Homebrew — My Servoelectric Guitar

Craft & Design Music
Homebrew — My Servoelectric Guitar


I live near Waukesha, Wis., the birthplace of Les Paul, a pioneer in the development of the electric guitar. Inspired by this fact, I decided to come up with a variation on this iconic instrument.

The fretboard was asking for re-imagining, given that chords and chord transitions are limited for a given tuning; fingers can only reach so far. I tried motors that ran rollers up and down a fretless neck (electronic “fingers”), but was discouraged by the distracting sounds the rollers made during note changes.

An alternative was to change the tension of the strings, but it seemed unlikely that the strings would hold up for more than a few semitones of high-speed stretching. I set about trying to find the “yield point” of a guitar string, the point where tension permanently deforms the wire. I ended up with an old handbook whose data was not at all encouraging.

On a whim, I bought a package of guitar strings to make the measurements myself. What I learned with a bathroom scale and a bucket of sand was that the handbook values are way too conservative for modern guitar string technology, and with a little care, one can coax out a full octave of note changes from standard guitar strings with a re-tensioning range that can be sustained indefinitely.

I was in business. I combined surplus Japanese gearmotors, slide pots from an audio mixer board, screen door springs, high-tension deep-sea fishing line wrapped around the motor shafts, and pulleys made from skateboard bearings, to make a guitar that provides rapid chord changes by changing string tension as you play.

The latest version, shown here, adds an Arduino controller that, over time, learns from its tuning errors to improve its tuning precision and speed, and a special nonlinear spring configuration that improves the tuning accuracy for low notes.

Because the transition between notes is smooth, it has a kind of gliding sound like a pedal steel guitar; but unlike a normal guitar, the notes don’t all slide together or in the same direction. It sounds kind of haunting, unambitious, slack, laid-back.

Of course a guitar is not all about sound; it’s also about looks. The web taught me how to bend masonite with a steam iron, sand it glassy smooth, and put on a dozen coats of lacquer to produce the familiar glistening curve we associate with this enduring musical instrument.

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Keith Baxter

Keith Baxter is a patent attorney practicing in Milwaukee, Wis. The design for this project is open source and described in more detail at servoelectricguitar.com

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