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I'm making an arch top, my first guitar, but I've only played on a classical and don't think that I'd like the extra string pressure. I didn't realize that arch tops were steel strings until I started building it after seeing and hearing a cool one on line. Sounded almost harp like. Besides that, I'm making it like what I'd think an Italian violin and cello maker would make an archtop in 1780 or so. He would have used gut, and nylon is about the same pressure.

I know it needs to be more flexible. That shouldn't be a problem. I read someone who makes nylon string arch tops, and he said that his regular arch top bellies are 400 grams, and a classical is about 250. Both of those numbers seem VERY high to me. My VERY thick back is 500 grams now! (it's not HUGE, only about 16 inch) The redwood belly I have roughed out ( I think it is 4-6 mm thick) is 350 g. now, and I figure it could easily drop 50 grams. It rings like crazy. Should be good. Cut out f holes, and add two braces, and you're about at the same place.

Making violins and violas I watch weight to an extent, but I glue them up, and finish the edge and recurve after; so I have no real idea where they end up. (60-70 and 90-110 I'd guess) I don't know where the finished tap tone is either, by then I'm tuning them as a unit to the body, and not a free plate.

Any helpful hints? Has anyone strung one up that way?

Oh, I'm thinking of no truss rod. See my blog for reasoning. It's more than the lack of a router, and not really wanting one!

https://kensarchtop.blogspot.com/2019/03/the-physics-sort-of.html

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Why be normal?

You won't find much info for weights used in archtop guitar plates. Archtop builders, generally use graduation, flexibility and tapping to achieve the sound\voice\tone of the plates. If you are attempting a carved archtop with nylon strings, I would say that you are in uncharted waters and you are on your own.

I have not done this put will put this out there for you to consider as a starting point.
This assumes nylon strings have approx half the tension.
A "standard/average" archtop plate is . 25 in the center I believe. It's been a long time sine I made one!
So if I have half the string tension, I might shoot for a plate that's half as stiff. That'd put it in the .2 range in the center.

Steel strings and classical roughly follow this rule. A classical top at .085 has approximately half the stiffness of a steel string top at .110.
Waiting for some comments.

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Jim Watts
http://jameswattsguitars.com

I make a nylon string arch top, but I went to a carbon fiber top to get the weight/stiffness right. I know that Alan Carruth built some out of wood, but I just don’t see how you could get it thin and light enough without imploding. I do notice that after only a few he apparently stopped making them.

I know what you mean about a good acoustic arch top having horn like evenness and balance. But It is REALLY challenging to get that tone. It is way to easy to end up with something nasally or barky. There is much less history with these instruments, and little accepted parameters to get the sound right. Acoustic arch tops existed from roughly the late 20’s to shortly after WWII. Compared to a steel string dreadnaught that’s a tiny time for development. Most archtop guitars since the early 50’s are truthfully electric guitars.

All I can suggest is carve it as thin and light as you dare. You can play an arch top in the white first before doing all the finish work. I met one guy who would use a very fine brad nailer to barely attach the top to the rim and check the sound that way before gluing the top to the box. Or so he claimed —I never saw that. In theory one could keep thinning the top and carving the recurve and braces, string it up, test, and repeat as needed.

In any case, a nylon string archtop is a fringe instrument and you are going to have to figure a lot out by just doing.


Sent from my iPhone using Tapatalk Pro

I have made two that I would consider successful, and would happily make more if there was more interest. I may do so anyway; I've got a couple of ideas I'd like to try out.

Keep in mind that arched plates get a lot of stiffness from the arch, both the height and the shape, as well as the thickness, of course. You can alter the timbre a fair amount by changing those parameters, and I don't think anybody has explored that whole space enough to give definitive answers for nylon string arch tops.

Nylon strings have high damping: they tend to 'eat' vibration energy much faster than steel strings. This is, in part, the nature of nylon: if you tap a piece of nylon it goes 'thud', where a piece of metal will tend to ring on. Also, nylon strings being lower in density than steel have to be fatter to get to a given tension. The fatter strings have to move more air simply in order to vibrate, and this is like trying to run in knee-deep water. Damping acts by reducing the energy by a certain proportion per cycle of vibration. Since high frequencies have more vibrations per second, they tend to die out faster. Thus nylon strings will have very little high frequency content a second or so after being plucked, where steel strings can have quite a lot. Thus the central problem in making classical guitars is to get the most out of the little bit of high frequency in the strings, where steel strings are trying to get enough bass to balance all the treble.

It may seem counter intuitive, but it's easier to make a powerful small guitar than a powerful big one. Power depends in large part on the ratio of vibrating area to mass; the higher the A/m ratio the more power. As you make the top and back larger you need to beef them up a bit to retain sufficient stiffness as the span increases (think about using larger joists on a wider floor). The mass tends to go up faster than the area. Part of the balancing act involved in designing guitars is to find the size that produces enough power and also give the right treble/bass balance. Since steel strings carry more tension, and have more energy, and more of that is in the high frequencies, steel string guitars tend to be made larger: they can get away with some added mass, and need the added bass response. Classical guitars are generally smaller: around the size of a 'parlor' steel string, which helps with both treble response and power.

