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Thread: Hump in center of Mandolin Top

  1. #1

    Default Hump in center of Mandolin Top

    I never noticed this before on my mandolin, but there is a small hump running straight down the center of it. It starts off flat near just under the neck, and then becomes more of a hump as it gets to the center and passes under the bridge.

    Its not just an optical illusion with the grain or anything - i can feel the hump in the top when I run my finger over it.

    Is this normal? It's a Gibson A9.

    Click image for larger version. 

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  2. #2
    Adrian Minarovic
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    Default Re: Hump in center of Mandolin Top

    double post
    Adrian

  3. #3
    Adrian Minarovic
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    Default Re: Hump in center of Mandolin Top

    Few questions first.
    Was it there from new? (some of thhese low end Gibsons have quite odd archings to start with) Or did it emerge slowly over time?
    I see a dark line around centerjoint in front of bridge. Is there a joint failing under finish? You may not see a fracture in finish as this one is very rubbery and flexible but tere may be movement of the joined pieces underneath especially if the top is bulging out. Take very close look as it is hard to see and perhaps even experienced luthier would have to look at it with magnifier so be sure. Knowing thickness of top would help but without proper equipment you cannot measure that.
    Answers to these questions and perhaps other pictures from angles that show the bulge better would help with diagnosis.

    Other than this I would add that design of "two footed" bridges has tendency to form such ridge that runs right through the gap between bridge feet. Especially if they were not fitted pre-sprung or nit checked for long period of time. The top and bridge settles and tension of strings tends to compress the top along axis which wants to bulge out but the feet are pressing against this so the whole stress gets concentrated in the center between the legs. Sometimes when you remove the bridge you can see deep indentations at the inner edges of bridge feet and sometimes you can see the ends of the bridge feet lift above surface. This tendency also depends on position of tonebars and thicknass/material of top and also shape of arch and bridge base (stiffness and fit to top) so in some mandolins this will develop over 50 years and in some in 5. My solution was switching to full contact bridge almost 20 years ago for each of my mandolins. I noticed very slight hint of such bulge on my first mandolin (after 20 years of use) and I filled the gap in base with ebony and refit the base so it gently pressed the top center back where it was when I made it and over few weeks the top arch was as smooth as new. I refit the bridge after few weeks again to make sure it fits well the corrected arch now.
    Adrian

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  5. #4
    Teacher, repair person
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    Default Re: Hump in center of Mandolin Top

    I'm going to guess that what you're calling a hump is what I would call a bump. If so, it could be a failing glue joint. That would require repair by an expert. If you've got good eyes, you can press next to the joint and see if one side is moving.

    Or it could be that the wood wasn't seasoned well enough when the instrument was built, and the fit has changed due to the wood drying out. If the glue joint is still good, that's just a cosmetic annoyance that can be left alone. But the joint should be checked from time to time to make sure it remains sound.

    If you do think the glue joint is bad, loosen the strings immediately.

  6. #5

    Default Re: Hump in center of Mandolin Top

    Quote Originally Posted by rcc56 View Post
    I'm going to guess that what you're calling a hump is what I would call a bump. If so, it could be a failing glue joint. That would require repair by an expert. If you've got good eyes, you can press next to the joint and see if one side is moving.

    Or it could be that the wood wasn't seasoned well enough when the instrument was built, and the fit has changed due to the wood drying out. If the glue joint is still good, that's just a cosmetic annoyance that can be left alone. But the joint should be checked from time to time to make sure it remains sound.

    If you do think the glue joint is bad, loosen the strings immediately.
    Yes, it is a bump. How would I check the glue joint? Know anyone in NJ/Philly area that could take a look?

