2017-02-07

Building a Telecaster - chapter 3, Dreaming of Wires

This being an electric guitar, it obviously needs some electronics. I've pretty much decided to go with a single coil bridge pickup and a splittable humbucker in the neck position.

Switching and flipping polarities

Truth be told, I'm a sucker for options. My old electric has three pickups in an HSS configuration, which I upgraded from simple on/off -switches to 3-position ones to allow for some serial/parallel switching and flipping the polarity (or phase, as it's often called) of one of the pickups. It did open up new possibilities...even if the number of sounds only went from 7 to 11 or something like that, the added ones were worth the effort.

For this new one, I'm torn between putting the switching behind push/pull pots and having some additional switches beside the pots. Push/pull pots really only have simple two position switches, which limits their usefulness a bit, but they can help reduce the clutter in the controls. And I'm going to have four pots already...

What I've got to work with is a neck humbucker with all coil ends available on separate wires, and a single coil bridge pickup. And a 3-way switch to select one or the other, or both. I'm aiming for splitting the neck humbucker into single coil, preferably in such a way that combining it with the bridge pickup will cancel out hum as well. But then I'd like a reverse polarity option as well, and due to its weaker sound that one at least should most certainly be hum-canceling...

Humbuckers 101
So, a humbucker is basically two coils, usually connected in series, one wound in reverse and with an opposite magnetic field. The reverse winding means any outside magnetic fields (usually from electricity) will induce noise in opposite polarities in the two coils which will cancel itself out. To avoid the same happening to the sound picked up from the strings, the magnetic field of the coils are also opposite.

Coil splitting is as simple as bypassing one of the coils, when the switch is closed both ends of B will be connected to ground and only A will sound in the picture above. Let's say coil A has its noise positive and coil B negative (N+ and N-). For just splitting into one single coil that doesn't make any difference, but combined with another pickup it will, at the latest when there's a need for the out of phase sounds. If I'd just split one of the coils off and added a polarity reversal switch, one of the positions would actually double the hum, as it'd have opposite polarity in the two positions and one of them would coincide with that of the single coil. Which means something a bit more complicated is needed.
Surprisingly simple
I had to sit down and think for a moment, but I figured it out. First I worked out an arrangement for two DPDT switches, as found in push-pull pots. One of them will select between full humbucking and one of the single coils, either A1-A2 or A1(or B2, same thing)-B1, which gives either N+ or N- in both single coil options. It'll just take a little trial and error to find out which way is the right way when combining with the bridge pickup, so actually the picture could just have two equal output leads without hinting at one of them being the "ground".

Then it occurred to me that this same switching can be achieved through a Fender-style 3-position switch, pictured in the lower left, or using an ON-Z-ON type of toggle (just move the rightmost 3 contacts to another row and you'll see what I mean). Might just go for a small toggle switch to make the user interface simple.

Z doesn't stand for Zebra
Using a 3-position toggle as drawn, A in the middle position will be the in-phase coil, which might not be what one likes. It is easier to just flick it from one side to another. That problem is easily remedied in the Fender 3-way case, though; just connect the right-hand points of the switch like: A2 B1 A2. (I don't think that order is possible with the ON-Z-ON switch).

More Circuitrickery

After this switching, the signal will go to a Gibson-style volume/tone circuit, i.e. one pair of controls for each of the pickups and a big selector switch to choose one, both, or the other. But (there's always a "but"), exactly which way should I connect those components? So many ways to choose from...

There's one way that makes the volume knobs independent of each other, which sounds like a good idea. I'm pretty sure I want that, to mix in a little bit of one sound with the other when the selector is in the middle position. What's the point of that position otherwise?

Then there's the difference between modern and 50s style tone controls, it's said that the 50s style doesn't roll off treble when you turn down the volume like the modern does, but looking at the pictures it looks to me like the tone controls will interact in the middle pickup selector position instead, as they are connected after the volume control. Looks like you can't win using passive circuits... I'll just have to pick one or the other and hope for the best, and looking at the measures I'm taking to have useful mixed sounds the choice is pretty clear; the modern wiring. Especially thinking of any out of phase sounds the separation of tone circuits from each other should be essential. There's always the "treble bleed" circuit to remedy the shortcomings of the modern wiring, I guess, but I won't bother with that until I've tried my luck without it. It's not like I've missed that feature much in my present guitar either.

The pickup selector will just connect the pickups in parallel, it could be that a series connection brings some advantages, but if so, someone else should've figured that out already and series should be more common than it is now. On the other hand, how long did it take before Fender went from 3-way to 5-five switches in Strats? And there's always the full humbucker option for series connection.

To sum it up, there will be 7 unique combinations: bridge single coil alone, bridge+humbucker, bridge+coil split in phase, bridge+coil split out of phase, humbucker alone, and two coil splits. One of the coil split positions will be pretty redundant, but all positions involving more than one coil will be hum-canceling. Not too bad, I think.

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