Official Luthiers Forum!

does a 5/8 thick x -brace on a twelve string dread sound too thick?????

_________________
MK5acoustics.com

Do you mean width or height.....?
Tom

_________________
A person who has never made a mistake has never made anything!!!

width....by 9/16 high...

_________________
MK5acoustics.com

Hoo! Oh boy yes indeed!

I've seen a lot of forum threads where people ask how much to beef up bracing for a 12-string, and never seen a really authoritative prescription, though some people say what they use, which often sounds excessive. ("I leave the top .140 and add an extra X brace in the lower bout," etc.). Alan Carruth had a very good post on this subject, I think, but I was unable to find it just now. Based on what he said, I've been trying to figure it out for myself. The basic idea is to get good data for the tension of the 12 strings and then figure out how much to stiffen the top to withstand them, using the formulae for stiffness rather than guesses.

If you go to a string tension calculator, such as http://www.mcdonaldstrings.com/stringxxiii.html, you can find the tension of the 12-string gauge and scale you expect to use and compare it to a 6-string plan. (Google will help you find other calculators for more gauges and various drop tunings, but you'll have to do your own arithmetic in terms of adding up the tension string by string.)

For simplicity's sake, consider a typical long-scale 6 string with medium strings. It has about 170 lbs of pull, while a 12-string light gauge has 252 lbs (both at concert pitch). So, using light strings, you would want a top and bracing system on your 12-string that was about 50% stiffer than a standard 6-string built for medium strings.

If you look at the formulae for determining how stiff tops and braces are (such as at http://www.ukuleles.com/Technology/bracemath1.html, and in Somogyi's book, and in lots of posts on the OLF, etc.) you see that you don't need to beef up braces and tops more than a smidge to get a big increase in stiffness. For example, Somogyi points out that a 3.33 mm guitar top is 33% stiffer than a 3 mm top, and a 5/8" high brace is twice as stiff as a 1/2" brace.

This makes a 5/8" x 9/16" X-brace sound like overkill. If you figure out what gauge, scale length, and pitch you want to make your 12-string for, and a 6-string plan you will use as a starting point, then you should be able to figure out about how big your X-braces should be. My current guess for the sort of 12-string I envision is something in the ballpark of .125 inch thickness for a typical spruce top (and no top is typical) and about 5/8" - 3/4" by 5/16" for typical spruce braces (ditto) with a moderate scallop or taper. Beefing up the upper transverse brace a little is also a good idea, it seems.

I'm sure some more knowledgeable people can chime in here. If you specify what scale, string gauge, and tunings you expect to use, you may get more specific advice.

_________________
Tim Allen
"Never hurry, never rest."

thanks...

_________________
MK5acoustics.com

What I did on my last 12 was to assume that it was pulling about twice the tension of a 6. Since the stiffness of the top and the bracing goes as the cube of the thickness/height, I made the top about 25% thicker than normal, and the bracing about 25% taller, and a little wider to keep the gluing surface in line with the height. The top was twice as stiff, and only weighed a bit over 25% more than a six-string top. The guitar was a cannon, and, from the string data Tim posted, it's still over built. That's fine by me, and the customer is happy too.

The key here is that the limit to how thin or light you can build is not _strength_, but _stiffness_. When you have just enough wood to keep the bridge from folding the top up it's 'way stronger than it needs to be. That's how the 'sandwich' tops work: they reduce the amount of wood so that the strength is more in line with what's needed, while retaining the stiffness. By getting rid of excess weight they get more sound.

I am certainly no expert on 12 strings but I have had an old Martin D12-28 for a long time and three years ago decided to build something a little more finger style friendly. Rather than doing the math, I simply poked around in all of the 12 strings I could find - trying to figure out what worked (and maybe what didn't). First, a couple of assumptions - a typical long scale 12 strung with lights (0.010) at concert or mediums (0.012) tuned down two half steps will have tensions around 260 - 280 pounds (according to the UMGF FAQ) which is less than double a sixer. My Martin and many others have bracing pretty similar to a six string - the top plate on mine was 0.115 thick, braces are 5/16 unscalloped. My guitar is 30 years old, developed a slight belly and needed a neck reset, but seemed to hold up well.

