Wednesday, December 02, 2009

Setting intonation on a three-saddle bridge

The guitar I bought Monday evening has an official name: "Squier Classic Vibe Duo-Sonic 50s Electric". It's part of a "Classic Vibe 50's" series that is intended to bring modern re-imaginations of 50's classics to life, with suitable updates for playability and modern sensibilities. As such it has a 24" scale rather than the original Duo-Sonic's 22.5" scale, and a modern neck rather than the original child-sized neck. But one of the things that has not changed is the 1950's style three-saddle bridge -- see above (clicky on the picture to embiggen it and you'll see for sure).

This presents an obvious problem when it comes to setting the intonation of each string. Intonation is the quality wherein a guitar string remains in tune as you fret it down the fingerboard. Thus if you fret an E string at the 12th fret, it should be an E, not an Eb or E#. The usual way of setting intonation is to move the saddle away from the nut (the thing the strings cross at the top of the fingerboard prior to entering the tuners on the head) if the string is sharp at the 12th fret, or move the saddle towards the nut if the string is flat at the 12th fret. But if you do that on a three-saddle bridge, you're changing the intonation of the other string too! This was the first time I'd ever encountered a three-saddle bridge, and at first glance was stumped -- there seemed no way to properly intonate the guitar!

So how do we resolve this problem? Well, by remembering that there are two things that determine the tone of a string: The string length, and the string tension. And the tension of a fretted string depends on how hard you push it down to fret it, which in turn depends upon how high the string is above the fret to begin with, i.e., its action.

So with that, we now know what we need to do: Set the action of the bass string of each pair to the lowest that you can go without getting buzz, intonate the bass string of each pair by moving the saddle towards if too flat or away if too sharp at fret 12 (remember to re-tune the string after each tweak!), then raise (or lower) the action for the treble string of each pair until its intonation is correct at fret 12. The reason we're raising or lowering the action for the treble string is because the treble string is smaller than the bass string, thus we actually can lower its action without getting buzzing if necessary.

So how does it all work out? Well, pretty much like the photo above, which isn't my guitar but is intonated similar to my guitar. If the fingerboard was flat you'd need to raise each treble string to sharpen it at fret 12, but the fingerboard is actually curved. So you end up having to raise the action of string #5 fairly significantly to overcome the curve of the fretboard and sharpen it up beyond that, *very* slightly raise the action of string #3, and actually *lower* the action of string #1 (because the curve of the fingerboard outweighs the natural flatness caused by the bridge being too far away). And remember, adjusting the action of the treble string very slightly adjusts the action of the bass string of the pair also, so you'll need to re-tune the bass string (and check its intonation to make sure it hasn't changed) as well as re-tuning the treble string after each tweak of the action.

Oh yeah, because intonation depends on the tension of the string, if you change strings to a heavier set you'll need to re-intonate the guitar (because heavier strings will have a higher tension). The Duo-Sonic comes with #9's (i.e. the high E or #1 string is .009" in diameter), most people say they're a bit too slinky and that #10's or #11's are more appropriate for this guitar. But now you know how to do it and can eyeball the above photo to see what a properly intonated Duo-Sonic looks like, so...

Oh yeah, tools required:

  1. Guitar tuner (duh!), I have a 20 year old Korg guitar tuner that works just fine. I prefer the needle-type tuners to the blinky-lighty ones because it's easier to see just how far you are from being in tune. If you want to experiment with drop tunings you might want to get a chromatic tuner, but mine isn't one of those and I have no problem figuring out how to do drop tunings with a regular guitar tuner (just fret the dropped string, duh).
  2. Small phillips screwdriver (to adjust the saddle distance to and fro)
  3. The small allen wrench that came with the guitar for adjusting the allen-head screws for the saddle height (action height).

That's all. No other tools or special knowledge or techniques needed, other than knowing how to use the tuner and how to use the tuners at the head to adjust the note made by a string. (And remember, tune *up*, especially important with these "classic" tuners). It's kind of twiddly because every adjustment made to one string also affects the other string, but as long as you start out with the saddles level and the bass string of each pair at the lowest action that will work for that string, it all seems to work out pretty well without as much twiddling as you'd expect.

