Top slighly dipping between braces of 70s Sadao Yairi Classical

[deleted]

Hello all,

I have a nice Yairi guitar that has been knocked around a bit. 2 cracks along the grain of the soundboard have been repaired, but the bridge has pulled off (of its own accord and very cleanly). It still needs to go back on. I noticed that the bare wood where the bridge will go has sunken in two places: there is a 'raise' at the centre point (corresponding to the centre brace), and either side of this has sunken in just a little - let's say half a mm (but it may be less). It seems to be as a result of the bracing.

The main bracing are three long pieces of thin wood (one down the centre, and another two on both sides at a slight angle so they cross the grain a little) - there are also a number of cleats that cross the grain). There is no plate beneath the bridge.

Basically, where the guitar doesn't have a brace the top has sunken slightly. Where it is braced, there is a slight bump. It is a pretty thin top, and I am sure deliberately designed this way.

Humidity of the guitar at present is running a little over humid (above 60).

The sunken portion is not only under the bridge region, but continues in a line toward the soundhole. The 'sink' is really minor, and nothing like the usual belly bulge.

My question is: is this something I need to try to address? Or will gluing the bridge on be sufficient to maintain structural integrity?

Any thoughts?

I can provide pics tomorrow but it will be tricky to show the problem as it is quite minor.

[deleted]

I have attached a few photos

On the shot of space for the bridge the dips are those bits that are stripped back to bare wood.

The attachment dips.jpg is no longer available

[deleted]

Apologies, can't seem to edit my posts. Below is the image.
In short, the board dips slightly between the braces, either that or the braces are raised.

[kiwigeo]

No bridge patch?

if it were me I'd stick some sandpaper to the bridge area and work the base of the new bridge over the sandpaper fore and aft aiming to get the underside of the bridge matching closely with the top. Unless you have a good wood to wood contact over the base of the bridge the glue job is going to be tenacious.

[Mark McLean]

Hi Deleted (do you have a real name?),
What you are describing is called "telegraphing" of the braces. It tends to happen in bracing patterns like this one where the braces are running parallel (or close to parallel) with the grain of the soundboard, and if the soundboard is thin and flexible. In essence, the soundboard is (partly) folding itself around the rigid braces. You don't see it so much with X-bracing because then the braces are running across the grain. It may get worse if the guitar is dry. I know you say it is not dry at the moment. But it could be worth trying to humidify it even more before attempting to reglue the bridge. Do this by placing the body of the guitar in a large plastic garbage bag and tape the neck of the bag closed around the neck of the guitar. Place some wet cloth or sponge in the bag and hang it all up by the headstock for a few days (so the wet cloth is sitting in the bottom of the bag, below the level of the guitar body, not in direct contact with the wood, but just humidifying the air in the bag). Nothing to lose by doing this for a couple of days to see if it flattens out.

Telegraphing is not always a problem. Some of the really lightly built old Gibson acoustics used to show this feature. But they all had a bridge plate which kept that part of the soundboard stiff and flat, so the bridge could get good contact across its whole footprint. If your Yairi doesn't have a bridge plate, and the bridge area is wavy - you have a problem. You could try to produce reciprocal waves in the base of the bridge by doing what Martin described. Or you could try to conform the soundboard to the bridge shape using a flat caul on the inside, where the bridge plate normally would go, and clamp it up with the bridge glued on top (bridge acting as a cross-brace to pull the sunken bits up).
Mark

[pat foster]

Not at all unusual for classicals built with thin tops (some are built thicker). I believe it's less common among x-brace guitars, since they're usually steel strings with much thicker tops, and as Mark mentioned, due to the braces running close to parallel to the grain. You could make a caul that mimics the dome of the top, without the dips, and they would flatten out. I use vacuum clamping and that works well too.

[deleted]

Thanks for the good info.
My name is Cameron.

The guitar top is pretty thin. I was running a small booklight in there to find out how much light escaped from the cracks before I repaired them... I was surprised to learn that everywhere I placed the light a red dot shined through.
I couldn't replicate this on my other guitars!

Kiwigeo, thanks of the suggestion. I don't think I would be able to do that in my case. The top is too flexible; I suspect applying pressure when sanding would just flex the dips lower.

I am pretty confident there'll be no problem at all getting those dips to raise up and get good contact on the bridge via clamping.

