Odd Intonation Question

[Bardagh]

I have a guitar that has an odd intonation issue that is the opposite problem people usually have. On one string, all the fretted notes are flat compared to the open note. This is an acoustic guitar and it’s the G string.

The guitar came with a graphtech tusq compensated saddle that was overly compensated on a few strings (they also had flat intonation) so the other day I shaped a new saddle myself to shorten the scale length on these strings. This improved the intonation for all but that one string, which I find baffling.

The thing is, if I use a capo and eliminate the open note, the intonation is acceptable, i.e. the 1st fret note is not too off from the 13th. What could cause an open note to be sharp if the intonation otherwise checks out?

 

[SpiderWars]

First thing that comes to mind is that the saddle is cut for an unwound G-string but you’re using a wound G-string.

That seems unlikely but I thought I’d at least get that out of the way.

 

[Chubbs]

To me it sounds like the nut could come up a smidge and or move the saddle forward. Kinda basic but I can’t think of anything outside of that. My ghetto solution for that is fill the nut slot up with pencil lead.

 

[DanTravis62]

I agree with spider, this sounds like a wound/unwound situation

 

[IDDQD]

Maybe it's better to take it to your guitar tech (if possible)? He will give valuable advice or solve the problem. It should be cheap.

 

[Bardagh]

Well, it’s an acoustic so of course it has always had a wound G.

Going by the old stewmac scale length calculator, the distance from the nut to the contact point on the saddle is very close to where it should be, but still slightly long so maybe it is still just a length issue. It’s just weird to me that the fretted notes are all close enough but the open note is so sharp. I’ve never seen it before.

Unfortunately this saddle I have done is as forward as it can be on that string’s position, which leads me to the conclusion that the bridge itself was glued down too far back on the body. This was not a cheap guitar either. It’s actually very well done in every other respect and it sounds incredible, so this is a major bummer.

I don’t have a luthier I would trust with this, so I guess I’m going to have to try and find one.

 

[Sermo Lupi]

If it's just the one string, and you can measure the intonation problem on a tuner, there's only a few things it could be.

If the string is seated as it should be (i.e. securely positioned underneath the bridge pin, no windings from the ball end coming up over the saddle), than it is likely either the bridge/saddle or the nut.

If it is the saddle, and all other notes sound in tune, it could be the saddle compensation. However, you've said the issue has persisted after you replaced the saddle with a non-compensated one.

That leaves the nut. Bear in mind that, if the nut is cut correctly for a compensated saddle, the positioning of the nut could be set for a compensated saddle and not a regular one. I'm not sure how acoustic manufacturers handle this specifically, but I know that Ernie Ball's compensated nut requires it to be placed in a different location (closer to the bridge) relative to a standard nut in order for the compensation to work.

Could it be something like that? Or something as simple as the G-string nut slot being cut incorrectly, or causing the string to bind? If you're fretting the strings and the issue goes away, that's a big red flag that the nut is likely the issue.

 

[Bardagh]

If it's just the one string, and you can measure the intonation problem on a tuner, there's only a few things it could be.

If the string is seated as it should be (i.e. securely positioned underneath the bridge pin, no windings from the ball end coming up over the saddle), than it is likely either the bridge/saddle or the nut.

If it is the saddle, and all other notes sound in tune, it could be the saddle compensation. However, you've said the issue has persisted after you replaced the saddle with a non-compensated one.

That leaves the nut. Bear in mind that, if the nut is cut correctly for a compensated saddle, the positioning of the nut could be set for a compensated saddle and not a regular one. I'm not sure how acoustic manufacturers handle this specifically, but I know that Ernie Ball's compensated nut requires it to be placed in a different location (closer to the bridge) relative to a standard nut in order for the compensation to work.

Could it be something like that? Or something as simple as the G-string nut slot being cut incorrectly, or causing the string to bind? If you're fretting the strings and the issue goes away, that's a big red flag that the nut is likely the issue.

I kinda don’t think there’s a problem at the nut, I think they either positioned the bridge too far back or cut the slot for the saddle at the wrong angle. It’s a bummer that this kind of thing would get by in QC as the guitar is otherwise so well made, sounds fantastic tonally and feels great.

I could hit the company (Furch) up as there is a warranty but then I’m almost certainly shipping it off somewhere with an uncertain outcome. I like how it sounds and plays enough that I’m OK spending money on it so long as the problem is solved and it’s done right. I had just been hoping it was an issue that could be solved with changing the intonation at the saddle. Some of the strings were far back enough on the original saddle that I was able to get there but the G string’s contact point had already been close to the front.

 

[Sermo Lupi]

I kinda don’t think there’s a problem at the nut, I think they either positioned the bridge too far back or cut the slot for the saddle at the wrong angle. It’s a bummer that this kind of thing would get by in QC as the guitar is otherwise so well made, sounds fantastic tonally and feels great.

I could hit the company (Furch) up as there is a warranty but then I’m almost certainly shipping it off somewhere with an uncertain outcome. I like how it sounds and plays enough that I’m OK spending money on it so long as the problem is solved and it’s done right. I had just been hoping it was an issue that could be solved with changing the intonation at the saddle. Some of the strings were far back enough on the original saddle that I was able to get there but the G string’s contact point had already been close to the front.

Sounds like it's time to get a luthier/tech involved, then. Maybe contact the OEM and see if they'll cover the bench fee for someone in your area. I don't know about that brand specifically, but many OEMs honour warranties through local dealers, who in turn have in-house people or contract out the work. At least then they can diagnose the issue for you before you can decide how to proceed.