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Thread: String Geometry

  1. #1

    Default String Geometry

    I have recently purchased a 'vintage' Gold Tone OM. It was built in 2006 and has been well cared for. I have recently come across articles regarding 'sunken tops' that caught my attention. It seems that flat-top instruments might be prone to this condition, but here's my dilemma:
    I replaced the existing strings right away with a lighter gauge set. Then, I noted that the low G will buzz slightly on the frets above mid neck. Probably due to a lighter softer string (.048). With a straight edge, i do note some sinkage in the top, but it seems slight. Normally, one could correct by a thin shim beneath the bridge. The bridge does not appear altered in any way, and the neck has a nice mild relief at tension. However, it seems that the bridge has an integral transducer in it, wired from below. So shimming is out. Next, I noticed the break angle of the strings from bridge to tailpiece (See the attached photos).
    Click image for larger version. 

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    Here is where I need advice... notice the steeper and shorter angle of the strings on the OM, compared to that of my Mandola. Does the steeper angle allow the string tension to exert more downward force on the bridge and top? (I admit the teachings I received on Trig and Vectors abandoned me about 50 years ago). If the tailpiece was adjusted to a softer angle, would that help the situation? I do like everything else about this instrument.

  2. #2
    Teacher, luthier
    Join Date
    Oct 2017
    Location
    Southeast Tennessee
    Posts
    1,866

    Default Re: String Geometry

    That does look like a rather low bridge. But buzzing can be caused by a number of factors, such as uneven frets, a mis-adjusted truss rod, bridge height relative to the gauge of strings in use, an underset neck, etc.

    It wouldn't hurt to get the instrument checked out by a competent repair person to find out what is causing the problem. It appears to me that your action is indeed below spec. And it is not unusual to have to raise a bridge when you lighten the strings. If that would happen to be the correct remedy for your problem, there still may be a way to shim the bridge and accommodate the pickup wire. If not, it may be possible to add material to the top of the bridge rather than the bottom.

    But the first thing I would check would for would be whether the truss rod needs to be loosened slightly to accommodate the lighter strings. After it is determined that the rod is correctly adjusted, then it's time to check the bridge height and confirm that the frets are level.

    The tailpiece is not the first or even the fourth place I would look for a problem, but you could temporarily adjust the string angle across the bridge by sliding a piece of thick felt or even something like a popsicle stick between the strings and the tailpiece cover to see if that makes a difference.

  3. #3

    Default Re: String Geometry

    Thanks for the reply rcc56. Maybe I have overthought the sinking top thing. My logic was that perhaps less string tension would allow the top (and bridge) to regain altitude. Not the case, apparently. I did remove tension and adjust the truss rod about 1/8 turn CCW, as that can easily be undone. Then retuned, and it seems to have helped. Might consider installing next fatter strings on the low G.

  4. #4
    Adrian Minarovic
    Join Date
    Oct 2003
    Location
    Banska Bystrica, Slovakia, Europe
    Posts
    2,702

    Default Re: String Geometry

    Low hmidity can cause temporary sinking of the top of flattop instrument.
    Adrian

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