Intonation question

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87echo
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Intonation question

Post by 87echo » January 2nd, 2015, 6:17 pm

Hello all,

I searched some but did not find a solution to this specific issue. I am having a problem on my kit strat. with bolt on neck where the open string is in perfect tune, the first fret will be about 4-5 cents sharp. The second fret, with normal pressure will be almost 10 cents sharp. The lightest pressure will yield at least 5-6 cents sharp. The intonation will correct about the ninth fret or so. My question is whether it's possible to dress the frets so the crown is back a bit to correct this. If not, any experienced advice would be appreciated.

Thanks.

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notes_norton
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Re: Intonation question

Post by notes_norton » January 3rd, 2015, 8:30 am

You adjust the intonation by moving the bridge closer or farther from the nut. Most good guitars have an adjustment for each string. Google how to set guitar intonation, I'm sure you will find one for your guitar with instructions on how to adjust the bridge, and which direction to move it.

I'm getting ready to go to an early gig, so I can't take the time to explain it here.

Plus as far as fretting goes, the lighter touch you have, the better the intonation will be. Accuracy trumps pressure.

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87echo
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Re: Intonation question

Post by 87echo » January 3rd, 2015, 1:45 pm

notes_norton wrote:You adjust the intonation by moving the bridge closer or farther from the nut. Most good guitars have an adjustment for each string. Google how to set guitar intonation, I'm sure you will find one for your guitar with instructions on how to adjust the bridge, and which direction to move it.

I'm getting ready to go to an early gig, so I can't take the time to explain it here.

Plus as far as fretting goes, the lighter touch you have, the better the intonation will be. Accuracy trumps pressure.

Insights and incites by Notes
Thanks for taking the time to respond, but I don't think you understand the question. The standard test for accurate intonation, the 12th. fret, is on. The issue was at the first few frets. It was enough to be very noticeable and distracting. I was able to remove the nut, and take down the edge of the fretboard slightly with a small flat file. I also polished the back side of the nut to remove any inconsistencies. Oddly enough, it helped bring it close enough to be acceptable. Perhaps this will be helpful to others who get the open strings in tune only to find fretted chords sounding too far off. I am also planning on dressing the frets a small amount.

Thanks again.

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NoteBoat
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Re: Intonation question

Post by NoteBoat » January 3rd, 2015, 4:44 pm

I think you're on track with lowering the nut. But lemme get into a some detail, because even if you know this stuff it might help somebody else...

The pitch of an "ideal string" is determined by three factors: length, tension, and mass. Your scale length is fixed by the nut and the bridge, and the mass of the string is what it is at the moment (more on that in a bit), so you adjust tension to get the string in tune.

Now you fret a note - which means you're changing the length of the vibrating string, and a shorter string vibrates faster if the other factors remain the same. In our modern 12TET tuning, each half step needs to shorten the string length by the 12th root of 2. Do that 12 times and you end up with half the original scale length, and you'll get an octave; do it 12 more times and you'll have 1/4 of the original scale length, or 1/2 the distance from the 12th fret, and you'll be two octaves above the original pitch - that's where the 24th fret would go.

You said it's a kit guitar; the manufacturer has done the math for fret placement, and with modern tooling they should be in the right places (I've seen a mis-placed fret exactly once in my life), so the only variable is the string length in relation to the frets. The guitar needs to be assembled so the 12th fret lies exactly halfway between the nut and bridge. I've seen a few guitars where someone put the bridge in the wrong place, and that screws everything up... but since you're in tune at the 12th fret, you're golden on this point. So we can rule out tension, and we can rule out length as the cause.

The third factor is string mass. This one is a little funkier. If there's a manufacturing defect in a string you can have a spot with too much mass (rare, but I've seen it). More common is dirty strings affecting intonation - as you play, they collect dirt and oil from your fingers, which makes the string heavier - and that makes it flat. So you crank the tension to get the open string back into tune. But now when you fret a string, you're supposed to have the same ratio of mass to string length - and you don't. If there's more dirt on the vibrating part, you'll be flat; if there's more dirt behind your fretting finger, you'll be sharp.

Let's assume you've got brand new strings, so that's not an issue. Up top I said this is how an "ideal string" behaves - and they don't exist in real life. Two other factors come into play. One is the stiffness of the string. A real string has both a "scale length" (distance from nut to bridge) and a "speaking length" (the length that actually vibrates). They're different because of the stiffness of the material.

Imagine picking up a wet noodle and waving it around, and doing the same thing with a branch from a tree. The noodle wiggles around freely... the end of the branch moves, but because it's stiffer, the part closest to your hands is moving a lot less than the noodle. The stiffer the string, the more the speaking length will differ from the scale length. That's what the intonation adjustments on the saddle are for - compensating for the difference. But because the adjustment is made at only one end of the string, you can really only get one fret truly in tune! You can do a few things like using a compensated nut and fanning the frets to minimize or eliminate this fact, but in most cases you can just live with (and maybe not even hear) the problem... because our tuning system isn't perfect either.

The second thing that makes real world strings differ from ideal strings is the fact that you've got to bend them to fret them. That changes the length a bit (you're forming a triangle, with the points of one side being the original position and the amount you have to deflect the string to fret it, and the third point being the saddle - the length is now the hypotenuse rather than the second longest side) AND you're increasing the tension by bending the string. If you think about how much the tension will change by bending it, a string takes the least amount of force to bend a given distance over the midpoint (the 12th fret), and the most force close to the ends.

What's a bit confusing is the fact that you're MORE out of tune at the 2nd fret than the 1st. Because of the physics and geometry involved, I'd expect the opposite.

Anyway... from what you describe the first thing I'd do is get a straightedge and lay it on your frets. Your first fret might be a little high compared to the others, which would mean you're bending the string more for the second fret. But you want to rule out any warping of the neck that might be a factor. Then I'd check the relief - the 'backbend' in the neck to make sure the vibrating string clears the frets without buzzing. That's a truss rod adjustment.

I'd do those first, because if your frets aren't in plane you've got a different problem, and a truss rod adjustment is reversible if that doesn't solve things. No sense creating a third problem if something else is to blame, right?

If that's all good, then your string is too high at the nut end, and you're increasing the string tension too much to make it meet the fret, making it sharp. Reducing the nut height will definitely help with that. Just don't overdo it :)
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