5.3 Guitar Intonation / Scale Length
................5.3.2 Checking if the Scale Length is Correct
................5.3.3 Intonation / Scale Length Adjustment
................5.3.4 Adjustment - Floyd Rose Bridge
The intonation/scale length refers to the distance between your nut and the point of first contact between your string and your bridge. Have you ever found that you tune a guitar perfectly but the higher frets don't play at the pitch they should? That could be caused by incorrect scale length (although it could be just due to a high action). The frets on a neck have been designed to a specific scale length and if this is not set as it should be on a guitar, the frets will not sound the string at the correct pitch (which is more apparent higher up the neck, e.g. the 17th fret).
The scale length should ideally be set so that the 12th fret lies perfectly under the centre of the string. One way of roughly checking this is by playing the 2nd harmonic - i.e. touch your finger on the 12th fret and pick it. See the harmonics section if you don't understand what I mean (3.5 Harmonics). Once you play the 2nd harmonic, compare it to the pitch of the string when playing the 12th fret. If these differ, you probably have incorrect scale length. This should normally be done using an electronic tuner to ensure accuracy.
The other way of checking it (more accurately) is to actually measure your scale length and see where the 12th fret lies in respect to the scale length of your guitar.
The scale length of a particular string can be adjusted from the bridge.
If you are adjusting a bridge that is like this one or a Strat (which is very similar), there are usually screws behind the bridge which can be tightened to increase (or loosened to decrease) the scale length.
When adjusting the scale length, you should make sure you loosen the strings to relieve these screws of any tension to make sure they don't get damaged in the process. Also you should keep in mind that if you loosens the screws too much it is possible that part of the bridge called saddles (the metallic blocks) will become detached.
As a general guide:
If the 2nd harmonic is flat compared to the12th fret, the scale length of that string needs to be increased by tightening the screw.
If the 2nd harmonic is sharp compared to the 12th fret, the scale length of that string needs to be decreased by loosening the screw.
The intonation/scale length is adjusted in a slightly different way on a Floyd Rose Bridge.
On a Floyd Rose Bridge, each of the saddles are in fact locked down by 6 individual screws (one for each saddle). These must be loosened to allow adjustment of the saddles.
One this has been done, the saddles can be moved back and forth using these "rods" at the back of the bridge. (However before this can be done the saddle locks must be loosened.) These should not be turned however as this will release the strings from the bridge.
The actual adjustment of intonation is done in a similar way as before:
If the 2nd harmonic is flat compared to the12th fret, the scale length of that string needs to be increased by slightly pulling the "rod" out of the bridge.
If the 2nd harmonic is sharp compared to the 12th fret, the scale length of that string needs to be decreased by slightly pushing the "rod" into the bridge.