MIMF

I was advised by a guitar dealer whose opinions I generally respect that I should build in some "fall-away" in the upper fretboard on my archtop guitars. I understand why this might be a sensible precaution for flattop guitars, which often develop a "hump" in that area, but I can't see how that would likely occur with an archtop, where the neck extension is cantilevered above the top and the end of the fretboard is supported by a good 3/8" of (in my case) hard rock maple. What do other people do? Has anyone had a hump develop at the end of their archtop fretboard?

IMHO the "hump" on a neck is not caused in particular, by the body humping up from string tension.
The hump is caused by having to introduce relief into the neck, using the truss rod, and by string tension.
The fretboard extension is not affected by the truss rod; perhaps even more on an archtop, as it does not contact the body. Thus, when you install strings, and adjust the truss rod to get a bit of relief, most times you get a hump at, or above the body joint.

Fallaway can be introduced into that area by filing a "ramp" into the frets.
In other words, you start filing the frets down at the body joint, or where the hump begins. The first fret may only have .001 fallaway.
Gradually increasing fallaway, you arrive at about .006 at the end of the fret board.

This helps to get the action nice and low in the middle of the neck, and stops buzzing in the hump area.

Thanks Gordon - food for thought. But if there is no problem when the neck is straight, and the truss rod does not affect the fretboard extension, I'm still having trouble seeing how introducing relief creates a hump. You've seen this happen on archtops?

On second thought, maybe we are talking about two different things. I can see that if you introduced a significant amount of relief into the neck with the truss rod (I like mine nearly flat, but no matter), and then tried to bring the action back down really low in the middle of the fretboard, the strings would contact the upper frets. But I can't see filing down the end of a perfectly good fretboard in anticipation of that kind of setup (unless, of course, a customer insisted).

I was talking about the phenomenon of an actual "hump" developing somewhere above the neck/body joint, as in a rise and fall. I am told this can happen in just about any guitar, including archtops, but I don't understand the mechanics as it pertains to archtops, and I'm not convinced that built-in "fall-away" is called for. But there's a lot I don't know, so I hope to learn from others here.

I am talking about the hump created by the "valley" in the neck because of relief.
I agree with you that a straight neck is the goal. Unfortunately sometimes we have to introduce relief in the middle section of a neck, to get a low action near the body.
The relief compensates for the vibration pattern of the string.
I usually end up with about .004 - .006 relief on an electric, and no more that .010 on an acoustic.

May i extend the topic a bit?
Sometimes the hump is too strong to allow compensation by filing the frets, e.g. on some bass guitars equipped with really heavy strings which thus require a relatively large tension of the truss rod and thus may develop a strong hump.

So how to compensate for the hump in those cases? take the frets of and sand the fingerboard itself? It should be done under full string tension which i find difficult.

Beate, I still wonder if we're all talking about the same kind of "hump." Do you have any pictures of what you have seen on bass guitars? And is this something you would only deal with in repair situations, or something you would build in to a new instrument? In other words, is this in the nature of a malfunction of the neck/fretboard, or the expected result of tensioning (or loosening) the truss rod?

I don't mean to be difficult, but I just have not yet seen this problem myself (except on flattop guitars).

I found a good discussion of this issue as it pertains to bass guitars over on the talkbass forum - they call it the "ski jump." Hope I'm not breaking any rules by posting this cross reference:

https://www.talkbass.com/threads/gettin ... s.1111265/

Still seems to me the archtop guitar is a different case due to the cantilevered neck extension.

John, no i would not build it into a guitar. But what Gordon describes and what i am observing in some of my bass guitars is the same as what You noted on archtops if the reason is actually the truss rod.

On the other hand i agree that the neck extension may cause similar problems even in archtops that do not have a truss rod at all. One of my guitars has developed it over the decades. The physical reason is quite similar: the neck bends under string tension, but this will of cause not occur beyond the heel. So the neck extension will remain straight which means relative to the main part of the neck sick out upward. This effect is actually unavoidable, dependent on the stiffness of the neck more or less pronounced. As soon there is a truss rod ending at the heel this will turn into a more or less pronounced hump, and again is the same situation as with the ski jump on solid body guitars - it is just the sudden drastic increase of the neck stiffness which is causing this.

In my eyes the compensation can be done only by a ramp over the neck extension.
Or maybe also by avoiding an adjustable truss rod at all but making the neck very stiff.

After thinking about your comments, I have attempted to diagram 3 different scenarios. This first is what I generally aim for as a builder, except that there is always a slight amount of "relief" introduced by string tension alone, unless you level the frets under simulated string tension, which I do not. To me, that slight amount of relief is about perfect and requires no planing of the upper frets. The second scenario is my understanding of what Gordon described, and the third is the "hump" scenario caused by a stress failure or warping of the neck due to string tension over time. Have I got this right, or is there something else going on? Thanks for your help!