  7. #6

    Default Re: Hump in center of Mandolin Top

    Was it there from new? (some of thhese low end Gibsons have quite odd archings to start with) Or did it emerge slowly over time?
    I see a dark line around centerjoint in front of bridge. Is there a joint failing under finish? You may not see a fracture in finish as this one is very rubbery and flexible but tere may be movement of the joined pieces underneath especially if the top is bulging out.
    No idea where it was bought new, I got it from Guitar Center a couple years ago. The bump could have existed then and I just didn't notice - I have no idea.

    The dark line you see running dead center of the top is the raised bump. I have no idea if the joint is failing, and not sure how to check.

    The bridge is one of those fishman piezo deals - it was already on there when I got it.

    If its relevant, the setup is overall pretty good - decently intonated and the action is not low, but is where I like it.

  8. #7

    Default Re: Hump in center of Mandolin Top

    Quote Originally Posted by JerseyPicker View Post
    How would I check the glue joint?
    >>>

    Quote Originally Posted by rcc56 View Post
    If you've got good eyes, you can press next to the joint and see if one side is moving.
    "I play BG so that's what I can talk intelligently about." A line I loved and pirated from Mandoplumb

  9. #8
    Registered User sunburst's Avatar
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    Default Re: Hump in center of Mandolin Top

    Instead of staying out of this discussion like I should, here's some speculation on what might be going on.
    Between personal experience and Adrians repair photos of a Gibson F-9, I think of them as rather poorly made mandolins. I assume this one was made using Titebond or similar adhesive, and it could be that the top center joint was not well made and there is glue filling a gap in the top seam. Titebond (et al) dries to a plastic substance rather than fully solid and so can move with time and temperature. Pressure from the bridge on the arched top could be "trying to" close the glue-filled gap and squeezing the glue out causing the ridge along the top center seam. Or, perhaps the wood has shrunk and caused the glue to protrude. As I said, I'm just speculating rather than staying appropriately quiet.

  10. #9

    Default Re: Hump in center of Mandolin Top

    Quote Originally Posted by sunburst View Post
    Instead of staying out of this discussion like I should, here's some speculation on what might be going on.
    Between personal experience and Adrians repair photos of a Gibson F-9, I think of them as rather poorly made mandolins. I assume this one was made using Titebond or similar adhesive, and it could be that the top center joint was not well made and there is glue filling a gap in the top seam. Titebond (et al) dries to a plastic substance rather than fully solid and so can move with time and temperature. Pressure from the bridge on the arched top could be "trying to" close the glue-filled gap and squeezing the glue out causing the ridge along the top center seam. Or, perhaps the wood has shrunk and caused the glue to protrude. As I said, I'm just speculating rather than staying appropriately quiet.
    Assuming your speculation is correct, is that something I can ignore?

    Quote Originally Posted by FLATROCK HILL View Post
    >>>
    Neither side of the top appears to move at all when I put pressure on it. No signs of movement or any crack/separation. I have pretty good eyes, but I also have never tried to look for something like this before.


    Also, one more thing that might matter: the wire coming off the fishman bridge goes directly through the top. A little hole is drilled right in the center where the two top halves meet, under the tailpiece.

  11. #10
    Registered User sunburst's Avatar
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    Default Re: Hump in center of Mandolin Top

    Quote Originally Posted by JerseyPicker View Post
    Assuming your speculation is correct, is that something I can ignore?
    Mostly. Probably should keep an eye on it to see if anything changes.
    The hole under the tailpiece is not related, most likely. After all, there are two factory cut more-or-less-F-shaped holes that are much bigger (though perhaps better placed).

  12. #11
    Adrian Minarovic
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    Default Re: Hump in center of Mandolin Top

    Now that the OP said that the dark line IS the bump/hump I agree with John that it might be the centerjoint slipped by a tiny amount. Could be because of variety of reasons like overheating, poor humidity control or poor materials etc... The joint can still be holding well just the pieces slowly shifted out of alignment and since this finish is quite rubbery it won't crack easily and you'll just feel/see the ripple.
    What I described in my post is larger area hump around 3x1" or so that many old arch top mandolins with two footed bridge show over time.
    Adrian

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