Official Luthiers Forum!
http://mowrystrings.luthiersforum.com/forum/

12 string bracing
http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10101&t=54300		 Page 1 of 1
Author:  banjopicks [ Fri Aug 06, 2021 1:03 pm ]
Post subject:  12 string bracing
Just in case I ever finish my 6 string, I've got to stop doing repairs I think, I want to build a 12 string. What do I have to change on my D28 plan or should I get a completely new plan?
Author:  phavriluk [ Fri Aug 06, 2021 1:54 pm ]
Post subject:  Re: 12 string bracing
If I was relying on a plan, I'd get a 12-string plan. So much of the success of a 12-string depends on the soundboard and its bracing, and no plan is going to definitively specify this for a builder. Experience and judgement enter the process, and that can't be found on a plan.

If I was building a dreadnought 12-string, I'd be using a short scale neck joining the body at the 12th fret and redrawing the dreadnought drawing to accommodate the relocated bridge and braces shifted to suit the changed bridge location. I'd also take some measurements to make sure that guitar is going to fit into a standard dreadnought case.

I've scratchbuilt one 12-string and it turned out to have a magical voice, all by accident as I sure didn't have any idea of what I'd have when I was done.
Author:  banjopicks [ Fri Aug 06, 2021 2:05 pm ]
Post subject:  Re: 12 string bracing
Not much experience here so I think a plan is best. Any recommendations?
Author:  Tim Mullin [ Fri Aug 06, 2021 4:07 pm ]
Post subject:  12 string bracing
I’ve only had the opportunity to build one 12. Part of a pair of dreadnought twins, long scale 14 fret to body, Sitka over EIR. The two guitars are identical, with exception that 12 has a top about 8 thou thicker and a single extra tone bar situated along the back of the bridge plate and perpendicular to centre line. Both guitars have been completely stable for 12 years — no need to adjust the setup over that time. The 12 was a cannon out of the gate, the 6 took a year to open up. They remain my two most effective demo guitars.


Sent from my iPad using Tapatalk
Author:  Alan Carruth [ Fri Aug 06, 2021 4:18 pm ]
Post subject:  Re: 12 string bracing
What we're worried about here is stiffness; any top that's stiff enough to hold up over a reasonable length of time will be stronger than it needs to be. I've never seen a top ripped off a guitar by string tension when it's first strung up. I've been testing my tops for long-grain stiffness for some time, and using that to determine the 'correct' thickness for the top. That's based ultimately on bridge torque load; the total string tension and the height of the strings off the top. Once I've got the top to thickness I work to get the bracing and the top stiffness 'balanced' in some sense. Several years ago I realized that this was not based on the number of strings, and that simply scaling everything in line with the load should work out fine for a 12, with no change in the brace pattern.

Stiffness pretty much goes as the cube of the top thickness or brace height. I want each part to take it's share of the load, so I don't go with a thin top and heavy bracing, but add something to each. Using D'Addario tensions as a 'standard', a 6-string set of mediums pulls about 189#, while a medium 12-string set pulls about 321#, so the 12-string set puts 1.7 times as much load on the top with the same string height. To keep the stiffness the same you'd need to make the top 20% thicker, and the braces 20% taller, with the same layout, to have a top that's stiff enough. The bridge has to be larger, of course, and the bridge plate as well, but you only need to add about 25% to the overall mass of the top as compared with a six string. But remember, the vibrating energy in the string is proportional to the tension all else equal: you've got 70% more energy driving a top that's only 20% heavier.

The first time I tried this I used a 'fudge factor': assumed that the string tension would be double, and made everything 25% thicker/taller, and added some width to the braces as well to get more glue surface. I hate it when things I make break. The top still ended up less than 30% heavier than a normal six-string. When I got it together I noticed it was LOUD: I had trouble singing over it. That was 12 years ago. I have seen the owner since then; he still smiles at me and his wife says it's still LOUD. I've done it the same way a couple of more times since, and they're still LOUD.
Author:  Smylight [ Fri Aug 06, 2021 6:04 pm ]
Post subject:  Re: 12 string bracing
Well Alan, that's some very informative post. Thanks for taking the time to share.


Pierre
Guitares Torvisse
Author:  bluescreek [ Fri Aug 06, 2021 6:48 pm ]
Post subject:  Re: 12 string bracing
the one thing I can tell you as I made about 10 in my career I do not scallop the X braces and you want to add a tone bar or use a higher tone bar
I do tops about .125 and slightly larger plate. Also using short scale is not a bad Idea

Guild make great 12 strings and they are a stout brace patter but they do make a terrific 12 string
Author:  meddlingfool [ Sat Aug 07, 2021 1:31 am ]
Post subject:  Re: 12 string bracing
What I do is find out what gauge and scale the client wants and find the overall tension with the many string gauge calculators out there.

I convert that into a % and apply it to my regular process.

So let’s say 12 strings were 60% more tension than 6. I add 30% more weight to my deflection jig and then add what I think is 30% more stiffness to the braces (and since I’m not capable of that sort of math, I add 3/32 to the brace height), knowing that I will be shaving the braces down to meet my monopole target, which doesn’t change on a model between 6 and 12.

Basically the same as Alan, trying to distribute the extra stiffness required between the top and braces.
Author:  Greg Maxwell [ Sun Aug 08, 2021 7:41 am ]
Post subject:  Re: 12 string bracing
I've made one, and followed the "add 20%" advice commonly given. It turned out way too stiff although it sounded good and the client was very happy. Keep in mind that many, many guitars are over-built, so adding 20% to an over-built design may be contraindicated. Trevor Gore has a case study on 12 string design in his book that makes a lot of sense.
Author:  Trevor Gore [ Sun Aug 08, 2021 9:28 am ]
Post subject:  Re: 12 string bracing
Greg Maxwell wrote:
Trevor Gore has a case study on 12 string design in his book that makes a lot of sense.


Greg, thanks for the mention. However, the case study is in the 3 day Modal Tuning Course (and associated notes), but not in the book. I use the case study to ensure participants are comfortable with the design principles involved (which are in the book), which cover most types of flat top guitars. Cutting a lot of discussion points very short, the key points are to keep the string tension down (so you can play the thing!), use enough brace/panel stiffness to keep the bridge rotation under control and aim for the same target resonances that you would for a six string. (Keeping the bridge rotation under control means increasing the stiffness, which raises resonant frequencies, but a typical 12 string bridge can be used to add mass to bring the resonances down again to where they belong on a six string). There are a lot of other tweaks you can do "around the edges", but those are the basic principles.
Author:  Freeman [ Mon Aug 09, 2021 9:15 pm ]
Post subject:  Re: 12 string bracing
banjopicks wrote:
Just in case I ever finish my 6 string, I've got to stop doing repairs I think, I want to build a 12 string. What do I have to change on my D28 plan or should I get a completely new plan?


My two cents. I own three twelve strings, two of which I have built. I consider myself a bit of a twelve string player. These guitar are very very different from each other - in the way they were built, the way they are played, the way they sound.

Attachment:
IMG_2023.JPG


On the left is a 1980 Martin D12-28 - not to far away from your D28. It has 5/16 unscalloped bracing with two tone bars - looking at the inside all you see it the bigger bridge plate. It is Martins short scale (24.9) and the top measures 0.115 at the sound hole. I typically string it with "lights" (0.010 to 0.046) and tune it down two semi tones to D. It gets pretty muddy if I try to go any lower. It is a great all around generic twelve string, happy being strummed and a bit of boom-chuck, but I never really bonded with it.

I was looking for something a bit better for finger picking, for down tuning and slide. I spent a lot of time talking to John Hall about what I wanted and we came up with the OM size and long Martin scale (25.4). Spruce top, we went a bit thicker at 0.120 and braced it with 5/16 unscalloped braces about 1/2 tall, maybe a bit more, and added a third tone bar. We added a bit more depth to the body to get more air in the box.

The guitar is typically strung with "mediums" (0.012 to 0.052) and tuned down two semitones to D, then further down to the equivalent of open G or D or C. I play a fair amount of Leo Kottke stuff and while I am not quite as low as he is the guitar works very well.

The last guitar is a very long scale (26.5) ladder braced monster - strung with cables (0.013 - 0,056) and tuned in the cellar (C usually). It has its own sound, you would know it if you wanted to build one of these.

I also did a fair amount of research when I built the OM - poked inside some Guilds and Taylors. The Guilds were mostly braced like most X braced guitar but fairly heavily, the Taylors all had 3 tone bars. The exception was the Kottke signature guitar which has very fat braces very heavily scalloped.

I also have a set of Ultimate Guitar Online's grand auditorium 12 string plans. They are 25-13/32 scale, 0.31 wide by 0.55 tall braces and three tone bars. The GA shape is more what I think of with Taylor and in my opinion would make a great 12 string.

One of the significant things is that with each guitar I am somewhere between 225 and 250 pounds of tension - not double as most people think. Actions can be set to reasonable levels and the guitars are very playable.

The Martin at about 25 years of age had started to develope a slight belly and the action was moving up - I had the neck reset to the bridge. For the past 15 years it has remained stable and very playable. The OM has a dovetail joint, the Stella clone a bolt on neck, neither have shown any signs of change in geometry or bellying.

So, to answer your question. Martin seems to feel that standard unscalloped bracing is OK for a 12, that is apparently what they built (at that time). Most other manufactures add the third tone bar. Making the top a bit thicker can't hurt. You'll need a larger bridge plate but don't get carried away (the D12-28 had it big rosewood b/p replace by a smaller maple one). Scale length works with tuning and string gauges - the first thing you should decide is how you will tune it and play it, that leads to scale and tension. I would shoot for something between 225 and 250 pounds, I think the bracing will be happy with that.

There are a few more considerations. The short scale D12-28 fits in a Martin long scale 6 string dread case - barely. The OM-12 and Stella clone required custom cases. You will have to decide the nut width and string spacing you like, there are other considerations at the bridge. Plan on a reset, it will happen. Play as many as you can, carefully noting how the owner has chosen to string and tune it.

I have lots of photos and notes from building mine, I'd be happy to answer questions.
Author:  Freeman [ Tue Aug 10, 2021 10:51 am ]
Post subject:  Re: 12 string bracing
I'll add one more thing to the above. I find this very helpful when thinking about strings and tuning and tensions. The 12 string stuff is at the bottom

https://umgf.com/string-tension-t5339.html#.VpFU2aNIi1s

They are based on 25-1/2 inch scale, if you change scales remember to square the ratio.
Author:  Pmaj7 [ Wed Aug 11, 2021 4:19 pm ]
Post subject:  Re: 12 string bracing
I have only made one. After gleaning as much info as I could and examining a few 12 strings; I made the X's 5/16 instead of 1/4 and slightly taller, adding small third tone bar behind the bridge plate and tucked the lower arms of the x, top about .125. OM size @ 4.5" & 24.9. plenty of bass.

I noticed nobody said they do anything differently with the neck. After examining a 12-string that I have had for many years that had a very stable neck, I made the fretboard .030 to .040 thicker and the neck .030 to .040 thicker for a total of about .070 thicker all together. Also put two 1/8x3/8 carbon fiber rods in. After stringing it to pitch with a perfectly flat fret plane, just a perfect skosh of relief emerged and has not budged since. So, pretty happy with that. It feels slightly thicker than my normal (.870@1st vs .800), but not bad.

Does anyone else feel it necessary to do any neck reinforcement on a 12?

