intonation problem

[Willie]

I have a Gibson made Flatiron and the problem is that when tuned to standard pitch the G, D and E strings note correctly at all positions, the A string notes high at the fourth fret and low at the 12th fret...I have tried different gauge strings. I have looked at the position of the A string where it rides through the nut and it seems to be touching the front of the nut...The bridge is the one that came on the mandolin, I tried a different saddle and it did the same....I am open to all suggestions at this time...I am not a "newbie" and this has me stumped....Thanks...Willie

[JGWoods]

Is it the 4th and 12th frets only that show evidence of incorrect intonation, or is it a trend from 4 to 12 that it gets a little flatter with each step?
and how old is the instrument...has the neck been removed at any time?
no luthier here, just curious.
best,
gw

[sunburst]

That's something you can't correct at the bridge. If you try to correct one the other will suffer.
It would be possible to help the situation if you could shorten the distance from the nut to the fourth fret. That means shorten the distance from the nut to the 1st fret, but since it's the 4th that you mentioned, it's the one that counts.
It's not real practical to do, but a "step nut" could be installed. That means that there is a piece of nut material that extends over the fingerboard a little ways to help compensate the string length. That is a method sometimes used by banjo players and also when applying the Buzz Featon(sp) system to certain vintage instruments.
You can also cut off a little of the board at the nut end and install a new nut closer to the first fret.
It is likely that all the strings note a little sharp at the first 3 or so fret positions, most instruments do unless the nut position is compensated toward the first fret a little.

[London Al]

I'm stumped about this one over in England too! If the strings low at the 12th the bridge or the nut arent supporting the string where they should be, thats logical. But the 4th fret problem? Might you be hearing a dead spot? this happens at C sharp on Fender Precision basses sometimes- just a thought to try and help.
Maybe the 4th fret has a ridge at the bridge side on the A string area
I' d check out the whole fretboard with a digital tuner.

Good luck with that one Willie

Al

[Willie]

I should have said that it is sharp at the 4th, 5th, and 6th, frets and notes good on the 7th and is flat at the 12th fret.....I have thought about sending this thing back to Gibson but its not off enough to be real bothersome...A compensated nut (step nut) on the A string should do the trick and that is my next experiment to try...Thanks for all of your comments....Willie