THE PROBLEM

I have always believed that a guitar that plays in tune sounds better. This probably sounds like a given, but I have played on some very expensive guitars, built by world renown guitar builders, that have marvelous sound. However, due to intonation problems, which are unfortunately inherent in classical and Spanish guitars, unless some sort of compensation is built into the design, even these guitars can be painful to play.

The problem stems from the fact that the strings have different thicknesses, and therefore do not stretch the same amount over a given scale length. The higher up the neck a string is fretted, the worse the problem gets. The main culprits are the 3^rd and 6^th strings, these strings being the thickest.

In order to compensate for these different stretch characteristics, the vibration length, or the distance between the string break points on the nut and the saddle, needs to be increased for the thicker strings. Since it is impractical to have a different saddle for every string on a classical guitar, the saddle needs to be altered to suit every string, if proper compensation is to be obtained.

The same problem that is encountered higher up the neck, also occurs on the nut end of the fingerboard, although not always as defined. The notes on the first few frets will play sharp, and subsequently the nut position also needs to be altered.

MY SOLUTION

 The way I compensate for these intonation problems, is as follows:

1. When I slot the fingerboard, I cut a zero fret slot before I cut the 1st fret slot. Once I level off the nut end of the fingerboard, I file the end of the fingerboard level or flush with the inner edge of the zero fret slot. In other words, the distance between the zero fret slot and the 1^st fret slot is now shorter by exactly half the width of the slot, or 0.30mm (0.012”), the slot being 0.60mm (0.024”) wide. I then remove another 0.50mm (0.020") from the end of the fingerboard.

Once I adjust the nut height, I ensure that the strings clear the top of the 1^st fret by between 1.0mm (0.040”) and 1.40mm (0.056”), depending on the type and tension of the strings that I use. This ensures that the nut compensation is adequate to ensure correct intonation on the nut side of the fingerboard.

2. When I glue the bridge to the soundboard, I measure the distance from the middle of the 12^th fret to the front edge (sound hole side) of the saddle slot in the bridge, ensuring that this distance is 327.0mm (for a 650mm scale length guitar).

3. Once I have strung up the guitar, I now adjust the string height above the 12^th fret to 3.50mm (0.138”) for the 1^st string, and 4.50mm (0.177”) for the 6^th string, by sanding off the bottom of the saddle at an angle, taking off more on the end of the 1^st string.

4. I now bring the strings up to pitch, and compare the harmonics and fretted notes at the 12^th fret, testing each string from the 1^st to 6^th string. The 2^nd and 3^rd strings, as well as the 5^th and 6^th strings, will normally sound sharp.

5. I now push the string aside, and file a small angle on the front edge of the saddle, exactly where the particular string breaks over the saddle, thereby moving the break point on the saddle away from the sound hole, and lengthening the string vibration length. I do this with every “offending” string, testing the harmonic and fretted note at the 12^th fret in between, until the harmonic and the fretted note match.

6. Please note that string tension, type, and manufacturer will result in more, or less, compensation required, so changing strings to higher or lower tension, or a different manufacturer, will most likely require adjustment.

7. Below is a photograph of an accurately compensated saddle.