Headstock angle and break angle at bridge on an archtop

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Alan Carruth
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Re: Headstock angle and break angle at bridge on an archtop

Post by Alan Carruth »

I'm sorry if I came across as curt. I see the contention all the time that more break angle gives more driving force on the top, and it seems to me we need to think it through.

Although I'm not a math whiz, I can follow it well enough to see that the main signal the strings produce depends only on the string tension and the angle the string is displaced through before you release it during the pluck. Also, I've wasted 'way too much time measuring the forces a plucked string generates at the saddle top, and the data points to the same conclusion. If you push a .014" high E string down by, say, 1/8" at a point 5" away from the bridge, and let it go, the forces it produces at the bridge top will be the same no matter what the break angle is, or what the bridge is tied to, for the most part. The only exceptions I can think of involve problems, such as not having enough break angle, so that the string hops off the top of the bridge during part of the cycle. Since the string is seldom, if ever, displaced by more than five or six degrees at the bridge when it is plucked, it would seem that we don't need much break angle to prevent string hops.

Another problem is 'wolf' notes, where the bridge moves so much at particular pitches that it no longer presents a 'stop' for the string that is firm enough. In these cases the length of the string is not totally defined, so the pitch is not either, and the energy tends to be extracted from the string too fast to be of any musical use. One can see how a large downbearing force at the bridge would serve to 'nail' the top and prevent wolf notes, but it can also kill any chance of the top vibrating.

It's possible that there are certain geometries involving the top arch height, neck angle, bridge height, and so on, that serve to strike the 'right' balance between too much bridge motion that yields a wolf, and too little, that produces no sound at all (a la Les Paul). I've seen diagrams of violin setups that claim to show this (usually also involving the sound post, which we don't have), but I'm skeptical.

If this _does_ work, the only mechanism I can think of would involve a balance of forces. String tension puts a compressive force along the length of the top, which, by itself, would tend to make it belly upward. It's possible that having an equal countervailing down force provided by the break angle over the bridge would serve to free up the top to move. Since both forces would be proportional to string tension in the same way the 'best' break angle would be determined by the geometry. One outcome of this balance would be that the bridge would not be displaced either upward or downward when tension was put on the strings. Thus it would be fairly easy to test the idea out. Naturally, I haven't done it yet!

If I'm right about this, then there is a 'proper' break angle for any given guitar, which may well be pretty high in some cases. This does not, however, translate into 'more is always better'. That is the argument of the idiot with the pepper shaker, and I don't think any of us are idiots. ;)
Michael Lewis
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Re: Headstock angle and break angle at bridge on an archtop

Post by Michael Lewis »

Alan, thanks for the perspective. I had not considered the string tension pushing the top up but do see that now. This makes me wonder how the deflection testing so many folks are wound up about really works out, as there may not be a benefit to have the top deflect under string tension load, except for the string vibration. You raised a lot of ideas, " the road not yet traveled". I haven't done such tests either because it takes a lot of time away from other stuff that needs to be done. Dang, now you have me thinking about making a frame to hold an instrument for measurements before and after string tension.
Mario Proulx
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Re: Headstock angle and break angle at bridge on an archtop

Post by Mario Proulx »

String tension puts a compressive force along the length of the top, which, by itself, would tend to make it belly upward.

On one of my early mandolins with the removable backs, I went too thin in the recurve at one point, and the top pushed-up as you describe. I then opened it back up, and placed a maple dowel between the head and tail blocks, fitted tightly. This dowel took the brunt of the compressive load, and the top never deformed after that. I don't recall what happened to the tone, specifically, other than it did improve. I then incorporated the dowel idea into the next few new mandolin builds, even using CF as the dowel, but have long since dropped the idea. It simply didn't add anything to a properly built mandolin, so my conclusion is that it would only be of benefit with a very light/thin/weak top, but my experiments with thin tops on any type of instrument always lead to the same conclusion, and that no matter the instrument, the tone became thin and "nasely", more "banjo-like".
Alan Carruth
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Re: Headstock angle and break angle at bridge on an archtop

Post by Alan Carruth »

Michael Lewis wrote:
"This makes me wonder how the deflection testing so many folks are wound up about really works out, as there may not be a benefit to have the top deflect under string tension load, except for the string vibration. "

Deflection testing during construction is not meant to mimic the deformation you'll see when it's strung up. It's a way to know just how stiff the top is. Top stiffness affects both the structural longevity of the instrument, and the timbre, as Mario points out. Given the wide variation of Young's modulus in wood both along and across the grain the only way to really know for sure is to measure. Some people can do this by feeling the flex with their hands, others like to have numbers. On a flat top guitar the stiffness is closely related to the bracing, and the arch has something of the same effect on archtops. With arching, of course, you have to look at both the height and the shape of the arch, just as you pay attention to the layout and profiling of braces, and top thickness come in in either case.
Michael Lewis
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Re: Headstock angle and break angle at bridge on an archtop

Post by Michael Lewis »

Yes, but some folks are using a per-determined amount of deflection as a means to "achieving" great tone. The apparent idea is to get about one or two millimeters of deflection under normal string tension ( for a mandolin), which to me seems too flexible.

