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I have noticed something peculiar about 2 of my guitars. When looked at in reflected light, the contour of the top in front of the bridge distorts, the bridges are twisting the top down towards the soundhole. If you look at the reflected image of the window you can see this contour. The Western Classic has a spruce top and my 1938 SJ-200 has an Adirondack top which does not show any of this contour. It is stiffer so I guess that is why. The J-45 Custom pictured also has spruce top, but my Southern Jumbo which is also spruce does not show this contour either. The only thing that is different about that pair are the bridges, the J-45 Custom has a bridge with the pins far back, silmilar to the 4 bar bridges on my Western Classic and SJ-200. I wonder if this bridge geometry contributes to the rolling of the bridge forward. Has anyone seen this kind of deflection in their top?

 

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PS: This Photobucket nonsense is not really worth the hassle anymore. When Is Gibson going to get an adequate forum that can handle downloading pictures?

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And all this time I thought distortion in regards to a guitar had to do with sound. In all sincerity, I hope it's really nothing to worry about. If one is distorted and the others are not, is the distorted one better and the others have issues, or is it the other way around? I really doubt I'll be checking any of my guitars. Too busy playing them. [thumbup]

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The coupling of string tension pulling up at the pins and applying a downward moment over the top of the saddle produces torque on the bridge. The back edge of the bridge is pulling up, and the forward edge is pushing down. This can cause a hump behind the bridge, and a depression in front of it. This is exacerbated by heavier strings.

 

This is just part of the physics of a guitar. The smaller the bridgeplate, the more pronounced the effect might be. Wide x-bracing could also make it worse.

 

As long as there is not a pronounced depression in front of the bridge, it is unlikely to be a problem.

 

I just pulled out my two slope-J's, one a 1948 and the other from 2007. Both have slight doming just behind the bridge--which is actually part of the design, if you look at the blueprints--and are almost dead flat in front of the bridge. There probably shouldn't be a real depression in front of the bridge.

 

Rather than relying on reflections, just pull out a 6" straightedge and check it. A deflection of 1/16" over 6" between the front of the bridge and the front of the soundhole wouldn't bother me at all. Maybe even a little more.

 

Adirondack spruce and Sitka spruce have a similar Young's modulus, which is one of the inputs used in calculating the stiffness (resistance to bending as a result of an applied force) of an object. That may vary a bit because of differences in physical properties between two pieces of nominally-identical material. Any structure added to the base material (like the bracing) is another input that determines the stiffness of the top.

 

As Tom Barnwell says, guitars distort, and they are generally designed and built with that in mind. There's a compromise between building a top that doesn't bend, and one that can't vibrate properly.

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The coupling of string tension pulling up at the pins and applying a downward moment over the top of the saddle produces torque on the bridge. The back edge of the bridge is pulling up, and the forward edge is pushing down. This can cause a hump behind the bridge, and a depression in front of it. This is exacerbated by heavier strings.

 

This is just part of the physics of a guitar. The smaller the bridgeplate, the more pronounced the effect might be. Wide x-bracing could also make it worse.

 

As long as there is not a pronounced depression in front of the bridge, it is unlikely to be a problem.

 

I just pulled out my two slope-J's, one a 1948 and the other from 2007. Both have slight doming just behind the bridge--which is actually part of the design, if you look at the blueprints--and are almost dead flat in front of the bridge. There probably shouldn't be a real depression in front of the bridge.

 

Rather than relying on reflections, just pull out a 6" straightedge and check it. A deflection of 1/16" over 6" between the front of the bridge and the front of the soundhole wouldn't bother me at all. Maybe even a little more.

 

Adirondack spruce and Sitka spruce have a similar Young's modulus, which is one of the inputs used in calculating the stiffness (resistance to bending as a result of an applied force) of an object. That may vary a bit because of differences in physical properties between two pieces of nominally-identical material. Any structure added to the base material (like the bracing) is another input that determines the stiffness of the top.

 

As Tom Barnwell says, guitars distort, and they are generally designed and built with that in mind. There's a compromise between building a top that doesn't bend, and one that can't vibrate properly.

Fine answer! Was going to throw in 5 cents' worth about parabolic tops, but now there's no need☺

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And all this time I thought distortion in regards to a guitar had to do with sound. In all sincerity, I hope it's really nothing to worry about. If one is distorted and the others are not, is the distorted one better and the others have issues, or is it the other way around? I really doubt I'll be checking any of my guitars. Too busy playing them. [thumbup]

Norlin era Gibsons often exhibit top issues, in part because of a flawed (insane?) attempt to make guitars with actually 'flat' tops. I quit being too concerned about distortion issues long ago.

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No, I have 7 acoustics, only 2 distort. Maybe you should put more thought into your quick answer.