What's interesting is that when arch top guitars really started to be made in any numbers they kept making them bigger and bigger, and never did run into the problem of declining power. What happened was that they stopped enlarging the boxes when they got to about 19" wide, simply because they were too hard to play.

The arch classicals I've made have all sounded like 'small' standard classicals. The stiffness of the arch, among other things, tends to enhance the treble, so that's not an issue, and also helps retain the proportion between area and mass. The first two I made were 15" wide; nearly an inch wide than normal classicals, and were in no sense 'too big'. The two that succeeded best so far were 16" wide, and although they still have a 'treble' balance, are reasonably successful in the standard repertoire.

All of the ones I've made so far have had 'X' bracing: the next one will probably be 'parallel' braced, and may be 17" wide.

One other thing to watch is the arch height and top plate thickness. On the first two I made the arched high; scaling them in proportion to the box length relative to a violin. I then thinned them out to get what I hoped would be the right stiffness. Then I read Schelling's paper on 'The Violin as a Circuit'. He pointed out in a footnote that in order to maintain dynamic similarity the arch height should scale as the thickness of the top, not to the box length. On the next two I more or less arbitrarily settled on 3mm as the 'proper' thickness, which gives an arch height the same as that of a violin. They worked much better.

The top masses on those two guitars were 201 grams and 191.4 grams. Both were Engelmann spruce, chosen for low density. These are heavier than the tops on normal flat top classicals, but they are also much larger in area. The backs were 289.5 g.(mahogany) and 316 grams (? maybe, in maple). The tops used a more or less 'flat' arch profile, 15mm at the high point measured over the arch, with 'curtate cycloid' cross arches and reverse graduation at 3.2-3.3 mm overall, with a thin area in the center. The 'X' braces were 11 and 8.7 mm tall at the crossing respectively.

Hope this helps.

P.S.
One on my students gave me an article about Francesco Moreno's gut strung arch top guitars dating from the 1820s. It's in 'Early Music', Vol. XLVI, #1.

In the Fall 1999 issue of the ASIA journal 'Guitarmaker' there is a section on the nylon string archtops of
four makers, including one of Alan Carruth's, that might be of interest.

"I didn't realize that arch tops were steel strings until I started building it after seeing and hearing a cool one on line. Sounded almost harp like. "

If that is the sound you are after I would suggest you find an archtop with a good set-up and see how much the additional fretting pressure bothers you. It's really not that bad, and I don't think you will find the sound you are after with nylon strings.

You know, I was just thinking about that last night at work. I don't really like the idea of the truss rod. I don't have anything to cut that kind of slot with. But I watched some videos of gut and nylon strung guitars this morning while writing my blog, and they sound good to me. Then I went back to the one that I liked. A parallel braced arch top. I can't say that it sounds brighter. But the notes are separate, and distinct.

http://koentoppguitars.com/blog/shop-concert-with-nate-wilkinson/

I found a guy playing a nylon strung archtop, and the sound is pretty good, even through the so so recording.

https://www.youtube.com/watch?v=4br9CIHiiuE

A bright, live top would be good. Thick sounds brighter, but is that true when it is played? I'm guessing maybe not. One of those trick things.

Of course it would be great to be able to play like Steve Howe or Jimmy Page, but now that I'm retiring, I have time to practice! But first I need a guitar.

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Why be normal?

Alan,
You brought a lot of great stuff. I'll have to think about that for a while. This one has high arches. My violins and violas mostly have high arches. The sound seems more complex to me. Maybe its just that I like the way they look. The back has cycloid cross arches, but they are extended in the middle because the guitar is so much wider in aspect to its height than a violin. More extension in the bouts, and less at the waist.
The belly is basically straight catenary arches. I didn't take them all the way to the edge. There is an arch coming up from the edge, and the point of inflection is closer to the edge than on the back. It lines up with the f holes, which are quite far apart as compared to a violin. It's kind of a cool shift in thinking from violins.
The cutaway changes things up a lot too. I would imagine a cutaway sounds MUCH different than one that isn't cutaway.
Both are constructed from the inside with diagonal cross arches. It made a very strong plate that hopefully can be thinned, and not get wimpy.

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Why be normal?

To me the Koentopp guitar sounds like what I think of when I think of an archtop. The nylon strung archtop sounds like the way I've heard Baden Powell or Laurindo Almeida play a classical guitar. Obviously that was influenced by the tune, but it really didn't sound much different than a mid level classical guitar, again that could be the recording.
I believe the Koentopp guitar was amplified, which is another thing that allows them to be set up with a soft action and played in a way the acoustic instrument was not originally intended to be played.

Yeah, I want to get that sound without the mic.

No problem, right?

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Why be normal?

Please re-consider the idea of a truss rod. It really is worth it especially for longer necks. I have a Chinese made plough plane that I modified and have used to cut truss rod slots in mahogany. But you can also make a three or five piece laminated neck where you design-in the slot instead of cutting it in.

