How to replace a bridge plate

[dantin]

Need advice. I have a pretty nice dreadnaught with a solid spruce top. There is noticeable belly bulge behind the bridge under string tension. The bulge almost disappears when the strings are loose.

This leads me to believe either there are loose braces or the bridge plate is not stiff enough. I checked all the braces as best I could by trying to slide the blunt end of a thin razor blade under them and they do not appear to be the problem.

I am thinking to either replace the bridge plate or try the JLD Bridge Doctor. I prefer to do the bridge plate replacement.

Any thoughts?

[mirwa]

Rotation of the bridge under tension distorts the top, the bridge plate whilst it does contribute some to the overall top stiffness, it is not there to prevent bridge rotation.

Have you got a photo of your bridge / saddle and top relationship.

Changing a bridge plate can be a tricky job, I would not advise for someone that has never done it before, most jobs I encourage but this is one I don't.

It is possible the braces may be loose, it may be possible your bridge is loose, lots of things can contribute to bridge rotation.

Bridge doctor is my least favour device, it works by bolting a block of wood to the underside of the bridge and then jamming a rod between that block of wood and the tail block of the guitar, it completely deadens the top of a guitar, reduced its ability to vibrate properly.

Steve

[redir]

Often times people thing top belly is a bad thing but in fact it could mean that you have a well built responsive guitar. Some makers believe that 2 degree bridge rotation is perfect for a responsive top. I used to build with an arch in the top and now I build flat and anticipate that there will be some 'natural' arch from string tension.

So my point is without seeing what you see it's hard to tell but it could be normal. If you take a pic with a straight edge going across the top parallel to the bridge to show the extent of the arching it would help.

If you have a small mirror and a light you can look around for loose braces, pushing on the top will expose the loose brace.

[jzach46]

To all the above, True dat! Hold a straight edge across the top immediately behind the bridge. There should be as much as but no more than 1/4" on both sides. Check it out.

Jon Z.

[ChrisN]

Originally Posted by redir

Often times people thing top belly is a bad thing but in fact it could mean that you have a well built responsive guitar. Some makers believe that 2 degree bridge rotation is perfect for a responsive top.

I've heard this before, and it makes sense. My concern is that it seems possible the twisting bulge is pulling/pushing against braces, and could eventually twist them enough to dislodge or crack them. I installed a bridge doctor in my '70s all-lam that had a bulge simply to prevent further or eventual damage. Sort-of a hijack, but am I worried needlessly, assuming no braces are loose/broken at present?

[redir]

Originally Posted by ChrisN

I've heard this before, and it makes sense. My concern is that it seems possible the twisting bulge is pulling/pushing against braces, and could eventually twist them enough to dislodge or crack them. I installed a bridge doctor in my '70s all-lam that had a bulge simply to prevent further or eventual damage. Sort-of a hijack, but am I worried needlessly, assuming no braces are loose/broken at present?

Possibly yes. You can get a small mirror and look inside to check for loose braces. Each case is different though. I'm curious to know what tonal differences you noticed when installing a Bridge Doctor? I have only installed a few. ON some guitars it makes sense, you just want a playable guitar and tone is frankly not really that important, usually cheap guitars. I installed one on a descent guitar before and the tonal change was not acceptable imho. But some people love them and claim they actually improve tone. Again, probably a case by case situation. One thing for sure though is that the B.D. does as advertised, it straightens out the top and lowers the action in worst case scenarios.

[ChrisN]

Originally Posted by redir

Possibly yes. You can get a small mirror and look inside to check for loose braces. Each case is different though. I'm curious to know what tonal differences you noticed when installing a Bridge Doctor? I have only installed a few. ON some guitars it makes sense, you just want a playable guitar and tone is frankly not really that important, usually cheap guitars. I installed one on a descent guitar before and the tonal change was not acceptable imho. But some people love them and claim they actually improve tone. Again, probably a case by case situation. One thing for sure though is that the B.D. does as advertised, it straightens out the top and lowers the action in worst case scenarios.

Re: Tonal Differences - It's my first guitar, purchased from a luthier who fixed it up - Martin knock-off OM all-lam - and to me it sounded great w/o the BD, but, even though I have several Taylors (GCs and a dread) I don't have enough experience to know. It had great bass and volume, even with a very small saddle. I installed the BD and listened for the tone changes others suggested could occur with the installation, but I didn't really notice any difference - again, though, a more practiced ear could reach a different conclusion. What killed tone on one of my Taylors was that bridge plate protector thing - even I could hear that one, and I returned it.

I found the BD actually raised the action due to the bridge rotating back to "normal."

I'm about to attempt a neck reset and may leave the BD off when setting things up, and hearing how it sounds now that I've got a little more experience. With this guitar, considering its age, I'm more concerned about keeping it intact, than getting the best tone from it, as it's the one I leave out, hence the original BD install.

Thanks for the input.

Originally Posted by ChrisN

...My concern is that it seems possible the twisting bulge is pulling/pushing against braces, and could eventually twist them enough to dislodge or crack them...but am I worried needlessly, assuming no braces are loose/broken at present?...

Your worry level is an order of magnitude too high.

[ChrisN]

Originally Posted by Todd Yates

Your worry level is an order of magnitude too high.

Roger that. Thanks. It's not worth much, but I do like the guitar and don't want something to happen to it because I didn't take action, if needed.

[Ned Milburn]

Originally Posted by dantin

Need advice. I have a pretty nice dreadnaught with a solid spruce top. There is noticeable belly bulge behind the bridge under string tension. The bulge almost disappears when the strings are loose.

This leads me to believe either there are loose braces or the bridge plate is not stiff enough. I checked all the braces as best I could by trying to slide the blunt end of a thin razor blade under them and they do not appear to be the problem.

I am thinking to either replace the bridge plate or try the JLD Bridge Doctor. I prefer to do the bridge plate replacement.

Any thoughts?

Don't use the bridge witch doctor.

The issue may not be a bridge plate issue. It could in fact be the internal bracing (or lack thereof).

Do you know, there is even a reputable company that now releases 12-string guitars with no transverse secondary braces between the main X braces in the lower bout behind the bridge?? (I have one like this in my shop right now. The proper way to fix it's belly bulge (the guitar in my shop, that is) would not be to replace the bridge plate, but rather, it would be to add the appropriate braces.

So unless there is more info, it is tough to determine.

[Truckjohn]

Tldr: The effect of the strings increasing the dome behind the bridge is normal and expected. Leave it be. If you add bracing - especially a big heavy bridge plate - the guitar won't sound as good.

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If you do some math - you can calculate the force pulling up on the top at the bridge due to string tension. The strings are trying to rip the top off the guitar.... Its about 50 lbs in the "up" direction for typical steel string guitars.. There is no magic here... Just math/physics..

That 50lb pull upward is what creates the "belly" in the lower bout.

Guitars are naturally a balancing act between durability and tone. At one end is the Electric guitar. Massively stiff and durable - yet without pickups you get no acoustic volume. At the other end are guitars built so lightly that they sound loose, flabby, and unfocused. Build too heavy and you can't hear it without pickups... Too light and they are too flexible/fragile... Warranty becomes a huge issue.

So builders build somewhere in the middle... Stiff enough to avoid "normal" warranty failures.. Fragile enough so you can hear them....

There's no free lunch. A builder can build heavy enough to nominally reduce the deflection to where it's not obvious - but that makes a super stiff and unresponsive instrument... He can modify the structure to try to spread out the belly more - but that often changes the bracing towards less of a traditional pattern - which moves away from the traditional sound...