How to adjust that old beater acoustic guitar for the best balance between playability and sound
You're visiting relatives in the heat of summer, bored out of your mind. Turns out some cousins have a banjo and a washboard. They're no great shakes, but they’ve got music in their veins. A jam session sounds a whole lot better than listening to your aunt talk about her irregularity. Diligent inquiry turns up an old flattop from the back of a closet. It looks OK but plays like a dog. With a few clues and a little courage you could get that thing up and running. Here are the clues:
Don't Expect Miracles
In general, setting up an acoustic guitar is a trade off between sound and playability. By and large, big strings and high action make better sound and worse playability. Don't start with the delusion you're going to set the thing up to play like a cherry Strat. It ain't gonna happen. Slap a set of .10s on that big ol' flat top and you're going to have problems. The head will pull back, the strings will be too close to the neck, and you'll have buzz city. So be reasonable. Light acoustic strings—say .12 to .54 gauge—are about the lightest strings you can go with and still get a decent tone out of your soundboard. Remember, all of your sound is coming from the mechanical action of those strings vibrating and transmitted by that board. You need some tension and some leverage—you need decent sized strings.
Check For Big Problems
Tune the guitar to pitch, leave your hand off the neck, and do some hard strumming on the open strings to listen for buzzes. If the strings are buzzing hard against the frets, you've got trouble. Either some idiot has cranked the truss rod way too hard (and probably damaged something inside) or the neck is majorly warped. You can try loosening the truss rod, but you'll probably have to take it to a pro (where the prognosis probably won't be favorable).
If something is buzzing around the headstock, be sure it's not just an uncut string, then make sure the nuts that hold the tuners on and the screws on the back are tight. If there's a buzz on the soundboard, it's either a foreign object inside, a loose pickguard, or—more likely—a loose or cracked brace inside. If you can find the problem with a compact mirror and a flashlight, you may be able to glue it up yourself with wood glue or even the gel type of cyanoacrylate (Super Glue). The advantage of the cyanoacrylate is that you can hold the thing tight till it dries (but be careful not to glue your fingers to the inside of your guitar). If in doubt, take it to a pro.
Straighten The Neck
The truss rod is located inside the neck in a groove beneath the fretboard. It runs the length of the neck and when you tighten it you pull the headstock back causing the middle of the fretboard to come closer to the strings. In most acoustic guitars you'll find the part of the truss rod you're after inside the sound hole just beneath the end of the fretboard. You'll need the right size Allen wrench to adjust it.
Point the head of the guitar away from you with the body at eye level and sight down the fretboard. Ideally you will see no curvature to the fretboard at all. If there's a bow so that the middle of the fretboard is visibly farther away from the strings than the ends, tighten the truss rod in 1/4 turns clockwise until the neck looks straight. If there are obvious twists or bumps in the fretboard, you may have to take it to a pro. Whatever you do, DON'T start filing frets, you'll just get yourself into a world of trouble.
If the neck is bowed so that the middle of the fretboard is closer to the strings than the ends, try loosening the truss rod. If this doesn't make an immediately visible change, you're probably out of luck. In any event DON'T LOOSEN OR TIGHTEN THE TRUSS ROD MORE THAN A COMPLETE TURN! If you break something in there, your guitar will need major work.
To check your neck adjustment, press the low E string down at both the first and last frets. There should be a tiny gap—around a 64th of an inch—between the string and the middle frets. Repeat the same process on the high E string. This should give you a clue if you're in the ballpark and also reveal any special problems. Don't be terribly alarmed if the gap is somewhat bigger; it might be because the fretboard takes a bit of a turn where it meets the top of the guitar.
Adjust The Saddle
Sighting down the neck again, see how far you can press your high E string down toward the body and still have clearance off the fretboard. If you can go more than an eighth of an inch or so, and the neck looks straight, you can probably bring the action down a bit without ill effect. But if it plays just fine for you, leave it alone. A little room for the strings to move is a good thing. As a general guide, you want the high E string about 5/64ths of an inch off the 12th fret and the low E about 7/64ths. But use your own judgment. If it buzzes too much, bring it up. If it's too hard to play, bring it down.
The piece of hard white plastic that inserts in the little slot on the bridge of your guitar is called the saddle or the bridge nut. Don't do anything to the top of this, you'll only screw it up. To bring the action up, you'll need a shim under the bridge nut. You can make one of these out of any number of thin, flat substances. I usually use the top of a plastic food tub. With a sharp knife, cut a piece out of the lid that’s the exact dimensions of the bridge nut's footprint. Tuck it in the slot under the bridge nut and tune back up. If the buzzing is gone, you're done. If not, try another shim. If that still doesn't work, try loosening the truss rod a little.
If the action can stand to go down, you'll have to file the bottom off the bridge nut. Being very careful to hold the bottom of the nut perfectly flat, file it down uniformly across the bottom. Don't take off more than about a 16th of an inch before trying it to see how it works. If you take too much off, you'll have to shim it. If you totally destroy it, don't despair; bridge nuts are pretty easy to come by. Whatever you do, don't try to lower the action by filing the grooves in the head nut (at the top of the fretboard). Other than widening the gap slightly for a string that's really sticky, you generally want to leave head nut adjustments to a pro.
Don't get obsessive. You'll always have SOME string buzz, that's the nature of fretted instruments. And you'll never get the action extremely low without more string buzz. It's not an exact science, but it's also not rocket science. With careful, small adjustments almost any old guitar can be made to play and/or sound better without resort to a professional.
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