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Thread: Bridge Compensation

  1. #1
    Registered User Jim Baker's Avatar
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    Default Bridge Compensation

    Can anyone tell me if there is a formula for bridge compensation based on scale length and string gauge? Making a bridge is next on my list for my octave mandolin. Thanks
    Jim Baker

  2. #2
    Registered User Jim Baker's Avatar
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    Default Re: Bridge Compensation

    Could I assume that, in order to measure compensation for each string one would need to make a temporary bridge, string up and check intonation on each string?
    Jim Baker

  3. #3

    Default Re: Bridge Compensation

    Not only gauge and length but material affects the compensation too. I suspect it may be too complex with all the various string types for a simple formula. I've never made an octave bridge, but I have hand compensated mandolin and mandola bridges. Small adjustment, tune up and check. Lather, rinse, repeat. It's such a small area to file that it's really the repeated removal and tuning up that takes the time. You can file with a bridge still on the top, but it's very easy to make one bad slip.

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    Default Re: Bridge Compensation

    I posted on here one time a few months ago, "why the compensated bridge " back in the early days bridges were straight across with no compensation...I made a saddle years ago with no compensation and installed it on one mandolin and it seemed to have the correct intonation, I sold the mandolin with that bridge on it too....I have had a few antique mandolins and all of them had non-compensated bridges so It`s beyond me why they have them...I asked it it had anything to do with the gauges of modern strings and some of the builders on here said gauge size didn`t matter, it was the length and the fret spacing that made it needed to be compensated....Good luck in getting an answer....Willie

  5. #5
    Luthier&Pickup maker ret. Soundfarmer Pete's Avatar
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    Default Re: Bridge Compensation

    I would suggest that gauge size (or more correctly tension) does matter (based on my guitar experiences)!
    Suppose you have a guitar that intonates perfectly at the 12th fret in standard tuning. Drop the low E to a D and check the intonation - it will be wonky. Since I play mostly DADGAD, I make up my string sets from singles with heavier gauges for the dropped notes.
    With the wound strings, I wouldn`t be surprised if different manufacturers use slightly different wire for the core so even if a string is say .040", it probably won`t have the same tension from manufacturer to manufacturer

  6. #6

    Default Re: Bridge Compensation

    String compensation is so complicated, I think the best way is just to trust those little bridge tops with the back-and-forth saddle positions.
    Just joking, but I don't know a better way to state it. I have also had success with a straight line saddle, slanted.

  7. #7
    Mandolin tragic Graham McDonald's Avatar
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    Default Re: Bridge Compensation

    There is not a simple formula, though Mike Doolin wrote a comprehensive article for American Lutherie a few years back on calculating compensation. An OM like your will need around 2mm compensation on the top e string and 3-4mm more than that on the bottom G. It gets a little more complicated depending on whether you want to use a plain or wound a string. A plain a string will need another 2-3mm compensation behind the e string, but a wound a, because of the thin core wire will want a shorter string length than the e. Compensation is much dependant on the diameter of the plain steel wire, whether as as the core of a wound string or a plain steel string.

    For a pin bridge octave I usually start by positioning the front of the saddle slot around 1mm past the theorectical scale length and start from there. For a floating bridge it will be a matter of little bits of wire to mark the saddle setbacks.

    Here is a diagram of a four course saddle if that is any help

    cheers

    graham
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  9. #8
    Café habitué Paul Hostetter's Avatar
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    Default Re: Bridge Compensation

    No fixed formula, because there are so many variables. That said, Gibson established a pretty good standard, if you have a typical 13-7/8" scale and J-74 strings.

    There are also many, many earlier threads on this issue - search the archives.

    .
    ph

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  10. #9
    Registered User Jim Baker's Avatar
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    Default Re: Bridge Compensation

    Good place to start. Thanks guys.
    Jim Baker

  11. #10
    Café habitué Paul Hostetter's Avatar
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    Default Re: Bridge Compensation

    Quote Originally Posted by Graham McDonald View Post
    Here is a diagram of a four course saddle if that is any help
    This is precisely the opposite of the orientation Gibson originally intended. In this image is a '20's F-5, w/original bridge. This is how the bridge was meant to be oriented.





    I find that the D, A and E work well but the G still tends to play sharp up the neck, so I often do a different setback for that:

    .
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  12. #11
    Mandolin tragic Graham McDonald's Avatar
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    Default Re: Bridge Compensation

    Maybe it is just my diagram, but it looks pretty close to the Gibson saddle as far as I can see. It is what I have been using for Octave mandos/ bouzoukis for a very long time. Jim B is building an octave not a standard mando, and the stringing is a little different.

    cheers

    graham

  13. #12
    Registered User Jim Baker's Avatar
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    Default Re: Bridge Compensation

    I'll be starting off with a set of J80's. I think they are 12, 22w, 34w, 46w. Just a starting point.
    Jim Baker

  14. #13
    Mandolin tragic Graham McDonald's Avatar
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    Default Re: Bridge Compensation

    For the J80 set the .022w will be the forwardmost (if that is a word) contact point, the .012 and .034 1-2mm behind behind, and the .046 back another couple of mm. As Paul points out, getting the G to play in tune can be tricky.

    cheers

    graham

  15. #14
    Registered User Jim Baker's Avatar
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    Default Re: Bridge Compensation

    Graham, can you tell me is that because the core of the A course strings will be smaller than the E course strings? Just trying to understand a bit.
    I have a 17 fret tenor banjo that I was having A string trouble with. When I switched to a wound A string it solved the problem.

