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I'm curious - what do most folks here use for the degree of angle for the neck/body joint for flat tops? Archtops I've been using 4.5 % per Benedetto's book, and flat tops I've used 1.5% based on my own measurements of some fine mass produced instruments (not martin or taylor et al, but some fine instruments produced in lesser quantities). I'd really like to know what this esteemed group of artisans uses, though.

Many thanx - Byron

That will depend entirely on the radius used for the top, and where the apex of the curve is on the top (ideally at the X in front of the bridge), and bridge height.
For me (20' radius, .350" to .375" bridge height) it's about 1.5º.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

As Laurent points out , it is matching you bridge placement. There is also the length of the body to consider. Top radii is ofter between 25 and 30 foot R. When you run the math for the best transfer of energy from the strings , the string height at the bridge in most cases will top out at 1/2 and inch. On most bodies of 19/12 to 20 inch length you want a 28 to 30 foot radii. this will get you a .312 bridge and .125 saddle. this will get you the 1 1/2 degree neck angle.
Another way to do this is to angle the top to match your neck angle. Most necks will run from 1 1/2 to 2 degrees depending on the supplier. So you can see there is no one answer. Just consider the inputs for the end result
john hall
blues creek guitars

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Angle doesn't matter much IMO; what's important is the bridge and action height. Try this:

Draw a side view of the neck of a guitar, with the strings on it at the proper height above the frets. Now make a transparency of the side view of the body with the bridge and saddle. Pin the top of the saddle to the end of the strings over the neck drawing. You'll see that there is a whole range of neck angles that will result in the action and bridge height coming out right. You can't go back too far, but there is a wide range of allowable up angles.

The question is; what effect does the neck angle have on the sound? So far the experiments I've been able to do suggest 'not much'.

I do know that too much break angle of the strings over the bridge on an archtop can kill the sound. This is only indirectly related to neck angle, of course. It has been suggested that the tension change that happens twice per cycle of vibration of the strings could push inward on the top if the neck angle is high enough, but when I tried an experiment to look for that tension change signal I didn't see it. I suspect the angle is too low.

Harps do benefit from this effect, since the strings pull upward on the soundboard at a very steep angle. The 'Millennium' is supposed to work in that way, but, again, when I tipped the neck up on one of my 'test mules' to give the requisite 5* angle, I saw no evidence of that in the output.

There is a difference in top loading depending on neck angle: obviously there will be a net upward or downward force on the top if the strings are not parallel to the plane of the top (whatever that is). You can see the structural result of this on guitars with a pronounced angle.

Acoustically having the neck pull upward on the top 'should' raise the pitches of the top resonant modes. It's like tightening a banjo head. That could effect the tone. I was not able to play the test mule with the neck tipped up, so it's hard to say whether it really had an effect: there's another experiment I need to do. Tightening the strings on an archtop guitar pushes the top down, and that does lower the pitches of the top modes.

As usual, the whole thing gets complicated if you start to think about it too much. In the end, I really doubt anybody would hear the difference that a half degree or so of altered neck angle would make, so long as the guitar was playable and the height of the strings off the top was not effected.

As Al said, there are many angles that have good action and a good saddle height. The problem that I have on most of my instruments is that the fret board extension often has to be tapered to stay flat on the body. This taper may be to make the fret board thinner as it approaches the sound hole or it may be thicker.

I don't like the looks of a tapered fret board. I'm experimenting with adjusting the thickness of the upper bout so that a fret board lays flat and there would be a good saddle height. You can do this with a simple sanding board. Once the fret board lays flat, measure the angle of the top in relation to the side at the neck join. That's your angle. Make your neck with that angle in the heel.

Another problem is that the top sinks in a little when the strings are up to tension and that ruins your action. Having a very solid upper bout bracing structure is very important. I'm adding 1/16" to my upper transverse brace and going to an A braced upper bout to minimize this effect.

To answer your question, however, I'd have to say that my neck angle is whatever it happens to come out to once I've adjusted the top to make the fretboard lay flat and have good saddle height.