Intonation Question - Fretted Notes Are Sharp

[fngrpepr]

So I picked up an inexpensive guitar which I am using for a rebuild project. I stripped it down and repainted, and am planning to replace the plastic nut with a bone nut.

I have the nut roughed out, but have this problem -- I tune the string to pitch, and confirm that the harmonic at the 12th fret is also in pitch, but when I fret the note at the 12th fret (or fret any note for that matter) the pitch is sharp. To confirm the problem didn't lie with my new nut, I put the old plastic nut back on... and same problem.

So what's my problem? Is it string height? The strings are kind of high.

[Woodenspoke]

So I picked up an inexpensive guitar which I am using for a rebuild project. I stripped it down and repainted, and am planning to replace the plastic nut with a bone nut.

I have the nut roughed out, but have this problem -- I tune the string to pitch, and confirm that the harmonic at the 12th fret is also in pitch, but when I fret the note at the 12th fret (or fret any note for that matter) the pitch is sharp. To confirm the problem didn't lie with my new nut, I put the old plastic nut back on... and same problem.

So what's my problem? Is it string height? The strings are kind of high.

So your saddle in in the wrong place. Funny how that happens on cheap guitars...

[fngrpepr]

But if the saddle were in the wrong place, wouldn't that change the scale length, thereby causing the harmonic to not appear at the 12th fret, but slightly higher or lower?

Joe

[Ken Bennett]

I've moved many a saddle slot for that very reason. The nut may even be in the wrong place. Sometimes on older cheap guitars you will even see frets out of position.

The scale length is fixed and determined by the fret spacing, not by the location of the saddle.

Moving the saddle only moves the 12th fret harmonic half as much. The intonation error is more apparent to the ear than harmonic is to finger position.

If you know what the scale length is supposed to be, you can calculate an acceptable position for the saddle. Start by measuring nut to 12th fret, multiply by 2, and add .1 to .15 at the middle of the saddle to find a starting point (If the nut is out of postition, that theory is no good). Then make sure the saddle is angled correctly (rotated around that point).

You'll love it when you're done.

Edited September 23, 2010 by Ken Bennett

[ihocky2]

I'll disagree slightly with what Ken is saying, only about what determines the scale length. Fret spacing is determined by the scale length, if you don't have your scale length ahead of time you cannot run the 17.817 formula to get your fret spacing.

Scale length is twice the distance from the nut to the 12th fret, hence forth the distance from the nut to the saddle. But it is not even across every saddle. The saddles are moved (when the saddles are not fixed) to adjust for the difference in string thickness. This is the compensated scale length, made famous by Gibson. Fixed saddle bridges are angled to adjust for the compensation, but are never exact for each string. On cheap guitars very little time is spent on setup so it will be common for the intonation to be off.

[Ken Bennett]

ihocky2 is right. But in practice you are not going to be moving the frets. So when you move the saddle, you don't change the scale length, just the compensation factor.

Edited September 24, 2010 by Ken Bennett

[fngrpepr]

Thanks for the comments and advice. Tonight I took some measurements- the guitar is intended to be a 25" scale (based on nut-saddle and nut-12fret measurements), but it appears to be about 1/32" too long. This is based on the nut-12 length as well as also checking some of the other fret distances from nut. My saddle is fixed not adjustable, so I can't just move the saddle positions, and in any case, this wouldn't change the nut-fret distances.

Questions:

1) Would the scale length being slightly longer than intended be consistent with a fretted note being sharp relative to the open string? I believe so, but need a sanity check.

2) I would like to solve this problem by sanding away about 1/32" of the fretboard where it meets the nut. This would appear to solve my problem, putting the frets the correct distance from nut and also putting the saddle at the right distance. Is this an acceptable approach?

[postal]

No.... the REAL measured length is supposed to be longer than the actual scale length. It's called "compensation".

1/32" is not enough, and thats why fretted notes are high.

generally, about scale length + 3/32" at high E, and +3/16" at low E.

Also, action being high makes it even worse....

So for example... a 25.5" scale guitar... low E should measure about 25.7... This is normal, and how it should be.

[Southpa]

If the guitar intonates ok at nut to 12th fret then move on. You mentioned high action, but combine that with heavy strings, high frets and pressing too hard, then you got a problem with notes sounding sharp. Analyze each of those potential problems and work from there. You say its still sharp with the old nut installed. Both nuts could still be too high. Do some reading at frets.com

Edited September 24, 2010 by Southpa

[Bionic Dave]

For starters you shouldn't be checking your intonation with a harmonic. Always fret at the 12th fret.

Check the distance from the nut to the 12fret whatever that is then double it that's the scale lenght. Then check the bridge saddles for rough distance the tune to open. Then fret the 12th and check for sharp or flat the intonat. Then retune and check again. So on and so forth.

Dave

[Workingman]

Also try new strings. They can make a differance if they are old and crudded up, particularly with harmonics.

[ihocky2]

Probably the best thing to do is do a full setup from start to finish in the correct order with new strings (if your current ones are old). The last step of a setup is intonation because everything else affects it. Once you are setup to your liking it might be less of a problem.