Well we have about 200, and they all distort. Anything that was built so heavily that there was no distortion at all would sound like playing a brick. If you don't think so, change from light to medium strings and measure the string heights.

 

It is true that the amount of distortion can certainly be different -- but they all distort. Stew Mac sells a tool set to measure the distortion under string tension so you can get the neck pitch to correctly match the top distortion of the particular guitar.

 

Let's pick,

 

-Tom

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Radius Top

The top of many “flat-top” guitars are under a lot of stress from the pull of the strings, which can eventually compromise the top. So, while most acoustic guitars are true “flat-top” guitars, all of the acoustics produced by Gibson in Bozeman, Montana have a radiused, or “tuned” top. Instead of being perfectly flat, a radiused or “tuned” top is raised slightly, and a special instrument is used to shape the top braces to the radius of the top. This process adds tension and strengthens the top, creating a less stressful joint where the top meets the sides and reducing the stresses of string pull. It also results in a “speaker cone” effect that maximizes sound projection, adding a significant boost to mid-range levels for a more balanced acoustic tone.

 

 

 

JC

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Every properly built acoustic I've owned has, over time, developed a touch of top distortion. I once owned an early '90s J45 which had very noticeable bridge roll and bellying, and was the best modern era J45 I've ever played.

 

Some distort more than others, but they eventually settle down and providing there are no issues with playability (something I've never encountered as a result of top distortion) there is absolutely nothing to worry about.

 

I understand the feeling of uncertainty and worry about it but it's all normal. Your SJ200 will be more prone to it than your SJ as the top is a larger expanse of Spruce and as a result is marginally less resistant to string tension torque. I've always kept my jumbos strung with medium lights (12-54) which are a little kinder to the top than full mediums, with this in mind.

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Every properly built acoustic I've owned has, over time, developed a touch of top distortion. I once owned an early '90s J45 which had very noticeable bridge roll and bellying, and was the best modern era J45 I've ever played.

 

Some distort more than others, but they eventually settle down and providing there are no issues with playability (something I've never encountered as a result of top distortion) there is absolutely nothing to worry about.

 

I understand the feeling of uncertainty and worry about it but it's all normal. Your SJ200 will be more prone to it than your SJ as the top is a larger expanse of Spruce and as a result is marginally less resistant to string tension torque. I've always kept my jumbos strung with medium lights (12-54) which are a little kinder to the top than full mediums, with this in mind.

That's a noteworthy point. There are folks out there who tend to assume that a really big guitar should be able to tolerate heavier strings than someone might use on, say, a 14-fret 000. It ain't necessarily so!

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Thanks for the many helpful replies. I do use Mediums 13-56 on all my guitars. I do think it is the bridges with the pins far back that is doing this. The further back the pins are the more leverage there is when they pull up. My SJ-200 with Adirondack certainly has a stiffer top, this can be heard, and does not show this bridge rolling. For one week I had an SJ-200 Standard with the 2 Bar bridge and I did not see this effect. Its pins are not as far back as the 4 Bar bridges.

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Thanks for the many helpful replies. I do use Mediums 13-56 on all my guitars. I do think it is the bridges with the pins far back that is doing this. The further back the pins are the more leverage there is when they pull up. My SJ-200 with Adirondack certainly has a stiffer top, this can be heard, and does not show this bridge rolling. For one week I had an SJ-200 Standard with the 2 Bar bridge and I did not see this effect. Its pins are not as far back as the 4 Bar bridges.

 

"It's not a tumor."

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Radius Top

The top of many “flat-top” guitars are under a lot of stress from the pull of the strings, which can eventually compromise the top. So, while most acoustic guitars are true “flat-top” guitars, all of the acoustics produced by Gibson in Bozeman, Montana have a radiused, or “tuned” top. Instead of being perfectly flat, a radiused or “tuned” top is raised slightly, and a special instrument is used to shape the top braces to the radius of the top. This process adds tension and strengthens the top, creating a less stressful joint where the top meets the sides and reducing the stresses of string pull. It also results in a “speaker cone” effect that maximizes sound projection, adding a significant boost to mid-range levels for a more balanced acoustic tone.

 

 

 

JC

 

 

Is the radius built in the top primarily behind the bridge?

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Is the radius built in the top primarily behind the bridge?

 

The top is actually domed about 1/8" (radius both transversely and longitudinally), but the radius is not constant. The high point (and I'm taking this off the construction drawings) is 2" or so behind the bridge, where the top in the center is about 1/8" higher than it is at the edges. Longitudinally, the dome tapers to zero at the tailblock and zero at the neck joint, proportionally to those distances (about 6" from high point to tailblock, 14" from high point to neck joint). This can give the impression of a pronounced hump behind the bridge, but a flat top forward of the bridge.

 

The back is domed as well, but dramatically more than the top.

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