JLD Bridge Doctor for ABG

[The13thFret]

Hi All

Has anyone tried a, JLD Bridge Doctor on an acoustic bass guitar?. I am wanting to correct a mild belly bow/bridge height issue, a search here on TB only found info relating to acoustic guitars.
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There are two types, one that uses brass replacement bridge pins, I guess converting the guitar to a pin less bridge, which is recommended for 12 string, however it seems unsuitable for bass guitar. And a screw in type that looks like it would work in theory.

No mention of ABG in any of the literature I can find online.

Cheers

 

[202dy]

The Bridge Doctor works. Use the screw down type.

The question that really needs to be answered is, "Why does the top have a belly?"

Acoustic guitar bridges are in rotation by design. (Some, like classical guitars and the Ovation style bridges much less so.) The braces should counteract these forces. Sometimes the belly occurs because the braces are loose. Either they were not glued properly at the factory (starved glue joints) or an environmental issue caused the glue to let go. If this is the case, then the correct procedure is to re-glue the braces.

Other times the problem occurs because they used the wrong materials to make the guitar.

• The top isn't stiff enough
• The braces aren't stiff enough
• Plywood top
• Plywood bridge plate - this one is very common

The bridge plate is a piece of wood that is located directly below the bridge beneath the top. The bridge plate and the bridge create a sandwich with the top in the middle. The reasons for the bridge plate are to stiffen the top and to prevent or at least alleviate rotation.* Bridge plates should be made of hardwood. Maple and rosewood are traditional choices. Plywood isn't stiff in all directions. It's not very stiff at all.

If the bridge plate is the problem, removing and replacing the plate is the answer. However, it is an expensive and time consuming procedure. It is best left to experienced repair people. People who have finely tuned woodworking skills, understand glue, and are good at working upside down, backwards, and in the dark should do well with this type of repair. If that's not you don't try bridge plate repairs.

If you're a casual tool user, take the guitar to a pro. If you have decent hand skills, the Bridge Doctor is a good choice.

Rotation: The bridge of an acoustic guitar is in rotation when the guitar is strung to pitch. The axis of the rotation is across the top of the guitar. Imagine inserting a rod from the bass side to the treble side of the bridge. Now imagine the force from the tuners trying to pull the strings out of the bridge. The bridge spins on the rod toward the headstock. The top collapses in front of the bridge and there is a belly behind it.

 

[bholder]

The bridge of a flattop acoustic guitar is in rotation when the guitar is strung to pitch

Fixed it for you. Archtop acoustics work on compression, not rotation.

 

[The13thFret]

Thank you for the advice.
I'm pretty sure I understand the theory, and after getting a USB inspection camera in there, nothing appears to have come unglued, the bridge seems solidly affixed. I think its a combination of some of the things mentioned above, (top isn't stiff enough, braces are not stiff enough & too heavy a gauge of string). The truss rod was maxed out and there was an obvious belly behind the bridge, it was low at the front (rotating forward under string tension)
While it sounds quite pleasant, it's not an expensive or rare instrument, and I can't get a bridge doctor locally (will have to get one shipped from the US). Also the bass is not mine, I'm just trying to help out as a favour.

Since the bridge doctor will need a 9/64" hole anyway, I got approval, drilled, and made something up out of what I had laying around, it seems to be a marked improvement. I'll order a bridge doctor, and maybe creep another 1/8th of a turn in 24 hours and see what happens.

 

[202dy]

Congratulations on confirming the method will work.

Visual inspections are okay. But loose braces don't always show themselves to the naked eye because the gap is to small to see even with high powered magnification.

Take a .005" feeler gauge, or smaller and slide the feeler gauge along the seam between the brace and the top. Try to force it (lightly but tightly!) between brace and top. Same for the top and bridge plate. Be careful not to wedge the feeler gauge between the wood.

Quite often brace that appears to be tightly glued isn't. It may not have been glued when it left the factory.

You'll be amazed at how many braces that appear to be glued are not.

 

[catalin gramada]

Thank you for the advice.
I'm pretty sure I understand the theory, and after getting a USB inspection camera in there, nothing appears to have come unglued, the bridge seems solidly affixed. I think its a combination of some of the things mentioned above, (top isn't stiff enough, braces are not stiff enough & too heavy a gauge of string). The truss rod was maxed out and there was an obvious belly behind the bridge, it was low at the front (rotating forward under string tension)
While it sounds quite pleasant, it's not an expensive or rare instrument, and I can't get a bridge doctor locally (will have to get one shipped from the US). Also the bass is not mine, I'm just trying to help out as a favour.

Since the bridge doctor will need a 9/64" hole anyway, I got approval, drilled, and made something up out of what I had laying around, it seems to be a marked improvement. I'll order a bridge doctor, and maybe creep another 1/8th of a turn in 24 hours and see what happens.

View attachment 3361194

Hello. I know is old, but despite the structural corection how the mod it sound please ?...it gains/loose sustain ? , is louder/quiet ? It loose or gain bass, mids ? Any appreciations about how it modified the tone in respect with original please ? Thank You.