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I have only built one arch top guitar (6 string)and am interested in building a 12 string acoustic. I think I want parallel braces . The one I have currently finished has dimensions (17in) and x bracing out of the Bedenetto book. I have limited knowledge other than the one I have completed. This one does not have a bright bass tone. Other than that I am happy with the way it turned out. I want a nice bright tone.
My question is what, if any, thickness are changed with the top & back and bracing for a 12 string arch top.

Is there any dimensional information on 12 sting arch tops out there ?

To quote Alan Carruth:



That is where I plan to start.

http://luthiersforum.com/forum/viewtopic.php?f=10101&t=28226

I'm not sure where Al got that "roughly twice the tension" - a quick look at the String Tension FAQ at UMGF shows that a six string with lights (12 - 54) is about 165 pounds, with mediums (13's) about 190. Twelve string string gauges are typically a little smaller than those on a six - "lights" are usually 10 - 47 and at concert are around 260 pounds. Add the fact that many of us tune a 12 string down one or two semi tones and the tension, according to the FAQ, drops to 230 or even 210.

http://theunofficialmartinguitarforum.y ... UAAT2jyuUl

Therefore I like to think that, depending on strings, scale length and tuning, a 12 string will be approximately 150 percent of a sixer, not twice.

I've built two 12 strings and own and work on others. However since my experience is with X and ladder braced flat tops I don't feel qualified to comment on an archtop, other than to suggest actually doing the calcs with the specifications you plan to use.