Using a Bridge Doctor to lower the action

Discussion in 'Acoustic Heaven' started by Tomm Williams, Jan 24, 2020.

  1. Tomm Williams

    Tomm Williams Tele-Afflicted

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    Today I was offered a Martin DCPA4 and case at a price that was too good to pass up. No I didn’t need it but when has that ever been a consideration? Anyway, it’s in very nice shape but the action is a bit high for me past the 9th fret or so. ( I like lower actions, I suspect this measurement is probably stock for this guitar) The neck looks good and there does not appear to be excess belling at the bridge. I want to lower the action a bit but the saddle isn’t that tall to start with so I’m wondering if I could use a Bridge Doctor to essentially flatten out the top thus bringing the bridge down to what is hopefully an adequate height?
    At $50 it seems like worth trying.
     
  2. Freeman Keller

    Freeman Keller Friend of Leo's

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    No, when I get a few minutes I'll tell you why. What the guitar needs is a neck reset and good setup AFTER the geometry has been corrected. A little belly is good, don't try to remove it. A DCPA4 should have their A frame bracing and M&T neck joint so the reset is slightly easier than a dovetail but its still not trivial.

    If you aren't convinced I'll come back later today and we'll talk about the bridge doctor.
     
  3. Tomm Williams

    Tomm Williams Tele-Afflicted

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    Nope, take your word for it!
     
  4. Freeman Keller

    Freeman Keller Friend of Leo's

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    I'm going to give you an answer anyway LOL. A pinned bridge acoustic guitar has about 170 pounds of string tension anchored at the bridge is such a way that wants to rotate it more or less around around the base of the bridge (I found this picture somewhere)

    TORQUE-LOAD-300x160.jpg
    That pulls the lower bout up and forces the area between the bridge and the sound hole down. That is the static condition just sitting there. When you pluck that string it causes the top to rock - the lower bout goes up as the soundhole goes down, then the other way. That is how they make sound. (which is different from an archtop or bowed instrument or anything with a tailpiece).

    The bridge doctor is a little internal BRACE that has a rod going back to the tail block of the guitar - these pictures are reversed from the previous one

    [​IMG]
    The purpose of that rod is to PREVENT the top from rotating under string tension and therefore to prevent the rocking action. Yes it will reduce the belly (but acoustic guitar have belly built in for a variety of reasons)

    Some models of Breedlove guitars had bridge doctors built it to them but they were extremely lightly braced - here is a picture from Frank Ford's wonderful site

    [​IMG]

    My point, these guitars sound good because the doc was designed in from the get-go, not added later.

    Last anecdote - when my wounderful old D12-28 developed a belly and the action started creeping up where I couldn't lower it any more I bought a bridge doc and installed it. Short story, not only did it not cure the belly, the guitar sounded like hell. I took the thing out (its somewhere in a box in my shop), had the neck reset properly and the guitar plays and sounds wonderful.

    Let lesson is simple - if you are shopping for a used guitar learn how to evaluate it, what normally goes wrong (like neck angle, frets, bridges...) and what it costs to fix it. Use that in your negotiation (the owner knows it needs a reset and doesn't want to pay to have it done)

    End of lecture, sorry...
     
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