Electric Upright Basses (EUB) typically have some sort of piezo transducer or pickup, although there are some that have magnetic pickups as well. The Libellula bass has a piezo transducer built into its high mass high compliance bridge. The combination of this bridge and its transducer along with the high internal damping of the orchestral strings used on this instrument results in a very authentic acoustic upright bass sound. This page explains how the bridge is built.
Last updated: Saturday, August 15, 2015
The bridge cup was routed out during construction of the body, and a shallow trench was made in the floor of the cup at the same time. The rest of the components of the bridge include a piece of 0.125” hard maple for the bridge blade, some thin brass sheet, a coax piezo transducer of the type used under the saddles of acoustic guitars, and some 100% silicone sealant (black, GE #103 or equivalent). Instructions for building the coax piezo transducer are found in this article.
The bridge blade is cut to size first. The wood is oriented so the grain runs up and down. The length of the blade is left a little long at this time. The sides are rounded over. The bottom edge will sit right on top of the coax transducer in the finished bridge and so it is hollowed out a bit using a small gouge.
The piezo transducer will be glued to the floor of the bridge cup. But first a hole must be drilled into the end of the cup for the transducer's cable to exit.
A small bead of the silicone sealant is run along the trench in the bottom of the bridge cup and the coax piezo transducer is glued down into the trench. The bridge blade wrapped in waxed paper is used to hold the transducer in place while the silicone cures.
The blade and the waxed paper are removed after the transducer is glued down. Then the bridge cup is filled 3/4 of the way full with the silicone sealant and troweled around to eliminate air bubbles. While the sealant is still liquid, the bridge blade is inserted into the sealant so the blade is sitting on top of the transducer. Pushing the blade into the cup will raise the level of the sealant so it is right up to the top of the cup or a little over. The blade is lightly clamped in a vertical position while the silicone cures.
Two small squares of brass sheet are glued to the bottom of the bridge cup at the points where the bridge height adjusting set screws will contact the bottom of the bridge cup. This keeps the set screws from puncturing the underside of the bridge cup.
The bridge assembly is temporarily installed into the body so that the bridge blade can be marked for trimming. With the set screws that adjust bridge height turned so the bridge is in its lowest position in the body, a straight edge is placed on the fingerboard and the front of the bridge blade is lightly marked with the curve of the surface of the fingerboard. A line is marked approximately 1/4” above that line. The bridge blade it trimmed to that upper line.
The silicone sealant is trimmed as necessary, and the bridge blade and the top of the bridge cup are scraped and sanded clean.