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I've been playing my arch top for 3 months now. I just noticed that the g intonation is sharp, Playing octave A's with the second string is jarring. Yes, I THOUGHT I had that all set when I set it up, but I changed strings from very light nickel? jazz strings to very heavy nylons. Funny, I can't even find the old ones now. I didn't touch the bridge at all besides raising it up for clearance, and placing it where the e strings played octaves at the 12th fret. I checked them the yesterday while practicing, and decided I had to fix it.

The clip on tuner doesn't work in cents, just has bars. At the 12th, the E's are off one, the A and B are right on, The D is off 2 and the G is off 5! It's a big, fat string. The top of the bridge is about 2-2.4 mm wide. How wide should they be? Or how thin can you take them. I have to file the one back quite a bit.

The strings are 2.15-2.7 off the 12th fret. The G was 2.6 but I lowered it some, it's 2.5, and I was filing it back, but I don't now how far to go.

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Why be normal?

You are always going to find that the required compensation is different for steel strings (need longer) versus nylons (shorter), and also different for wound strings versus plains. If you went from a wound steel 3rd string on your previous set, to an unwound nylon third string in the new set - it is not surprising that the G string is the one now showing the greatest change. Unwound strings need more compensation than a wound string (different ratio of stiffness:mass). This is why you see most steel string saddles move the contact point further from the nut (increased string length) for the unwound B and top E strings, and some makers like Lowden split the saddle to achieve this difference. If your nylon set also has an unwound G string then it too will need longer length (compared to what a wound G string would need). Additionally, you raised the bridge a bit, and higher action needs more compensation too.

You could adjust the contact point on the saddle for the G string to be more towards the tail piece. However, your 2mm saddle does not give you a lot of wriggle room. On the guitar I am building at the moment, a seven-string multiscale, I am using a saddle which is 1/4 inch (6.3mm) wide so that I have lots of room to play with compensation. If you switch back to steel strings later it will revert to the previous situation. You might need two different saddles - one for steel strings and another for nylons.

I'm sticking with the nylons. It sounds pretty good with them, and it was supposed to be what an arch top from 1780 or so as a response to his sons flattop guitars. I think the light steel strings sounded great, and could be set lower, but I thought I saw the neck move when playing; that prompted the string change. I bought both sets to see which would work. It's built pretty lightly, so they just overpower it. A small neck block was probably not a wise approach; I don't know much about guitars, but a neck block with feet on it, and cut away in the middle might be better.

I agree that I will run out of real estate, filing it back a mm has only taken care of 2 of the 5 bars. I think I'll take a pragmatic approach, and glue a small piece of rosewood on the back of that section. I'll work on the D too being sure that one doesn't need more wood, so I can glue them on at the same time.

It won't be a hack butcher job, I can probably find something that will match fairly well. Has anyone had success with that approach?

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Why be normal?

Plain nylon G strings do need a fair amount of compensation, so it's not surprising. They do make wound G's. I used to like the Savarez 'red card' strings with a wound G for sound, but the winding is plastic, and wears out very quickly.