Acoustic Problem

[sammyreynolds01]

My acoustic has developed an arch on the top i say a couple inches behind the bridge. i initially thought it was the bridge had come lose but it not the bridge. can anyone help?

[Freeman Keller]

This might be a better topic for the Acoustic forum but I'll answer anyway. "Flat top" acoustic guitars are not flat on top - they have a slight arch or dome built into them. This is done for several reasons, a dome is much stronger than a flat plate, it allows the top to shrink and expand with humidity without cracking (at least for moderate humidity swings), and the tension of the strings pulling up at the pins rotates the top around the axis of the bridge which pulls the lower bout up and pushes the area between the sound hole and bridge down. When you pluck the string the top rocks even more, that is what produces the sound.

 

So, its normal, but how much is normal? With a well hydrated guitar if you put a straightedge across the lower bout behind the bridge you will normally see 1/8 to 1/4 inch gap at the sides under the ends of the straightedge. That is normal and the sigh of a healthy guitar.

 

If you have much more than that there are several things that can be going on. If its an older guitar it is kind of the normal folding of the top under string tension - mostly we associate that with a change in neck angle but the top comes up too. The fix is to reset the neck. A second problem is a broken or detached brace - you can look around inside the guitar with a flashlight and inspection mirror and often see this, tapping on the top you can sometimes hear the brace rattle. The fix is to repair the brace (easier said than done). Another slight possibility is that the guitar is very over humidified, that causes the top to expand and push itself up. You would probably have other problems.

 

So for now, how about putting a 24 inch strightedge across and taking some pictures and measurements and tell a little more about the guitar (type, age, how you care for it). Also try working a very thin feeler gauge under the bridge on the pin side - lets make sure its really not the bridge, which unfortunately is a very common problem. And just for drill, put that straightedge on the fretboard and take a picture of the end next to the bridge. Report back

[Freeman Keller]

By definition a lifting bridge is a structural issue. The fix is to remove it completely, clean up the mess and reglue it. Unfortunately it is probably the most common structural repair that I do.

[DeepEnd]

When I talk about a "structural problem" I mean something like a bent frame vs. a dented fender. Sorry if you use a different definition.

[sammyreynolds01]

I'll have to find a straight edge. I don't have one on hand. You can see the bridge is still glued down. The reason why I asked was because the action has gotten noticeably higher. I just measured the height the 12 fret and it's about 9/64 and there's a pronounced bubble on the left side behind the bridge. I'll check and see if there's a detached brace. The guitar is a Washburn acoustic that's a bout 18 yrs old.

[mrbrown49]

Before you jump to any conclusions,check the setup. How much relief is in the neck?

[Freeman Keller]

As I've said a bazillion times before, before you do anything measure everything. But that starts with the geometry, hydration, structural issues, then all the setup stuff - you've heard it all too many times.

[sammyreynolds01]

I took it to a friend of mine and he determined that the neck is the problem and needs to be reset. He can't do it and if even if I find someone to do it, the will cost more than what the guitar is worth.He also measured the arch and said the arch isn't anything abnormal and to be honest it probably always there and I just didn't notice it. There's nothing structurally wrong the guitar inside.No loose braces or anything. Thanks for the help everyone.

[mrbrown49]

We've heard it over and over, but it really is the best course of action. I keep a spreadsheet of all my guitars and my prefered setup, and where applicable, manufacturer's recommended setup as a reference point.

[Freeman Keller]

Going rate for an average dovetail reset is $3-350 depending on what other work you have done. I've never done a Washburn so I don't know if its "easy" or "hard" (Martins are easy, Guilds are hard, Yamahas are impossible). Don't let anyone talk you into shaving the bridge - if you do and decide in the future to have the neck reset correctly it will double the cost.

 

One possibility is to find a "luthier in training" like myself - I haven't done enough resets to promise perfect results but I also need practice. There might be cosmetic flaws depending on the finish, but generally I can get the geometry dead on.

