Official Luthiers Forum!

Hesh mentioned in another thread his experience in setting brace height on 6-string instruments.

Any SWAG as to how much higher a 12-string's braces might be at the 'x' intersection as compared to a 6-string?

Thanks!

_________________
Peter Havriluk

I built a 12 string version of the Kinkade OM and just used the same brace and top thickness sizes as the plans, which are quite well known to be over braced for 6 strings, and it worked out fine.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I like to scale both the top thickness and brace height to the overall tension load, so that both the top and the braces take 'their share' of the load. Most 12-string players use a 'light' set. According to D'Addario's catalog they pull about 250# total tension, which is about 50% more than an 'light' set of six strings, or 33% more than a 'medium' set for a six.

Stiffness is the issue, rather than strength per se: any top that is stiff enough to hold up under bridge torque is more than strong enough to avoid breaking any time soon. Stiffness of a plate or beam goes as the cube of height: thickness of the top of height of the braces. To make the top 33% stiffer you only need to make it 10% thicker, and the same goes for brace height. Making the top 26% thicker, and the braces 26% taller, makes the system twice as stiff. For a 12 you will, of course, need a larger bridge and bridge plate, and I like to make the braces wider (say, 5/16" rather than 1/4", to get more glue area), and that adds stiffness as well as weight. Still, the weight doesn't go up as fast as the stiffness.

The power in a plucked string goes as the tension and the square of the amplitude. With a 12 you tend to drop the action height a bit to make things easier on the left hand, and that drops the maximum power available. You don't usually have to reduce it enough to more than offset the tension rise, though.

When I thought of this stuff some years ago I made up a 12 with the top and bracing about 20% thicker/taller than my usual six string (I didn't want to push it the first time out). The top ended up around 25% heavier than a six string for the same size, It was the first guitar I had made that I could not sing over. I saw the customer and his wife at a show a couple of years later and asked him how the guitar was doing. He said it was fine and his wife volunteered that it was loud. "Give 'em volume and they'll hear tone."

Its been quite a while since I built my OM sized twelve string but what I remember was leaving the top a bit thicker than normal (I want to say 0.110 or 0.120) and using non scalloped 5/16 braces. I could measure the top thickness if you would like. I did add a third tone bar. Braces are standard height (1/2 at the crossing if I remember correctly)



Guitar is long scale, always tuned two semi tones down and is 18 years old now. Top has the little belly that was built in, neck angle has remained perfect, guitar is still a delight to play.

I've done two shrunken Dreadnoughts (93% of a Herringbone outline), 12-frets-to-the-body, 24" scale length. They fit into a Fender 'Classical' gig bag and look entirely normal. And they're sure comfy to play. I'd asked in anticipation of doing a third one with a bit more than a 'TLAR' design.

There's a whole heap of braces in that picture!

_________________
Peter Havriluk