Excessive soundboard "bellying," caused by string tension, can make an acoustic guitar difficult to play. It can even damage the top, bridge plate and bridge. Replacing the original bridge plate to correct the problem is a time–consuming job that can unfortunately reduce the value of a vintage instrument.
Respected luthier and restoration expert T.J. Thompson developed a simple tool set, the Thompson Belly Reducer, that can fix soundboard bulging without permanent alterations to the top or bridge plate.
The Thompson Belly Reducer is a simple tool designed to get rid of the bellying without making permanent alterations to the original guitar top. This tool is a lifesaver for many vintage guitars, conserving their ever-increasing value. They are effective for bridge plates that have been installed with hide glue, aliphatic (yellow) glue, or PVA (white) glue.
This set includes 5 precision-machined aluminum tools, for the most common bridge and bridge plate shapes:
Instructions for use
1. After bridge is removed, select the proper bridge and bridge plate tools based on the footprint of the bridge and the size and shape of the bridge plate.
2. Heat the two parts to 150 degrees; immersing them in the water of a hide glue pot will work perfectly.
3. Dampen the exposed bare wood of the bridge area on the guitar top and the bridge plate inside the guitar with warm water using a sponge or paper towel. Check for loose areas of the original bridge plate. Inject warm water in gaps to reactivate original hide glue.
4. Place the bridge template on top, convex side down. Gloves will be necessary.
5. Hold the bridge plate flattening tool in place on the bridge plate with concave side facing up.
6. Insert a bridge clamp through soundhole and clamp the two parts together, maintaining alignment of both tool parts.
7. Finish by adding two more clamps through the soundhole, spaced evenly. Tighten all clamps as much as possible.
8. Leave the assembly clamped in place for at least 24 hours. In cases of severely bellied tops, longer clamping time may be necessary.