Guitarhacking

Handmade Fender style 'casters

The Tone-caster

Over 2019 I worked on several guitar projects, the Relicaster and the Hotrod, and I jokingly started referring to these projects as two of the guitars in the Vintage Trilogy. There were vintage aspects to each, faux relic’ing on one and an old hotrod inspired paint job on the other. In addition, the both were fitted with maple necks and a satin finish with a ‘vintage’ tint.

Then when 2020 began and it became clear the virus was going to alter our lives for some time, and I had some time on my hands, I started thinking about building a real third leg for this vintage trio. It would include a vintage tinted maple neck, like the others but what about the rest?

Eventually I came to rest on the idea of a black body with a cream binding and gold hardware. To me it was reminiscent of the 1960s/70s black Gibson Les Paul which is so iconic, and the gold hardware accentuates the look. But like all guitars I make, I needed something to set the guitar apart. Then I happened on the Lace Fingerburner humbuckers and I thought they’d be a great contrast with the traditional look I was seeking. Next were the electronics.

I happened to stumble across an article about the Fender TBX tone control system used on some earlier Clapton Stratocasters. What, in particular, caught my eye was the custom potentiometer Fender used, and which could still be ordered today for roughly $10.

This potentiometer had a center detent and could be turned left and right. With the shaft facing away from you and the lugs facing up, the pot closest to you is a 250K pot and the other pot is a 1M pot. In the detent position, the 250K pot is open, when measured between the left lug and the center lug.

The same detent position on the 1M pot measures 0 Ohms resistance.

From this position, if the shaft was rotated clockwise the resistance between the lugs on the 1M pot would rise from 0 to 1M while the 250K pot remained open. If the shaft was rotated in the opposite direction, the resistance between the lugs on the 1M pot remained at zero while the 250K pot changes from open to 250K, then decreasing in resistance as the shaft is rotated.

This seemingly strange behavior was actually brilliant! It meant one could build a circuit in which while the 1M pot was in use, the 250K pot was disconnected, and vice versa. And, with the device set in the center detent, it could effectively remove itself from the circuit!

So I started trying to design a circuit in which turning the shaft one direction cut the treble of the guitar and turning the shaft in the other direction cut the bass response.

Everyone who takes electrical engineering in college studies RC circuits and passive filters in their first or second year and so I expected this would be fairly easy to design. My first surprise came when I tried to calculate the resistance and capacitance values. The formula to calculate the cutoff frequency, the frequency at which a low-pass filter begins to attenuate higher frequencies (your standard tone control), is equal to

frequency = 1/(2* Pi* RC)

If you enter typical values for an electric guitar tone tone circuit, you’ll see the cutoff frequency this formula produces is about 29 Hz. Clearly that is not correct because signals below 30Hz are not even audible for most humans. As I looked into this I realized that we were not talking about simple RC circuits, as I had naively assumed. The pickups are inductors, the volume control is in the circuit, and the overall situation is vastly more complicated than I was able to calculate. So I was forced to simply pick values commonly used in guitar building and hope for the best. For the bass cut circuit, I used the 1M pot and a 0.0047microF capacitor. For the treble cut circuit, I used the 250K pot and a 0.047microF capacitor.

This was the circuit I eventually came up with.

You will notice that the bass and treble capacitors have been crossed out and re-entered. After building the guitar and testing the sound, I was disappointed to find the effects very subtle and barely noticeable. At first I thought I had mis-wired the circuit but eventually I decided to try different capacitor values. Eventually I landed on a value I liked.

The bass cut effect is actually quite nice – it cleans up the sounds, especially on the neck pickup, making it less muddy and somewhat clearer. The treble cut is not a tone I am likely to use much, it just makes the pickups sound a little dull.

Anyway, here are some shots of the guitar build in action. Wiring it up.

Then debugging the circuit which didn’t seem to work.

The Fingerburner humbucker!

Finally assembled and set up to play.

And the Trilogy, all together!

This may be my final project. I no longer find as much amusement in designing and building as I used to. Perhaps I will focus on playing from now one.

Ciao.

Categories: music

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  1. The Psychadelicaster – Guitarhacking

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