currents will change the tone. It's not really the capacitance, but inductive coupling.

That's what I'm saying, you want both ends to be open, neither end to be connected to anything. If either side is connected to anything else (itself or the circuit), you get unwanted L and/or C. Popular coil tapping wiring schemes with three lead wires require that one end be in-circuit, and there's almost nothing that can be done about it, unfortunately.

[David Schwab]

That's what I'm saying, you want both ends to be open, neither end to be connected to anything. If either side is connected to anything else (itself or the circuit), you get unwanted L and/or C. Popular coil tapping wiring schemes with three lead wires require that one end be in-circuit, and there's almost nothing that can be done about it, unfortunately.

If one end is open that's the same as not being connected. No current is going to flow.

[Chuck H]

If one end is open that's the same as not being connected. No current is going to flow.

Except that it's still a copper shield in close proximity to an inductor connected to ??? and forms a capacitor of sorts.

[Antigua]

If one end is open that's the same as not being connected. No current is going to flow.

Like Chuck says, because it's an AC voltage, a capacitive reactance occurs between the primary and secondary coil, and because of the large area and extremely close proximity of the two, the capacitance is rather substantial, unlike a humbucker, where the coils are farther apart, and the surface area between them a lot smaller.

[David Schwab]

Like Chuck says, because it's an AC voltage, a capacitive reactance occurs between the primary and secondary coil, and because of the large area and extremely close proximity of the two, the capacitance is rather substantial, unlike a humbucker, where the coils are farther apart, and the surface area between them a lot smaller.

If the secondary coil is open, it's just some metal near the primary coil. Capacitance implies two conductors separated by an insulator. But in this case the secondary coil is not connected. So you can't have mutual-capacitance, since no current is flowing in the secondary coil. Maybe it can have self-capacitance? But again, no current is flowing, as the secondary coil is open. So what's the potential of an open coil compared to the primary coil? Anything at all?

Sorry, I'm just not seeing it.

[David Schwab]

Except that it's still a copper shield in close proximity to an inductor connected to ??? and forms a capacitor of sorts.

Not it's it's open. A capacitor has each conductor connected in the circuit, and the dielectric between them. If you lift one leg on a cap, say in a tone control circuit, what do you get? Nothing.

[David King]

Seems like this could be settled with a quick experiment. Then again that might short circuit a lot of discussion based on not totally well understood science. Carry on!