Since I have not taken any measurements of deflection I have no idea of how much or little my instruments actually deflect (deform) under string tension. I do know that over time instruments tend to deform in specific ways from the constant string tension on the structure, and as a counter measure I have developed my approach to arching to prevent so much deformation. So far it is working nicely, but it took a few failures to show me what to avoid.
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Beate Ritzert
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Re: Headstock angle and break angle at bridge on an archtop

Post by Beate Ritzert »

Back to the original topic?
Robert Kroon wrote: The instrument i put the bigsby on is acually an isana!. It's an acoustic archtop and far as i can tell its made to be one. It's very old and unfortunately
some one drilled a bunch of holes in the top for pickups.
Old Isanas are actually very interesting instruments, not top of the notch, but good. I can easily imagine these old German guitars for a Rockabilly sound; i would even think they have been built for it (at least the electrics). But of course they are not free of feedback problems.

You should be very careful with deep modifications, especially mostly the bridge angle (which would envolve modifying the neck angle of these guitars). Increasing the load from string tension might destroy the top, especially if it has already been weakened by holes. Under no circumstances You should cut through the bass bars in order to be able to mount pickups if they are too narrow.
Alan Carruth
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Re: Headstock angle and break angle at bridge on an archtop

Post by Alan Carruth »

Michael Lewis wrote:
"Yes, but some folks are using a per-determined amount of deflection as a means to "achieving" great tone. The apparent idea is to get about one or two millimeters of deflection under normal string tension ( for a mandolin), which to me seems too flexible. "

Maybe I have not been paying close enough attention, but my memory is that deflection testing is usually done as part of the carving process, either on or off the rim. Off the rim I've seen holders being used, and on it the readings are generally taken with the instrument unstrung. They may duplicate the down load using a weight, but that's different from the load with strings because of the end pressure. Some folks do, I guess, work on the recurve with the instrument strung in the white, but I
m not sure they are measuring deflection in any regular way; usually seeming to rely on finger pressure to gauge the edge flexibility. I suppose that every imaginable process is used by somebody somewhere, and there's probably a maker who measures top drop with the strings on. I also put my conjecture out more as a musing than anything else: it seems plausible that a load balance would work well, but other things might work even better.
Robert Kroon
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Re: Headstock angle and break angle at bridge on an archtop

Post by Robert Kroon »

I would never modify an old instrument like that. I cust the poles off a p90 and wedged a strip of wood i mounted to the bottom of the pickup between the bridge. As far as my build goes. The top sounds about as hard as a skateboard deck when i tap it with my knuckles. It had some taptone before i glued it to the sides and braced it. I think il stick with a soundpost to kill some feedback and see how it works out.

Interesting read the last couple of posts.
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Mark Swanson
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Re: Headstock angle and break angle at bridge on an archtop

Post by Mark Swanson »

Yes Robert, but much of the talk here is about acoustic instruments and you're talking about an electric guitar. So that much anyway is like apples and oranges.
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Beate Ritzert
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Re: Headstock angle and break angle at bridge on an archtop

Post by Beate Ritzert »

Well, to my understanding, the guitar in question is not originally an electric, but indeed an acoustic archtop, and among other things Robert is modifying the guitar in order to be able to play Rockabilly.

Many of these german archtops have been built with pretty strong and thick tops and therefore had a percussive and bright sound which makes them interesting for electrification.

The Isanas play a somewhat special role as far as Elvis Presley has owned such a guitar when he served in the army in Germany. That guitar was also an acoustic archtop which had been electrified later on. See for example http://scottymoore.net/ep58isana.html
Bob Francis
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Re: Headstock angle and break angle at bridge on an archtop

Post by Bob Francis »

Beate Ritzert wrote:Well, to my understanding, the guitar in question is not originally an electric, but indeed an acoustic archtop, and among other things Robert is modifying the guitar in order to be able to play Rockabilly.

Many of these german archtops have been built with pretty strong and thick tops and therefore had a percussive and bright sound which makes them interesting for electrification.

The Isanas play a somewhat special role as far as Elvis Presley has owned such a guitar when he served in the army in Germany. That guitar was also an acoustic archtop which had been electrified later on. See for example http://scottymoore.net/ep58isana.html
There is a red Isana at the Hard Rock in Tamps labled Elvis Presley
Robert Kroon
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Re: Headstock angle and break angle at bridge on an archtop

Post by Robert Kroon »

Mark,

The Isana is an acoustic instrument and im building it along those lines. It may not be fully acoustic because its laminated but
my Isana sounds fine. So this thread is kinda a grey area. If there's a point your trying to make please expound.
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Mark Swanson
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Re: Headstock angle and break angle at bridge on an archtop

Post by Mark Swanson »

The point I was trying to make is that if you're going to put in extra braces, and route holes in the top for pickups and drill holes in the front for knobs then the "acoustic" side of things gets compromised and what you are really dealing with is a hollow electric guitar. You couldn't expect a guitar to retain a good acoustic tone after adding all those things.
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Robert Kroon
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Re: Headstock angle and break angle at bridge on an archtop

Post by Robert Kroon »

Not cutting or drilling any holes in the top Mark. Except for the f holes obviously. Mabee ill add a sound post if it's needed but my Isana works fine without it.
Nice little bit of controlable feedback at high gain. Haven't we had this whole "different beast" discussion already.
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