12 String Top radius?

[Joel Hagle]

I am an old, life long wood worker starting my third instrument build. I am working from a couple of plans that combined give me what I want and if all goes relatively well this will be a 12 string, 12th fret, Grand Concert ~ size guitar. Or it may end up being an ashtray, not really, it will be a guitar and I will play it even if I need protection in case it self destructs.

My immediate issue is finding information on putting a radius on the top of a 12 string. Is this a no-no because of the string tension adding lift already, seems to me a radius adds strength and would be a good thing.

Any input would be appreciated.

[Alan Carruth]

I've always use the same radius on my 12s and on the 6s; 25'.

Don't over build. I've found that using the same bracing pattern, making the top of a 12 about 25% thicker than I would for the same wood on a 6, and making the braces 25% taller and a little wider, ends up being plenty stiff enough. Since the stiffness of the top/braces is proportional to the cube of the thickness/height that's about twice as stiff, but with a lot more energy to drive it and only 25% greater mass (or a little more, due to the larger bridge plate and bridge) the guitar ends up being REALLY LOUD.

[Joel Hagle]

Thanks for the reply.

Happy to hear this because all my instincts were telling me to do it, but I thought I should ask those who have been there already. I have been using 30' because the course I was following recommended it.

Seems like a lot of people use 25'. 30' to 25' is pretty small, won't have an effect on the bridge height?

Along that line, no pun intended, do you keep the top flat above the sound hole?

[Alan Carruth]

II use an arched UTB, but that's unusual: I think it may help stabilize things in the long run, but who's to say? The neck set ends up with a bit of an over stand, and there's a wedge between the fingerboard and the top. I prefer to fit the wedge rather then removing material from the end of the fingerboard.

[Freeman Keller]

Joel, I have built two 12 strings, a long Martin scale OM sized guitar that is tune two or three steps down and a 26 inch scale 000 that is tuned to C. I also have an old Martin D12-28. The Martin has a bit of a belly and the neck has been reset to whatever dome the top has, the other two were built with a 24 inch dome and seem to be pretty stable ( they are about 15 years old). Bracing on the OM is 5/16 unscallop with a third tone bar and 0.120 thick top, the 000 is ladder braced.

Do your tension calcs for whatever tuning and scale you wand - most of the time 12 strings end up in the 220 pound range.