Taylor, on the other hand, uses a slightly thicker top plate and adds a third tone bar to their standard models, the LKSM has wide scalloped braces. The Kottke is standard scale, but meant to be strung with heavy gauge strings (0.013) and tuned to C#. I'm pretty sure the Taylor braces are also 5/16.

When I built my deep OM sized 12 I used 0.120 top plate, 5/16 unscalloped and added the third tone bar. The narrower waist should add a little stiffness there over the dread. You will also have a larger bridge plate (and the bridge itself is deeper to allow for the extra pin holes). I made my OM short (24.9) scale, but if I were to do it again, I would go to 25.4.

That guitar is loud, reponsive and balanced across the courses. I string it with 12's and down tune to D, then further down to some open tunings. After 3 years it shows no signs of any structural changes.

I have also built a very long scale (26.5) ladder braced 000 sized 12 string designed for tuning to C or even lower. Based on plans and measurements from old Stellas, I went to a 0.150 top plate and fairly thick ladder bracing - but this is a tailpiece guitar and not subject to the top rotational forces of a pinned bridge.

A lot of great information...thanks...

_________________
MK5acoustics.com

I took the Martin Factory tour earlier this year and asked a supervisor in charge of
assembling the boxes,
"What is the differance between a dred six string body and a dred 12 string body"
He said,
"Other than the bridge plate, nothing"

Gregg

_________________
Gregg

That would explain why Martin 12 strings sound like they do. Nearly every one I've ever played or heard sounded like a handful of nickels thrown into a harpsichord.

I go beefier with my 12 string tone bars and my X. Even a little more mass on the upper transverse. My x is about 11/32" wide by 11/16 high plus a .080 (Plus or minus) cap.

_________________
michael propsom
www.propsomguitars.com

These two quotes about sum it up. If you want a "harpy" 12, build a Martin with a larger bridge plate, string it up with lights and tune to E.
If you want to end up with a 12 string with guts, you will have to use heavier strings, tune down to C#, C or even B (you will need a long scale for that), and either beef up the bracing, make a thicker top or both.
I have been making long scales and regular scales with very heavy strings, thick tops (.150"-.170") depending on the guage of strings and mostly tune to C, (B on the long scale). All tops have laminated bracing. I have also been using 3 tone bars lately (6's too) and like them. I think my next 12 will be a very short scale with HEAVY strings. I've been missing 12's and can't play long scales or regular anymore.

That last 12 of mine has a 24.5" scale, but it's tuned to pitch. I have to say that the 12-fret neck with the short scale and long head had a little of 'Thor's hammer' to it. It sounds like Thor's hammer, too.

here is what I ended up with...all my braces are drawn on cad , glued to wood and then cut out...
x braces are 3/8 wide by 9/16 tall at center
tone bars are 5/16 wide by 9/16 tall at the first high point for the lower tone bars
the upper tone bars are 5/16 wide by 3/18 tall
bridge plate is approx 3/32 by 3 inches wide with a brace at the rear ...
utb is 1/2 wide by 5/8 tall
top is .125 thick
weight is 376 grams my last OM soundboard was 262 grams...
I am thinking I might be too stout..the board has a nice bell sound to it..
any thoughts??????

_________________
MK5acoustics.com

I would say if it has a lively tap to it your are about there...125" sounds a little thin for my taste, but looks like you are going the heavy brace route. If you want to loose some more weight, take all the braces down to triangular (a lot of work after the bracing is glued on, but worth it).
Don't forget, you can't compare 12 string construction to 6 string. You get a lot more power out of 12 strings working that top...