Intonation issues

[LARRY KLOSE]

A friend was given a 2002 Sigma (Martin import from Korea) guitar with a fishman preamp/under saddle pickup.  It is in good shape but has a couple of issues.  The missing tuning key was replaced at my LMS.  It also had a decomposing plastic nut and the plastic saddle was too narrow for the slot, so it leaned towards the nut.  This made everything about 10% sharp at the 12th fret.  25 3/4" scale.

I made a new nut and a saddle that's the same thickness as the slot (about

2.25 mm) so it stands up straight.  With the radius of the saddle down the middle, it still is about

5 cents sharp at the 12th.  I re-configured the radius so that the peak is about 3/4 towards the butt end of the guitar, away from the nut but it's still 3-4 cents sharp.  Things start to get sharp at about the 4th fret and it goes up from there.

The question is whether I can put a still sharper angle on the top of the saddle, very near the back edge, and whether that would do any good.  any mechanical problems with this approach?  This thing isn't worth enough to have a luthier fill the slot and have a correct saddle slot routed.  Could the nut be too close to the top of the fingerboard?  Things were pretty sloppily done up there.

[Len Biglin]

Hi Larry , I would say if the intonation is good on 1st couple of frets , the nut must be ok . You might be able to get a small piece of saddle and wedge it in behind the saddle , to check how it will go . Some manufacturers do have their saddle cut back to a sharpish edge on the B and sometimes E6 . 3 or 4 cents will probably come in , but you may find that a wound G will want to be near the front edge.Hope that helps , you seem to be on track anyway.Len

[FRANK]

have you compensated the saddle ? the last time i saw the 4 th fret area out of wack it was the nut height was high

[Pierre-Antoine Roiron]

You can check the nut : it should be half the string scalescale from the 12th fret. But I'm quite sure the problem comes from the saddle. I assume that the string height is OK. I would make a larger saddle (see Len hint or enlarge the slot) and work on it with a file string by string. Good luck!

[Frank Ford]

I wouldn't hesitate to angle the top of the saddle all the way back. You'll need to lengthen the compensation by a bit over .040" to take up 3 cents, so there will be limitations.

There's no need to try it experimentally - here's a simple formula to calculate intonation correction:

http://www.frets.com/FRETSPages/Luthier/Data/compcalc.html

[FRANK]

hey frank your message is gone it is not there ?

[LARRY KLOSE]

I followed Frank's suggestion and angled the top of saddle as far as practical to the back edge. Problem mostly solved. This bridge has the saddle slot angled more than a Martin, it resembles a Gibson, so compensating the saddle didn't make much difference in the over all scheme of things as it would on a Martin.

Now that I have the saddle done and the intonation correct (well, all but the low E string, still sharp by 3 cents), it's apparent that this guitar has a fretboard problem. everything goes sharp about the 3rd fret, progressing up several frets toward the 12th. At least this is true with my electronic tuner, the now ubiquitous FZone FT-800. Maybe I should try my new Intellitouch PT 10 to see if I get the same results. If I had a Peterson would I get much of a different result?

One thing I learned is to seat the saddle firmly in the slot. The first time with the last setup I didn't push it in very hard and and it leaned enough to still be sharp. Corrected by releasing tension and pushing it down to seat firmly.

Larry, happier