    Thanks
    Jim Baker

  16. #15
    Mandolin tragic Graham McDonald's Avatar
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    Default Re: Bridge Compensation

    The core of the .022w string will be something like a .008, and so needs less compensation than the .012p e or the .034w D, which will have a core around .010 or .012. There will hopefully be someone here who can explain the science behind it, but my understanding is that it is the core diameter (or plain string diameter) which has a major part in how much compensation is needed, rather than the overall diameter of the string.

    cheers

    graham

  17. #16
    Café habitué Paul Hostetter's Avatar
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    Default Re: Bridge Compensation

    Wound strings intonate very differently than plain ones. Using a typical Gibson mandolin bridge—right-side up or upside-down—with its compensation for two pairs each of plain and wound strings on an octave mandolin, which has only one course of plain strings and three pairs of wound strings, strikes me as a recipe for hopelessly intractable intonation. I replace such bridges all the time. Depending on the scale, action and so on, I use either of the two fairly similar patterns below for octave mandolins:

    .
    ph

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  18. #17
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    Default Re: Bridge Compensation

    Mandolin scale is much shorter than OM and is much more sensitive to intonation issues. The shorter the scale the more critical the fret placement and compensation. As explained, there are so many possible variations that there is no rule or established pattern to follow so one needs to adjust the compensation for each course of strings while on the instrument. The saddles Steve Smith supplies with his Cumberland Acoustic bridges have a nice wide area on the top so there is room to make the needed adjustments. Once you set the intonation you can remove the excess material on the back edge of the saddle.

  19. #18

    Default Re: Bridge Compensation

    I just strung up a 20.75" scale OM that I made and I used a mandolin bridge as a starting point to set the intonation and it ended up being spot on. I don't think I could get it any better and I did use a .022 wound A string. I am still working on the best for the G string. By the way, along these same lines of setting up a bridge, what type of spacing do you use on your longer scale instruments? My fretboard looks way too large from the 12th fret up because the spacing on the mandolin bridge is too narrow. Should I consider increasing the string spacing or narrowing the fret board on the next one?

    By the way, I used on of Steve Smith's bridges that he uses on his Redline Travelers.

  20. #19
    In The Van Ben Milne's Avatar
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    Thumbs up Re: Bridge Compensation

    Quote Originally Posted by Paul Hostetter View Post

    Interesting stuff... where is the mando-matic bridge from, Paul?

    EDIT - found your answer Here...

    So I gather If you had one of your hypothetical mando-matic bridges, you'd basically use it to intonate each instrument, and then duplicate the compensation in a fresh cut...
    Last edited by Ben Milne; Apr-19-2010 at 9:26am.
    Hereby & forthwith, any instrument with an odd number of strings shall be considered broken. With regard to mix levels, usually the best approach is treating the mandolin the same as a cowbell.

  21. #20

    Default Re: Bridge Compensation

    Vanner98,
    That is a reality. Here is what I came up with for my electric mandolins. I've made them in brass and wood, but the wood ones are easier to make since I run them on the CNC. I don't see why you couldn't make a base for them to determine compensation. The only downside is their height. The saddle piece itself is 1/2" tall although I have sanded up to an 1/8" off them. I can't recall what the hole spacing is , so I don't know if they will fit on a standard mandolin base.

    Note: The saddles are tilted because I don't have the correct ones in there.
    Andrew
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  22. #21
    Mandolin tragic Graham McDonald's Avatar
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    Default Re: Bridge Compensation

    My octave/bouzouki fretboards are 34mm wide at the nut and 44m at the 12th fret. The spacing between the outermost strings at the bridge is about 44mm, but adjusted as required to have the strings almost parallel to the outside of the fingerboard. The line of the outside strings almost follows the flair of the fingerboard, which uses the same proportional flair as a guitar fingerboard. The top e is 2-2.5mm from the edge and the bottom G about 3mm from the edge at the nut. This does mean that a standard mandolin bridge (with an outside string spacing of around 40mm) can be a bit tight, but I don't use mandolin bridges on OMs/bouzoukis anyway.

    It seems as though setting compensation at the bridge can be a variable thing, and does suggest that different instruments do need different setups. Ah, the joys of lutherie

    cheers

    graham

  23. #22
    Registered User Jim Baker's Avatar
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    Default Re: Bridge Compensation

    Thankyou guys. I'll post a picture when I get it working.
    Jim Baker

  24. #23
    Café habitué Paul Hostetter's Avatar
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    Default Re: Bridge Compensation

    Quote Originally Posted by Vanner098 View Post
    So I gather If you had one of your hypothetical mando-matic bridges, you'd basically use it to intonate each instrument, and then duplicate the compensation in a fresh cut...
    Exactly. But in its absence, any bridge will work: you position the bridge so that the intonation is precisely where you want it for each course, one at a time, starting with the E, and then you measure from nut to point of contact. Or if you want to start with a block for a saddle, get the E properly set from the forward edge, and adjust the other courses from there. Kind of hard on the strings taking it off and on, but you at least know you have it. Or, as Michael suggests, get one of the Cumberlands and start there and tweak away.
    .
    ph

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  25. #24

    Default Re: Bridge Compensation

    21.5" scale length gdae, daddario phosphorbronze.048-.034-.023-.013 YMMV
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  26. #25
    Registered User Jim Baker's Avatar
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    Default Re: Bridge Compensation

    I have a new question about bridge compensation. I have converted my first octave mandolin to a bouzouki with a 25" scale. I am octave stringing the G and D courses.

    I now have to make a new bridge.

    Here is my question: In the G and D course is there a different compensation between the wound string and the plain string?
    Jim Baker

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