How to Intonate on a 3/4 Scale Guitar
Learning to set the intonation on your 3/4 size guitar gets your instrument perfectly in tune. 3/4 size guitars are generally made for children, or people with small hands, and they have a scale length of around 22.75 inches. Intonation is basically "fine tuning" your instrument, and involves checking that a natural harmonic at the 12th fret has the same pitch as the fretted note. You can generally correct this by adjusting the scale length slightly.
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Instructions
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Play a natural harmonic at the 12th fret on the low E string. See what note this registers as on your guitar tuner. If you have a tuner which tells you the exact frequency of the note, write this down on a piece of paper or commit it to memory. Lightly touch the string over the 12th fret marker and pluck it to play a natural harmonic. You should get a high pitched tone as a result.
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Fret the note at the 12th fret and pluck the string. Compare the tone produced by this to the tone produced by the natural harmonic. This is easier if you can check the frequencies. If the note is the same, then your 3/4 size guitar is correctly intonated. If it is higher or lower, you will need to make some adjustments to your instruments. Repeat this test with each string.
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Locate the small screw used to adjust the saddle position on most electric guitars. Gibson style guitars which feature a "Stop Bar" have a small bridge between it and the pickups, which has screws below each string on the pickup side. Fender style instruments have screws on the opposite side of the bridge.
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Turn the screw clockwise if the note at the 12th fret is too high and counter-clockwise if the note is too low. Turn the screw in tiny increments and keep checking the intonation. Each string has its own dedicated screw which you can turn to adjust the scale length.
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Check the action on acoustic guitars. Most acoustic guitars have a fixed bridge, which isn't easy to adjust to correct the intonation. If this is the case, check the action and neck relief before replacing the entire bridge. The action is how far the strings are from the fret board. Measure the distance from the 12th fret to the low E string. If the gap is more than 7/64 of an inch, you may need to lower the action to correct the intonation. The additional distance you have to press down the string to meet the fret could be effectively "bending" it and ruining the tone (see Resources for action adjustment guide).
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Check the neck relief if the action isn't the problem. Place a ruler over the neck of the instrument. You will be able to see how much your neck bows in relation to the straight edge. If there is too much bowing, the string may bend down too much when fretted and warp the pitch. In this case, you need to adjust the neck relief to straighten out the neck a little (see Resources for full guide).
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References
Resources
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