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I've been thinking about building a classical nylon string archtop someday soon. I fell in love with this sound on Joe Pass's album "Unforgettable". It's gorgeous. I know a little about the guitar he used... It was a flat spruce top with a carved maple arched back. The only braces were on the inside of the top. Not sure if they were parallel or X. I know almost nothing about Classical guitar building. How thin is the top usually thicknessed to? Could I use a No. 4 plane to thickness a Classical top, or is that too small?

What you're describing sounds like a Buscarino 'Cabaret' model. The top would be the same as a 'normal' flat top guitar, and only the back is arched. If that's indeed what it is (I have not seen the album) then there would be no bracing on the back; the stiffness and strength come from the arching and thickness. This is a neat way to go, in part because the arching of the maple back helps it behave acoustically more like a rosewood. I've built a steel string that way, as did one of my students, and the sound is interesting.

I have built three nylon string guitars with carved, arched tops. They were intended to be used as 'classical' instruments, but the first two did not have an acceptible timbre for the standard repertoire. The third one is much better, although the sound is still not exactly 'traditional'. It's sort of a tricky balancing act to get the arch height and graduations just right to make the timbre come out the way you want it, and also have the top be light enough to have good volume AND strong enough to not sink.

Alan, all interesting points. The guitar I spoke about in connection with the Joe Pass album is a Borys. I believe it's called a Borys Jazz Classic. I can attach some pictures of one that was recently for sale.

This type of guitar is something I would love to build someday. I've been thinking about the design elements bit by bit every day. My two inspirations are the Borys and a nylon string acoustic D'Aquisto built. My plan is to use a nice stiff Tyrolian Spruce soundboard, Euro Maple back, and Honduras Mahogany for the neck. Everyone always goes straight to Maple for the neck, but I believe in this case the Mahogany will sound better.

Since this is kind of a black-sheep archtop there are some uncertainties that I wanted to get your opinion on. What's a good thickness for the top? For my guitar the top will not be arched. It will be more in the style of a classical guitar like the Buscarino you mentioned. I will probably X-brace it. I'll arch the maple back. Even though the guitar will have a lot of inherent archtop qualities I think I should learn a little about Classical construction before starting this project. What's a good book that covers the basics?

BTW, here's a picture of the Borys. The sound of this model is what made me consider this hybrid build.
-John

There are several good books available, I realy like "Making Master Guitars" by Roy Courtnall. Not cheap, but worth the money!

Typically, classical tops are 2-3 mm thick, mostly around 2.5mm for spruce. And yes, you will have to get used to measuring in millimeters as that is pretty much universal for classicals.

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There are a number of good books out that give details of making classical guitars. Basically I'd follow what they say except for the back. You will probably want to make the sides shallower, though, as the back arching adds some depth.

The decision about how to brace the top will depend on a lot of things, including the sound you're looking for and the size of the guitar. If you stay with a 'normal' classical size of about 14" across the lower bout, I'd use fan bracing. The added stiffness of X bracing on such a small top tends to make the sound too 'bright' for various reasons. I've had good luck with X bracing on 000 size classicals, 15" across.

My third arch classical had a mahogany back. It was something of an experiment, and I didn't want to use an expensive piece of curly maple: next time I will. I think the sound would have been better with a heavier back.

The arch height, arch shape, outline, and thickness of the back are all interelated: you can't specify any of them by themselves, and all of them will depend on the wood you use. Just as a starting point, my last arch classical had a mahogany back that was 3.2mm thick overall. The arch rose about 12mm from the top of the edge to the high point of the arch. The back weighed 289.5 grams before I glued it to the rim. The lengthwise arch was determined using a sprung stick, and the crosswise arches were laid out using 'curtate cycloids'. The guitar was built on a 16" 'Small Jumbo' platform. Again, next time I'll use a denser wood for the B&S; the assembled 'main back' mode was a bit higher than I wanted it to be, but I didn't want to make the back any thinner or lighter than I did. Remember that the arching and thickness graduations of the back are serving much the same purpose as the bracing on a flat back.

I am really glad to see this post and these responses, because I have been pondering this sort of guitar for nearly 20 years. If I ever build it, mine will most likely be a steel string flat top with a cutaway and a carved, arched back. I posted a query about such an instrument on another forum some time ago, and didn't get much response. Recently, another hobby builder posted a similar query and got a lot of encouragement. I am excited to read about people thinking about such things. I really like this forum for that reason. It's all about exploration and experimentation--sometimes with better results than others, but that's why we keep starting new instruments, isn't it?
Patrick