Sigma soundboard belly advise sought

[Jerry Ryan]

I'm a novice in these repairs (still don't charge for work, but someday...). Learning from books recommended here, Frets.com, StewMac Tips and from posting a few times, always received good advise & input. Every project requires more tools, new supplies!!

Question: Where to start? A friend brought me an nice old Sigma dreadnaught guitar with a bad soundboard belly issue. Typical from what I have seen, dips ahead of the bridge visably, about a 1/4"+ rise behind the bridge.No cracks, finish is still good. Bridge is lifting from the back, the outside edges are mostly still attached. I used my IO camera to inspect the inside, bracing is all solid, bridgeplate look good, but thin apx 1/8".

I have used the JLD brace for similar issues, on 'run of the mill' guitars, but not as dramatic a belly as this one. However, I know there are better ways and someday I'll use my skills on 'Good' instruments. My reading says to start by removing the bridge and replacing or re-inforcing the bridge plate?

[Greg Mirken]

Is this a ply top guitar? Look at the edge of the soundhole for the telltale dark middle layer. Because that middle layer comprises about half the thickness of the top, and its grain orientation is side-to-side, ply tops are prone to distortion from string tension. A Bridge Doctor may help, or at least stabilize it, but otherwise it's not worth whatever heroic measures you might have in mind, in my opinion.

[Jerry Ryan]

Thanks for responding.

The soundboard is solid spruce, reasonably fine grain. I try to fix these cheaper instruments in the same way I should an old Martin. Gotta learn, practice methods, techniques.

Would you replace the bridge plate or ....?

[Greg Mirken]

Yup; pull off the bridge and bridge plate. Clamp that sucker flat and put on a new plate (you might orient the grain parallel to the top grain while you're at it).

[Antonio Cotichini]

Greg, why parallel? Wouldn't be, perpendicular, a stronger joint? Just curious.

[Greg Mirken]

Frank was the first person I heard suggest this- for the same reason the top grain goes lengthwise. Being stiffer along the grain than across it, the bridgeplate will better resist the torque of string tension with that orientation. Also less apt to crack between the pin holes.

[Jerry Ryan]

Thanks!

I'll get 'er done! I have some rock maple for the plate, anything better?

Ideas on clamping flat? I have some thick plexiglass plates I have used before to flatten cracks in a soundboard.

Jerry

[Mike Kolb]

(hmmm, I think we've been down this street before, but here's my 2-cents).... When I replace a bridge plate, maple is used and the grain orientation is 45-degrees to the top. This (IMHO) captures the best of both worlds... the strength of cross-grain, coupled with the lack of a tendency to split along the pin-hole line. Haven't had any come back yet....

[Jerry Ryan]

Thanks for the input.

Question on clamping was in reference to the belly zone (area behind the bridge) Is the belly reduced by clamping that area flat? Or is the bridge area alone dealt with?

[Antonio Cotichini]

Yes, Greg, what you say makes perfect sense, but also Mike's 45 degrees theory is interesting. Should be the perfect compromise between parallel and perpendicular advantages! Anyway, I don't want to start a new argumentation here.
Thanks

[Jerry Ryan]

Thanks Guys,

I did find an explanation of clamping the warped soundboard warp in the bridge plate process. The parallel orientation makes the most sense in creating resistance to the desire to bow/warp

Jerry

[Ned Knepp]

Not a recommendation. Just a question. If the issue is grain direction, why not use a very thin sheet of plywood, at least on an inexpensive guitar? The hobby shops carry some very thin, three ply, birch plywood.

Ned

[Jerry Ryan]

Ned:

I want to learn the best way, the right way to solve the bellying issue. So I can repair a good Martin or Gibson someday using the technique and materials recommended by experts, and know I can fix it.

Jerry