Non- Adjustable Bridges (Pictured above) -Mandolins with non-adjustable bridges (solid wood without thumbwheels) must be sanded and carved in order to lower the action at the bridge. When a great deal of adjustment is necessary I like to distribute the removal in order to maintain a balanced look, a small amount from the bottom, a small amount from the top. The compensated top on some will discourage a great deal of removal, as will the pickguard mounting hole near the base.
If more height is necessary due to a sinking top or overset neck I will often laminate wood to the base in order to build height and retain the original bridge.
Thumbwheel Style
Thumbwheel Bridges - Bridges with thumbwheels are considerably easier to adjust. The thumbwheels are used to raise or lower the top portion of the bridge thereby raising or lowering the strings height above the frets.
When attempting to set the action at the bridge of a mandolin with one of these bridges it is far easier to start by placing the bridge too high and bring it down. After a Mandolin is strung to pitch there will be so much tension bearing down on the bridge that adjusting the thumbwheels up will take bionic strength and finger tips of steel, yowee! And don't let me catch you with a pair of pliers in your hands. A lapse of common sense and impatience can provide hours of heartache. BTW, there's no such thing as "touching up" a sunburst finish, someone out there knows what I'm talking about.
* Ideally the feet of the bridge should contact the top across their entire foot surface. Good coupling normally equates to better sound.
* Strings should not be deeply "inlayed" into the bridge. The strings will of course wear the notches deeper over time but once the bass strings sit below the top surface of the bridge it's time to do a little sanding and remove a little mass.
*A bridge that has been shimmed or adjusted exceedingly high may be indicative of an instrument that has an incorrect neck angle or one who's top is sinking.
Replacing Bridges On poor quality thumbwheel bridges the top portion of the bridge may split and collapse. Ideally the grain's orientation should run parallel with the base to ensure adequate strength, but many inexpensive replacements and instruments come equipped with cheaper bridges that need replacement.
New bridges must be sanded or carved to fit in most cases. The feet of the bridge needs to match the curvature of the top and shaping is necessary. Also, if one is replacing a very short bridge the new one will need to be altered to maintain the action height.
Intonation
The position of the bridge on the top of the mandolin effects it's intonation as does the overall string height (action).
Intonation- Proper intonation gives an instrument the ability to play in tune while moving up the neck. The position of the bridge on the top affects the strings overall length and therefore affects the mandolins intonation. Compensation or adjustment of the strings scale length is necessary to properly intonate any instrument.
The twelfth fret is the center point of your scale and one octave higher than the same string played open. If the fretted note is sharp, then the bridge must be moved away from the fingerboard, lengthening the string. If the note at the 12th fret is flat, then the bridge would be moved towards the fingerboard, shortening the strings length. The tricky part to adjusting mandolin bridges comes from the great amount of pressure holding them in place. They usually don't budge easily, and great care must be taken when moving them so the top finish is not damaged.
Intonating Your Mandolin
For beginners Before making intonation adjustments to any instrument, one should make any necessary adjustments to it's action first. Changing the action (height of the strings above the frets) can affect intonation as the strings height above the frets change the strings stretch. (higher action=more string stretch=sharper notes). A poorly cut nut slot can also either change the strings length or create excessive sharpness if the string sits too high above the first fret.
After any necessary action adjustments have been made we proceed with intonating the instrument. What I normally recommend is that you tune just the outer E and G string to pitch first. Without the other 6 strings tuned to full tension you will be able to "scooch" the bridge back and forth much easier. Adjust the bridge angle for proper intonation of these 2 strings using the above method. The treble side of the bridge will ordinarily sit closer to the fingerboard than the bass side. This angle is needed to compensate for the fact that we have a much larger diameter string on the bass side than the treble. Tune the instrument and check the tuning at the 12th fret.
For the advanced Fine tuning simply requires a bit more time and patience. After bringing the instrument to full pitch and perfectly tuned, fret each string on the 12th fret and make a note of it's tuning, whether it be sharp, flat or dead on. Most bridges will sit on the top slightly slanted with the treble side closer to the fingerboard than the bass side. If we can not adjust or angle the bridge in a way to get perfect intonation on all 8 strings (highly likely) then I will strive to position it in a spot that achieves the greatest majority. So if I can have 6 perfectly intonated I'll leave the bridge there and adjust the other 2 differently...when possible.
So lets say our initial findings show the A's, D's and G's to be sharp and the E's to be right on. We'll then move the treble side of the bridge further from the fingerboard in order to lengthen the string and flatten the sharp intonation.
Once that adjustment has been made (and you may need to detune in order to move the bridge safely) we check our intonation again. Once we are satisfied that we have arrived at the best compromise we look at adjusting the bridges string slots rather than it's position.
The ability to dial in our intonation will depend on the width of the top of the bridge where the string lies as we will be altering the strings contact point for and aft of center. In effect we are changing the point at which the string notch peaks. By cutting and angling those string notches we are able to slightly increase or decrease the strings length depending on the overall width of the bridge. This is the exact same concept and purpose to having a compensated bridge.
Once correctly positioned many players opt to replace strings one at a time to avoid moving the bridge. Ugh...Ya think?
Other factors that may alter your instruments intonation are:
