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12 string cedar top
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=28226		 Page 1 of 1
Author:  P@uL [ Thu Jul 15, 2010 8:18 pm ]
Post subject:  12 string cedar top
i am wondering for a 12 string how thick are you all leaving your cedar tops. i would assume 10-15 thousands thicker than usual? or would most of the added strength come from my bracing, was thinking of double x. i have not done a cedar top so i am completely in the dark.

i saw a recent post on nut width and string spacing by Haans:
Quote:
Nut: 1-7/8"
String spacing: 1-13/16" ctr to ctr.
Between pairs: ctr to ctr 5mm bass, 6mm treble
Between string and octave: 3mm bass, 2 mm treble

Saddle: 3"
String spacing: 2-1/2" ctr to ctr
Between pairs: ctr ot ctr 8-1/2mm bass, 9mm treble
Between string and octave: ctr to ctr 4-1/2mm bass, 3-1/2mm treble


is this pretty standard? and has anyone made a 12 with a shorter nut width than that? the guitar is being built for a female player with smaller hands.
any other helpful info for 12 strings in general?
Author:  TonyKarol [ Fri Jul 16, 2010 7:22 am ]
Post subject:  Re: 12 string cedar top
Hi Paul,

I just finished a 12 last month ... spacing at the saddle was 59mm outer to outer, each course was a 56mm spread, 3mm between each pair. A the nut, I was 1 7/8, and then used the stew mac nut spacing rule to get the pairs lined up once I had the offfset fomr the FB edge, about 1/8.

I can see for a poerson with smaller hands going a 1/16 narrower at the nut, but thats about it as the pairs will really get squeezed ... mark it out and see if that will work, its possible that 1 3/4 may work too, you just need to try it on paper and check the spread.
Author:  Steve Kinnaird [ Fri Jul 16, 2010 10:21 am ]
Post subject:  Re: 12 string cedar top
Paul, the most thunderous 12 I've ever heard was a cedar topped Breedlove. (That's just to say that it is possible to use cedar on 12 strings.) Not sure what the top thickness was, nor how it was braced, but Breedlove does take advantage of the Bridge Doctor system.

Steve
Author:  P@uL [ Fri Jul 16, 2010 11:16 am ]
Post subject:  Re: 12 string cedar top
thanks for the advice guys i really appreciate it. i will be giving my old teacher a call today to make an order anyway so i will have to ask him what he would shoot for in a cedar top thickness. im pretty sure hes done a few 12's over the years.
Author:  Alan Carruth [ Fri Jul 16, 2010 1:26 pm ]
Post subject:  Re: 12 string cedar top
First: a twelve is carrying roughly twice the tension of a six, so you need to make the top about 25% thicker: the stiffness goes as the cube of the thickness, more or less, and 1.25^3 is almost 2.

Cedar runs lower in density on the average than spruce, and the stiffness along the grain at a given thickness will be pretty much proportional to the density. If you have not got the wherewithal to actually measure the stiffness of the wood you have, I'd suggest leaving cedar about 10% thicker than your usual spruce.

So: if you would normally make a spruce top for that size of guitar with six strings at, say, .110", then with cedar you'd go .110" x 1.1 x 1.25 = .151".

I know some folks will say that's excessive. In my defence, the last twelve I built (with a spruce top) I followed the 1.25x thickness rule, and ended up with a cannon. Remember, you're driving 25% more top weight with twice the power. I've also gotten good results leaving cedar tops thicker: it's enough lighter than spruce that even at 10% thicker it still tends to weigh less.

Keep in mind that cedar has more of a tendancy to split than spruce. This can be a real problem on bridges. Making the bridge wider (that is, deeper along the line of pull of the strings) will reduce the peeling stress at the back edge a lot. I often use a lighter weight wood for bridges on cedar tops, so that I can make them bigger without incurring a weight penalty.
Author:  P@uL [ Fri Jul 16, 2010 5:01 pm ]
Post subject:  Re: 12 string cedar top
great input thanks Allan. just got back from measuring some 12's at the guitar shop got lots of info that i needed other than the top thickness thing so thanks again. I remembered the cubed rule but never thought of applying it to my top thickness just bracing. just as clarification if one were to beef up the thickness by about .25 percent what would be done with the bracing? anything very different?
Author:  jfmckenna [ Fri Jul 16, 2010 8:34 pm ]
Post subject:  Re: 12 string cedar top
Double X is not a bad idea imo. That and a shorter scale length
Author:  Jeff Highland [ Sat Jul 17, 2010 12:30 am ]
Post subject:  Re: 12 string cedar top
My Guild F212Xl from 1975 has a top about 134 thou thick at least near the soundhole.No deformation at all.
That's a 17" Jumbo in sitka tuned to concert pitch with standard light strings for 35 years .
It's not quite as bad as an extra 100% string tension when you consider that a light guage set for a 12 string is 10's compared to 12's on a six. More like an extra 70%
I dont think there would be many using mediums on a 12 unless they are tuning down.
I would want an extra 10-15 thou for cedar which gets up close to Alan's 150 thou.
Author:  Haans [ Sat Jul 17, 2010 9:43 am ]
Post subject:  Re: 12 string cedar top
You should try to consider what guage of strings you are going to TRY to use and the scale length before any speculation about top thickness. My spruce red tops run anywhere from .155" for mediums to over .165" for heavy strings. Instruments are tuned anywhere from B to C#.
Author:  P@uL [ Sat Jul 17, 2010 9:55 am ]
Post subject:  Re: 12 string cedar top
Well this Is for a lady who I doubt will be using many alternate tunings. And I can't see her ever going below a drop d. Also was planning on a 25.4 scale. So light strings are the way to go it sounds?
Author:  jfmckenna [ Sat Jul 17, 2010 1:28 pm ]
Post subject:  Re: 12 string cedar top
P@uL wrote:
Well this Is for a lady who I doubt will be using many alternate tunings. And I can't see her ever going below a drop d. Also was planning on a 25.4 scale. So light strings are the way to go it sounds?

Paul, why a long scale length if she has small hands?
Author:  Alan Carruth [ Sat Jul 17, 2010 6:06 pm ]
Post subject:  Re: 12 string cedar top
Jeff Highland wrote:
"My Guild F212Xl from 1975 has a top about 134 thou thick at least near the soundhole.No deformation at all.
That's a 17" Jumbo in sitka tuned to concert pitch with standard light strings for 35 years ."

Sitka can be quite a lot denser and stiffer along the grain than cedar tends to be, so it could work at that thickness.

Hans wrote:
" My spruce red tops run anywhere from .155" for mediums to over .165" for heavy strings."

Which points up the notion that we all do things differently, and what woorks for one person might not for another. That's why it's nice to have a way to scale things for the way you work, rather than just a number.

On that last twelve I made the braces 25% taller as well, and a little wider to be sure there was plenty of gluing surface. It seems to have worked.
Author:  P@uL [ Sun Jul 18, 2010 4:42 pm ]
Post subject:  Re: 12 string cedar top
jfmckenna i was thinking on the wrong wavelength that day. must have been thinking of my other build which is the larger of the martins. i have a stew mac ruler for 24.9 and was planning on using that for this 12.

most likely going with 1 13/16 nut width unless i can figure out a way to make 1 3/4 work.

thanks again for all the tips on top thickness. i will be feeling for stiffness when i start getting towards the 155 range. i still plain down my tops by hand and have graduated the last few tops i have built just because thats the way i was taught. so this thing could very well end up in both the 150 and 135 range with a 10-15 thousand graduation at the edges.
Author:  Tim Mullin [ Sun Jul 18, 2010 11:40 pm ]
Post subject:  Re: 12 string cedar top
P@uL wrote:
most likely going with 1 13/16 nut width unless i can figure out a way to make 1 3/4 work.

thanks again for all the tips on top thickness. i will be feeling for stiffness when i start getting towards the 155 range. i still plain down my tops by hand and have graduated the last few tops i have built just because thats the way i was taught. so this thing could very well end up in both the 150 and 135 range with a 10-15 thousand graduation at the edges.


Not sure why "figuring a way to make 1-3/4 work" would be difficult. The bigger issue is at the saddle, where string clearance around pin heads is the major limiting factor determining how close strings are paired (unless you use 2-strings per pin).

I stayed out of the discussion on cedar top thickness, as I really don't know cedar. As a point of reference, the Scott Antes 6-string and 12-string dreadnought plans are very similar (although I think both are over braced). His 12-string plan calls to a 0.125" thick spruce sound board, while the 6 states 0.109". His bracing on both is identical, with the exception of a larger bridge patch on the 12-string, an extra brace across the bottom of the patch, between the X-brace arms, and realigned lower tone bars. For my own guitars, I followed the Antes bracing pattern, but both tops and bracing were lighter than his spec -- it's strung with x-light 12's and hasn't budged since I strung it up a few months ago.

Have fun -- 12 strings are a blast.
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