Mark, the guitar shows symptoms of having been too dry. Even now, there's a slight gap (seen through the soundhole) between two pieces of timber for the guitar's back. It's a laminate back, though and the next piece is intact.
I have recently moved to New Zealand (wellington). I bought a hygrometer, placed the sensor in the soundhole and sealed it in, and was surprised that it was high: 65%. But it turns out Wellington humidity is usually in that ballpark (often 60-70%).
Do you still think it's a good idea to try humidifying?

re: clamping. You may notice a neodymium magnet (n42) in one of my photos - I used it to glue the cracks. I have 10 of those and am considering experimenting with them to glue the bridge on. I would cut 3 separate thin pieces of wood as cauls to fit between the braces inside the body and put some padding on each to adsorb the contours of the wood. On top of the bridge I can use spacers to determine how tightly the magnets grip (spacing the magnet more means weaker grip). I know it is unconventional, but done carefully any reason this wouldn't do a good job?

Thanks guys.
On the basis of your info I feel comfortable that the 'telegraphing' is not really a problem.

[kiwigeo]

re: clamping. You may notice a neodymium magnet (n42) in one of my photos - I used it to glue the cracks. I have 10 of those and am considering experimenting with them to glue the bridge on. I would cut 3 separate thin pieces of wood as cauls to fit between the braces inside the body and put some padding on each to adsorb the contours of the wood. On top of the bridge I can use spacers to determine how tightly the magnets grip (spacing the magnet more means weaker grip). I know it is unconventional, but done carefully any reason this wouldn't do a good job?

Thanks guys.
On the basis of your info I feel comfortable that the 'telegraphing' is not really a problem.

I don't think you'll get adequate clamping pressure from those magnets....I'd be using bridge clamps and a caul under the bridge. Careful attention to the caul(s) and clamping will be the key to a good glue up.

[Mark McLean]

Like Martin said, if you want to lift the "sagging" bit of soundboard between the braces and keep it in contact with the bottom of the bridge during gluing, nothing will do that as effectively as a long throat bridge clamp and a caul on the inside. It would be be best to have three clamps (if you can fit them in there), one in each span between braces - and short individual clamping cauls that are short enough to fit between the braces. Or you could have a single long caul which is notched to fit over the braces, so that it bears on the soundboard itself and not on the braces. I don't think the magnets will do it for you.

[kiwigeo]

For making up a caul for a guitar with top already fitted I stick a light source inside the guitar and trace the braces onto a piece of paper. This then becomes the template for the caul. I face a block of MDF with cork sheet at least as thick as brace height and then cut channels in the cork to match the braces. When the clamps are applied the clamping pressure is between the braces

[deleted]

Hi

Thanks for the tips. I thought the magnets might be strong enough. Didn't think I would need a huge amount of pressure.

But on your advice I will make some long reach clamps. I do want to do it properly!

It must be common, then, for a lamp to shine through a soundboard, illuminating the braces? That's a good tip to trace em that way, kiwigeo.

I have glued bridges a couple of times in the past (in Oz with my tools). But each time one learns more and (hopefully) gets better.

One question though: The bridge is not perfectly flat. It has a slight contour - as I believe it should to match the contour of the sound bound. When I use a caul underneath (I will pad it with left-over shoe innersole rubber) won't the caul press the soundboard and the bridge completely flat? In which case, would I lose the contour of the body?

Also: any suggestions for grit # to use when sanding the soundboard bridge space before gluing? 240 be fine?

[kiwigeo]

One question though: The bridge is not perfectly flat. It has a slight contour - as I believe it should to match the contour of the sound bound. When I use a caul underneath (I will pad it with left-over shoe innersole rubber) won't the caul press the soundboard and the bridge completely flat? In which case, would I lose the contour of the body?

Also: any suggestions for grit # to use when sanding the soundboard bridge space before gluing? 240 be fine?

Normally when I'm building I make up a convex sanding board that has the same curvature as the (concave) dish I use when gluing up the braces to the top. I work the base of the bridge over the sanding board and it ends up concave. If any fine tuning of the fit to the soundboard is required then I stick 120 grit sandpaper over the top at bridge location and work the bridge until fit is better. The caul I use under the top when I glue up the bridge also has a convex upper surface that fist against the underside of the top.

In your case if you line the caul with cork sheet you can then shape it to match the underside of the bridge by temporarily sticking sandpaper to the bottom of the bridge and working it over the cork face of the caul.

[deleted]

In your case if you line the caul with cork sheet you can then shape it to match the underside of the bridge by temporarily sticking sandpaper to the bottom of the bridge and working it over the cork face of the caul.

Cheers,

[kiwigeo]

In your case if you line the caul with cork sheet you can then shape it to match the underside of the bridge by temporarily sticking sandpaper to the bottom of the bridge and working it over the cork face of the caul.

Cheers,

If you use thick enough cork sheet on the face of your caul the cork will usually deform enough to conform to the shape of the bottom of the bridge.

[Jim]

This might help too- Stewmac tip:
https://www.youtube.com/watch?v=aWW4aKxfSJw

Good luck

Jim