Pat
Author:  Freeman [ Wed Aug 11, 2021 5:03 pm ]
Post subject:  Re: 12 string bracing
Pmaj7 wrote:
Does anyone else feel it necessary to do any neck reinforcement on a 12?

Pat


The D12-28 in my pictures has the Martin non adjustable bar truss rod. It has about 12 or 14 thousands of relief with the strings and tuning I described. I would like a smidge less but there is nothing I can do about it. I don't usually play high enough on the neck that the relief is a problem. Fwiw, it has a nice low fingerstyle action.

The home made ones each have a single LMII double acting truss rod. Both of them have relief adjusted in the 5 to 7 thousands range. Necks are wider of course but roughly the same depth as what I use on other guitars. Fretboards are standard 1/4 thick, nothing special. Necks are mahogany built with stacked heels and scarfed head. The OM is a dovetail, the Stella-clone a bolt on. Both guitars are very stable setup wise and the OM even more or less stays in tune.

Guild 12 strings as you know have two truss rods. I have repaired a Guild twelve that had a broken head, I added a backstrap to the head and neck - splines would have been nice but with the 2 rods there wasn't much room.

Short answer, reinforcement wouldn't hurt, I didn't.
Author:  Smylight [ Wed Aug 11, 2021 6:29 pm ]
Post subject:  12 string bracing
Pmaj7 wrote:
I have only made one. After gleaning as much info as I could and examining a few 12 strings; I made the X's 5/16 instead of 1/4 and slightly taller, adding small third tone bar behind the bridge plate and tucked the lower arms of the x, top about .125. OM size @ 4.5" & 24.9. plenty of bass.

I noticed nobody said they do anything differently with the neck. After examining a 12-string that I have had for many years that had a very stable neck, I made the fretboard .030 to .040 thicker and the neck .030 to .040 thicker for a total of about .070 thicker all together. Also put two 1/8x3/8 carbon fiber rods in. After stringing it to pitch with a perfectly flat fret plane, just a perfect skosh of relief emerged and has not budged since. So, pretty happy with that. It feels slightly thicker than my normal (.870@1st vs .800), but not bad.

Does anyone else feel it necessary to do any neck reinforcement on a 12?

Pat

Hi guys,

I have never made any twelve strings since I own a repair shop, but I thought I could chip in with my 2 cents as I've seen my fair share of sick twelvers.

AFAICT from what came across my bench, neck problems have been really rare, and those seemed to be related to long-neglected guitars with badly-adjusted truss rods left alone for a long time. I'd say not any worse than on 6-strings. Pivoted bridges, along with their inevitable friends deformed tops form the vast majority of my 12-string patients.

Being sixty-plus myself, I can remember the times when all 12-string guitars were kept tuned lower to ease tension, and I'm led to believe there is wisdom in that, because I've seen many relatively recent (say 10 years) guitars with deformed tops, from much any manufacturer. Just in the last weeks, I had a lower-end great-sounding but unplayable Martin, and to my surprise a very stoutly-built higher-end Guild Jumbo. This one had a very thick top and IIRC an additional tone bar. I can't offer any measurements since it's out of the shop, but I can say it was obviously very much built like a tank. And yet, the bridge had pivoted like crazy without any loose bracing inside. This guitar was now pretty much unplayable as well.

So that's what I can offer up. This leaves me wondering… maybe those guitars are better off tuned lower after all if they are to survive a few decades. I guess the OP will make decisions and will report back in 2040. ;-) Now if I was playing an acoustic 12-string myself, I know I'd probably keep it tuned down.


Pierre
Guitares Torvisse
Author:  banjopicks [ Thu Aug 12, 2021 1:17 am ]
Post subject:  Re: 12 string bracing
I'm getting some great input here. I'll re-read it all a few times before I put a plan together. Thank you
Page 1 of 1 All times are UTC - 5 hours
Powered by phpBB® Forum Software © phpBB Group
http://www.phpbb.com/