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Intonation Problem

I have a problem wherein I made a small cookie-tin instrument about the scale of a soprano uke with steel strings and the first fret sounds two half steps above the open note, as opposed to one (if I tune to C, the first fret plays a D). The rest of the frets increase the tone the usual half-step. I was thinking it was mostly an action-based problem, but as I file the nut down further, it doesn't seem to be changing much... I'm going to continue filing, as I assume any other cause would require replacement of the nut anyway... I was just wondering if anyone else had similar problems on any of their builds. And possibly an solutions! Here's some other things I thought might be the culprit:

High string tension - Combined with the high-ish action, this would seem to be a logical problem... With pressing to hard, it could cause the note to go sharp.. But I would think this would be a more widespread problem than just the first fret.
A thick nut - I made the nut out of a section of bone a little over 1/4" thick.. I thought maybe the string was making contact with the wrong side of the nut, so I sloped my filing to make sure it contacted the string where it was closest to the fretboard... It didn't seem to matter much.
All-over incorrect intonation - I checked, rechecked, measured, and remeasured the bridge placement.. Seems ok...
String width - This kind of goes aloing with tension and action and all that, but I have medium D G and B acoustic strings on this thing.. Perhaps lower gauges would be better?

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Permalink Reply by Dan Sleep on July 27, 2011 at 1:17pm

On my builds, as a starting point, I slip a penny onto the fretboard and cut the slot until it just reaches the penny. This has seemed to give me a nice action. If you slip a penny under the string that is causing you grief, how close is the top of the penny to the bottom of the string?

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Permalink Reply by kickuntilitbreaks on July 27, 2011 at 2:08pm

I'd say around half the thickness of the penny away from the penny itself. It's kind of a guess, because the fret spacing is too small to fit in a penny and the action raises further down the neck.. Would that be a problem? Needing to lower the action on the bridge end? I hadn't tried it yet because I used ball-end strings that push the tailpiece a little high for the bridge to go any lower.. But I just thought of using either tie-end strings or loop the ball-end around to the other side...

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Permalink Reply by Dan Sleep on July 27, 2011 at 2:22pm

It does sound like the string height is too high, thus causing the notes to go sharp on you when you fret a string.

Any chance you can post a picture of the nut and the bridge?

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Permalink Reply by Outhouse Instruments on July 27, 2011 at 3:30pm

Two words. Uke and steel strings. Ok that's more than two words.

But steel strings and extremely short scale ( saprano uke ) don't mix well. Now I know someone's gonna say they do it, but Ive never seen steel Uke strings for sale. The string height will have to be insanely low to keep the first fret from going too sharp.

My suggestion .... get real Uke strings. The nylon is stretchier, and won't go as sharp.

AFKAM

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Permalink Reply by kickuntilitbreaks on July 28, 2011 at 3:36am

I thought about that too, I was just hoping to use steel strings.. The instrument I made is only 3 strings and I was hoping for a more mandolin-style metal sound.. So I used the steel strings..

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Permalink Reply by Dan Sleep on July 28, 2011 at 3:57am

Using the nylons strings would probably help, but looking at the pics, it looks like the action at the nut is still pretty high.

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Permalink Reply by kickuntilitbreaks on July 28, 2011 at 12:17pm

Ok guys.. Thanks a lot! I'll keep messing with it, then!

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Permalink Reply by Mark Bliss on July 28, 2011 at 6:38pm

It sure looks to me like the distance from the string termination at the nut to the first fret is a bit short too. In the photo it appears to be shorter than the distance between fret 1 and fret 2, when it should in reality be very slightly longer.

If lowering the action a little further doesnt help much, maybe modifying the nut into an "L-shape" by removing a little material at a time from the finger board side.

Nylon strings are more forgiving as far as these problems go, but with some tweaking this should work. Mando scale lengths for instance are very short too.

Keep at it.

Mark