[mrbrown49]

I took it to a friend of mine and he determined that the neck is the problem and needs to be reset. He can't do it and if even if I find someone to do it' date=' the will cost more than what the guitar is worth.He also measured the arch and said the [b']arch isn't anything abnormal and to be honest it probably always there and I just didn't notice i[/b]t. There's nothing structurally wrong the guitar inside.No loose braces or anything. Thanks for the help everyone.

 

That doesn't make sense. Why would it need a reset if something didn't move?

 

Did he check the setup also? How much relief is in the neck? How worn are the frets etc?

 

[DeepEnd]

That doesn't make sense. Why would it need a reset if something didn't move?

 

Did he check the setup also? How much relief is in the neck? How worn are the frets etc?

A neck reset involves the neck angle. That can be bad even if there's nothing wrong with the bridge. It's the neck that "moved" relative to the body.

[Freeman Keller]

 

That doesn't make sense. Why would it need a reset if something didn't move?

 

Did he check the setup also? How much relief is in the neck? How worn are the frets etc?

 

MrBrown, The neck on every guitar is "set" at some sort of angle relative to the body - that is a fundimental part of its geometry. The angle might be zero in the case of a Fender, 3 and a half degrees for a LP or somewhere around one degree or a little more on an acoustic. That is done to make the string plane fall at the right place on the bridge so that the action and playability can be adjusted within a reasonable range.

 

Acoustic guitars with pinned bridges are under about 165 - 180 pounds of tension, with the ball ends of the strings pulling upward on the bridge plate. The entire top is subject to some fairly large rotational and shear forces - pulling the upper bout up, forcing the area between the bridge and sound hole down and rotating the neck and heel block around its axis.

 

Over a period of time the neck block slowly deforms, the angle changes. The guitar isn't "broken" or for that matter "warped". It has nothing to do with relief (altho that might be changing too) - relief stops at the 12th or 14th fret. But that critical neck geometry angle changes, maybe only a half a degree, and the string plane is no longer in the sweet spot. We can lower the saddle for a while to get acceptable action but at some point we need to take the neck off and readjust that angle.

 

This often happens at 25 or 30 year of age and should be a consideration with any used guitar. It is also the very first thing I check with any guitar that comes into my shop - see my little rant in your post above - if a guitar isn't structurally correct, have the right geometry and have perfect frets a "setup" is a waste of time.

 

By the way, its not just an acoustic phenomena - look at the number of telecasters with some foreign item in the pocket trying to make the bridges adjustable. You don't see it very often on solid body Gibsons with that big long neck tenon but I sure see it on these fancy new shredder guitars where the body meets the neck at the 19th fret or something similar.

 

If I was in the market for a used acoustic guitar the first thing I would always check is the neck angle. If its bad (and the owner probably knows its bad, thats why he is selling, eh?) that doesn't necessarily kill the deal but it becomes a negotiating point.

 

Last two comments - once a neck has been reset it rarely needs to be done again. And some manufacturers, Martin for example, warrants neck resets to the original owner for the life of the guitar.

[Freeman Keller]

This is a good neck angle for an acoustic. The fret plane is just touching (or a hair over) the top of the bridge, that allows a reasonably tall bridge to be installed which will provide accept action and the ability to lower it in the future as the angle changes

 

 

This guitar has a bad neck angle. Its kind of hard to see but the straightedge is below the top of the bridge. The saddle is very short so there isn't much adjustment and the action on this guitar was very high. It needed a neck reset

 

 

This is very unusual, a neck that is "over set". The fret plane is way above the top of the bridge, what that means is that to get reasonable action you will need a very tall saddle (remember that for X action you need 2X at the bridge). The cause of an overset neck is frequently a very dry guitar where the top has flattened, taking the bridge down with it.

 

 

The second and third guitars were brought to me for a "setup" - in both cases the geometry needed to be fixed first.

[sammyreynolds01]

The relief is dead straight. It's not a relief problem with the rod. Over time an acoustic the neck will put up because of string tension.

[sammyreynolds01]

That's Freeman. I only paid like $200 for the guitar 18 yrs ago. Putting 3 to 350 in it doesn't really makes sense to me. I've asked around and there's no one around that can do luthier work around here. If I still lived in the north part of the start I could take it to Memphis around here there's nothing.