If you are going with nylon strings you could laminate a 1/4" thickness of ebony in place of the adjustable rod. Carbon fiber bars are another option.

A plane? Didn't even think about that. I just finished a very cool convex hogging plane, and was planning on making a little router for inlays. The blunt chisel idea should work good for that. A sharp edge for the plough plane. Making a plough plane would be more of a challenge than a guitar!

Clay,
I guess you're saying that an acoustic NEEDS the tension to get the sound? I read somewhere that said to set strings up and find where they sound the best, and if it isn't the note that you want, use the tension charts to find the one that fits. Much easier to do, especially price wise, on a guitar than a violin. It seems like you would have to do all at the same time, because the tensions change as the adjacent strings are tightened. Does everyone do that?

Maybe I'll go to a guitar center and try some arch tops out.

Are the nygut strings that much different from regular nylon strings?

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Why be normal?

I was thinking about that plane last night at work, and I see that it is just a little thing. A tiny little jewel of a tool. I'll make it for 3/4" stock, and can make it out of 1/4" hardwood from Woodcraft or Rockler. They're both about an hours drive. I'll check to see if I have a 1/4" tool bit by my lathe in the basement. If not, I'll have to buy one of them too.

I see that their stock is 1/4" x 3" x 24" Cool, that should leave leftovers. Just leave the center 1/4" and put the shims on either side of the tool bit. Then I don't have to plane them flat.

Oh, this won't be hard at all.

But will it work?

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Why be normal?

"I guess you're saying that an acoustic NEEDS the tension to get the sound?"

Hi Ken,
No, what I am saying is the original archtop (plectrum) guitar was made to be played with a pick and provide rhythm back up for dance bands. The action was usually set moderately high so it could be strummed forcefully and not buzz on the frets. After amplification was introduced the un amplified version of the guitar lost its popularity.

That would explain the gigantic pic guards. I've never used a pick.
So a finger picking archtop is a more modern invention, and usually amplified. I'll get it to work. That's our job, isn't it?
If an acoustic steel string, or classical can have great sound, why shouldn't an acoustic archtop?
I'll put a truss rod in, just in case it needs over 100 lbs on the strings to make it work.

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Why be normal?

A friend of mine got to try out one of Freddie Green's guitars at a vintage show. It was a 19" wide Stromberg. The strings were set up 1/4" off the fretboard at the 12th fret, and the low E was about a .070". (This guy has a masters in Jazz guitar from Berklee: he knows what he's looking at). He asked the owner if that was the original setup and was assured it was. Once my friend managed to actually fret a chord he hit the strings. Everybody in the hotel ballroom turned toward him and said: "What was THAT?".

The power in a vibrating string goes as the tension times the square of the amplitude. Tension goes as the cross sectional area, which is about 56% greater than 'normal' strings, so they're carrying 1.56 times the tension. That action being about twice as high would allow for about four times the power for a given set of strings before it would start to buzz. So that guitar could theoretically make about six (in round numbers) times as much sound as your normal box, assuming the numbers are correct. You'd hear that. Would you want to play a gig on it? Not me.

I've been taking stock off the back to have it close to size before I glue it on the back. I started at 500g and had a tap tone at close to 400; just above the VERY THICK belly that is 392. I just listen to an online tone generator. I got it to 430, and it was 330 hrtz. I was thinking that it would drop in frequency like that. Now at 375g it taps a tone below the top at 368 hrtz. It freed the back up and let it ring? I'll take another 40g or so off if I find where to do it. Right now it isn't quite even but it goes from 3-6 mm with 4 plus on the edges.

Maybe it will drop in tone instead of rising?

The belly is old curly redwood. It is .38 sg. It is very resonate stuff, but it seems like the cross grain stiffness is low, I think I'll keep it thicker. But thinner than the 5.5-6 mm that it is at now. Even that thick I can just barely flex it some. I'm careful, wouldn't want to snap it! Recurve would easily take 50 grams off it, and 3.6 - 4 mm might get it to 200 grams. Or something like that.

Would cross bracing be a better choice for wood like this?

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Why be normal?

Now THAT is a cool story! Thanks for sharing.

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sweat the small stuff.

It would interesting to know how that guitar was tuned. It seems that if it was tuned using the open strings, every fretted note would be very sharp. Maybe it was tuned to fretted notes and played with no open strings. A jazzer could get away with that I suppose.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

My two most successful archtop classicals were both built on a 16" small Jumbo platform. The first was spruce/mahogany, and the second spruce/European maple. The tops weighed 201 grams and 191.4 gm, respectively, and the backs were 289.5 gm and 316 gm.

I tend to be suspicious of old curly redwood. Some of the stock like that I've gotten has had low long grain stiffness and high damping, which makes me suspect compression failure. Basically, in a very large tree, the compression loads near the center at the base can exceed the strength of the wood, and the cell walls fail by micro fracturing. You can't see the damage, but it's similar to 'wind shake' in that the wood is already broken. The 'settling' of the tree can cause a type of curl. I'm not saying all curly redwood suffers from this, bur it's something I watch for.