Tuesday, February 28, 2012

Guitar Action Adjustments

I've been doing a lot of guitar tech work on my various instruments lately. Let me tell you a little bit about guitar string action.

I used my new Dremel rotary tool to carve a replacement nut for my acoustic bass guitar, but I'm having trouble completing the setup process.

I corrected the neck curvature to 0.012", which made a huge difference, but it doesn't seem to be enough.

Here is a diagram showing the three adjustments that affect string action--the amount one must press down to fret a note--on a guitar.

guitar-adjustments

Basically if the action is too high, the intonation will be (more) sharp because your fingers will have to push so far that they actually change the tension in the string. In addition the player's fretting hand will hurt. (More.) And if the action is too low, then the string won't have enough room to vibrate--when it vibrates it will hit the next fret up, causing a buzzing sound, which is usually unwanted.

Nuts are slotted, and the action is set by filing the slot deeper--you can only reduce it. On an acoustic, the bridge is a single piece of plastic as well, that must be sanded to the right height. You set the nut and bridge heights for each string separately. But if you file them too low, you can raise it by cramming shims of wood (or sometimes razor blades) underneath--but this possibly slightly changes the sustain and tone of the guitar. Also, shimming raises the height for all of the strings, not just the one you're working on.

Anyway, 0.012" still seems to be too high. I sanded down the bridge until the strings started buzzing, but the action is still actually way too high, in terms of feel and intonation. So I guess the curvature is still the problem, even though the general recommended measurement is anywhere from 0.012" to 0.002".

Another problem with adjusting the bridge on this particular guitar is that it has a piezo pickup in it, which consists of two thin sheets of metal with four little ceramic blocks between them, and the bottom of the bridge is funny shaped so that the blocks fit between them.

Basically what I'm saying is that shimming is out of the question, as is buying a slab of plastic and carving the bridge from scratch myself.

The curvature is adjusted by manipulating a sort of tension rod that goes down the inside of the neck, called a truss rod. I'm a little nervous about tightening it any more than have already because it's kind of an adjustment that you don't want to have to mess with much, because if you break it you've basically ruined the neck, possibly "totaled" the guitar. But I guess it's what I have to do. If I get this right then the acoustic bass will hopefully cease to be the least favorite instrument I own.