-- Badtux the Music Penguin


  1. Well, even though we're talking about intonation, I've never heard the verb "intonate" before. In the brass world, we just "tune."

    This dissertation was highly confusing to me, but I am physically and mentally used up. I'll check it again tomorrow.

    Intuitively, it seems the three saddle bridge would give you more tuning flexibility, not less.

    JzB the baffled trombonist

  2. Jazz, modern electric guitars have six-saddle bridges, where you can individually adjust the intonation at each string's saddle. Once you understand that, my befuddlement at initially looking at the above three-saddle bridge makes more sense.

    In the end it's all physics -- the tone created by a string is a function of its length, tension, and string diameter. The initial tension is provided by the tuner at the guitar head (the screw thingy that pulls on the string more or less depending on how you turn the knob), which is used to tension the string to the note that's appropriate for the string. The open length and height of the string above the fingerboard has to be such that when you fret the string to reduce its length (and also apply additional tension to the string by pushing it downwards onto the fret), you move to the next note upwards. My initial mistake upon looking at the three-saddle bridge was forgetting about the tension part of the equation and its relationship to the height of the string above the fingerboard. Once I remembered that, it became more obvious what I needed to do so that I'd get a correct step upwards when I fretted to the next step -- if I couldn't adjust the length of that next string over, I could adjust the tension applied when fretting the note by adjusting the string's height over the frets to compensate for the fact that the string's length is slightly too long (which would flatten the note when I fretted if it were the same height as the other string).

    - Badtux the Physics Penguin

  3. "So how do we resolve this problem? Well, by remembering that there are two things that determine the tone of a string: The string length, and the string tension."

    Yeah, and, as you said in your comment, string diameter (and also: material composition) are all factors, of course.

    All things being equal, the 12th fret should be *exactly* an octave above the open string. If it's not, you have a problem. Yes, the fretboard is curved, which is problematic. It could also be curved the other way (head-to-pickups, the long way) which is annoying and causes buzz as well.

    Of course the actual instrument (like most actual instruments) is not ideal, and you have to make adjustments, like you describe.

    Instruments are strange and wonderful.

    --SA (the music theorist)

  4. String diameter and composition aren't adjustable once you put your strings on your guitar, so not all that useful for setting intonation ;).

    Yeah, you need to make sure your truss rod is set right for the spec'ed lengthwise curvature to the neck before you start doing anything else with action or tuning. Otherwise you'll never get things right. Luckily mine was right on spec when I set a straight-edge on it (the spec is downloadable off the Fender site in the same three-page PDF document that has the wiring diagram and parts list). Messing with truss rods is no fun... but that's all part of the setup of a new guitar.

    Knowing some of the physics of vibrating strings is useful for any guitarist, because knowing *why* you're getting the sounds you're getting out of the infernal machine helps you figure out ways to make it sound better, different, or both. You don't actually have to memorize equations or some shit, but you should at least know what the variables are in how this stuff all works. It just makes you better at doing what you're intending to do, assuming that your intention is to make music...

    -Badtux the Scientific Penguin

  5. Just a quick note - a tech at Music Man guitars (I own three of them - they're great) said to use the 19th fret to check the harmonic versus the fretted note for intonation, not the 12th. And he said to use your ears; if it sounds like the intonation is good between the 19th fretted/harmonic, it's probably good to go. Seems to work for me.

  6. Music Man guitars have a six-saddle bridge, this guitar has one of Leo Fender's less bright ideas, a three-saddle bridge. Entirely different method of setting intonation there. MM's are also full-scale, this is a 21-fret short scale guitar, fret 19 is basically right there by the neck pickup. I'll stick with Leo Fender's directions for how to tune his guitar, they worked, the guitar is in tune all up and down its neck now.

    As for tuning by ear, has never worked for me. I need a reference note to compare to. I can decidedly hear that the note is "off", but unless I have a reference note, I can't tell whether it is "off" on the high side or the low side. Thus why I use an electronic guitar tuner. Herr Korg has been my best friend and pal for 20 years now so I'll stick with him for a while longer.


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