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Home : Tools : Clamps, support tools : Thompson Belly Reducer

Thompson Belly Reducer

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A lifesaving tool set for older acoustic guitars! Helps correct soundboard bulge caused by string tension, without replacing the bridge plate.

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Thompson Belly Reducer

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1380 Thompson Belly Reducer Yes

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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 set of five precision-machined aluminum tools: convex flatteners for vintage–style belly and rectangular bridges, and concave flatteners for the three most common bridge plate shapes.

They're designed for use with the bridge removed from the instrument, and are effective for bridge plates that have been installed with hide glue, aliphatic (yellow) glue, or PVA (white) glue. Simply match the appropriate flatteners to your guitar, heat them to 150 degrees F., dampen the wood on the bridge area and bridge plate, and clamp the flatteners into place for a day or so. Instructions are included (view instructions online).

T.J. Thompson recommends using three Ibex bridge clamps.

Fixing a potbelly with the Belly Reducer
Fixing a potbelly with the Belly Reducer
Fixing a potbelly with the Belly Reducer Read more

In this issue:
Dan Erlewine’s got a Martin with a potbelly: string tension has pulled the top into a hump on this 1928 0-18. To make things tougher, the guitar top’s encased in a thick layer of epoxy finish! It’s an interesting puzzle...

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 Ratings and reviews
Thompson Belly Reducer
Thompson Belly Reducer
4 out of 5 stars
Based on 13 ratings Write a Review
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Displaying 11 – 13 < 1  2
5 3-21-2011
A great tool!

I've been using the belly reducers with great success for about a year. They are well made, and straightforward to work with. They are intended to work best on guitars asembled with hide-glue, but testing on PVA glued guitars has shown good results.
Some dexterity is required while working with hot cauls inside of guitars, but they're easy to get used to.
I've also found the convex (outer) cauls excellent tools to un-cup bridges that have cupped from age or heat. Just heat up both the caul and bridge and then clamp together with just enough force. Don't overtighten though as you run the risk of cracking the bridge.
I plan on ordering a third set for our busy shop here at Folkway Music.

Mark Stutman  Ontario, Canada Read my reviews (4)
4 2-19-2011
Effective but awkward to use

Apart from the construction of acoustic steel guitars I now and again get the opportunity to repair vintage ones. Most require neck reset and bridge problems resolved. I've used the TBR on a 1931 Gibson LO and various Martins including circa 1890 2 1/2- 18, 0-28 and a 1926 0-18.

The difficulty I found is getting the inner plate in position while hot and keeping it there until the top plate is firmly clamped. Thin gloves help, the use of thick gloves is just prohibitive. Where bellying is behind bridge area on straight bridges, usual, the use of the belly bridge clamp has proved effective. Inner clamps would benefit from either a removal handle or built in positioning clamp.

Martin Pollard  Sussex UK Read my reviews (4)
Customer supplied image for review
1 10-10-2010
Save Your Money

Tried it three times with no noticeable difference. By the time you get it installed it has lost all heat content. The guitar I used it on is not severely bellied. If you closely inspect the bellying on a guitar you will see that it is primarily behind the bridge, not under the bridge.

Mason R.  CO Read my reviews (6)
Displaying 11 – 13 < 1  2 /\ Top of Reviews

Shown on page 28 of our latest catalog.
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