12 string build advice

[Abstraction]

I'm building a 12 string with my 16yo grandson. Hopefully making memories as much as a guitar. I've built folk harps and electric basses before - I'll post elsewhere maybe. This time I actually bought plans. I did build my son's electric bass with him years ago. Bit of a learning curve on an acoustic guitar though, which is interesting.
But I still work full time in overseas aid, heavy travel and work commitments, other commitments and also seriously paint in oils so struggling to get the headspace around all the issues in this project. There seems to be a certain amount of dogma and I want to understand.
Really appreciate advice from luthiers. Especially evidence-based advice and real experience.
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12 string Dreadnought acoustic
Plans GenOne Luthier Services
timbers
Soundboard: Sitka spruce. Bought it 30 years ago. Two harps from it already.
Neck and body: Sassafras. (Josh's choice).
Fretboard: gombeira. (Two basses from this plank so far.)
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Neck advice
Here's my thinking thus far and questions.
1. Laminate neck for greater stability and stiffness? Or stay with my single piece?
I've attached pics of the two ends of the piece of sassafras. It's 765x75x22mm. Neck piece effectively quarter sawn 40mm then on one side gradually curves a little and the increases by the other end of the board (the horizontal pic).
Does this risk twisting? Truss rod is more for correcting bow is my understanding. I'm not sure how I could cut this piece to laminate anyway.
Also, which side face up to fretboard? I'm assuming side with less curve so I'm removing that timber.
2. Scarf joint for headstock. 15° I think it is. Not sure what the overlap works out at but I expect to cut about 600mm from my piece before
3. Truss rod: recommendations? for this neck for 12 string. I'm on budget but won't skimp if expense is really warranted. I've heard about carbon fibre reinforcement, is that worth reading up on?
Brett

[kiwigeo]

Welcome to the forum.

Re 12 strings - I treat a 12 string build pretty much the same as a 6 string build with the following exceptions:

- if using conventional X bracing I add an extra tone bar across the lower bout and a short brace along back of the bridge plate.
- standard truss rod.
- saddle compensation can be a challenge because of the pairing of thick and thin strings. See here for discussion on same: http://www.anzlf.com/viewtopic.php?f=1&t=890
- bridge c0mplexity can be reduced by using a single bridge pin for each string pair.
- I use Schaller M6 mini tuners to keep the head stock from getting too large or crowded

[Mark McLean]

Hi Ab (do you want to share your real name - most do around here), welcome to ANZLF.
A build with your grandson sounds great! I did my first with my son when he was age 15-16 (he is 32 now). One caveat is that he was all into spending time in the workshop until his first serious romance began and then I found myself there alone a lot of the time. Good luck with that!

Some random advice:
It is good that you are following a plan. I had not seen those ones at GenOne before - but I went and checked them out and they look pretty good. Their 12-string plan adds an extra tone bar brace (compared with a 6-string) as Martin has suggested. It looks like they suggest a bolt-on mortise and tenon neck joint? That would be a good choice. I would certainly recommend that you avoid a dovetail and definitely employ some sort of bolt-on design, and the M&T is fine.

Having a plan is essential for dimensions and design facts. However, you would also really benefit from having a book that walks you through the build method. A plan shows the "what", but the book will give you the "how". You obviously have lots of useful skill and experience already, but a step-by-step guide for this project will really help. There are lots to choose from. For your purposes I would recommend Jonathan Kinkead's book or the fairly recent one by R.M Mottola. No so much the venerated Cumpiano and Natelson book, which is a bit outdated.
https://www.amazon.com.au/Build-Your-Ow ... 0634054635
https://www.amazon.com.au/Building-Stee ... 1734125616

Regarding the neck - this is the Achilles heel of a 12-string because of the additional string tension. I am concerned about the grain of that piece of sassafras. It would have great potential for cupping or twisting. I also have not heard of that timber being used for guitar making before. For the backs and sides of the body it doesn't matter much - lots of different timbers can do. But for the neck, and especially a 12-string neck, you want to choose carefully. You will be investing a lot of hours in this project and it is worth it to also spend a few $ on reasonable materials in the critical areas. Whatever you use, laminating it as a 3-piece or a 5-piece would be a good idea for stability, and it can also look cool if you have a contrasting colour strip included. You definitely need a 2-way adjustable truss rod. If I was building a 12-string I would put a truss-rod down the middle and two CF rods - one on either side of the truss-rod.

You will get lots of advice here if you share your build (lotsa photos). Good luck with it and enjoy the project!
cheers
Mark

[Abstraction]

Thanks Martin and Mark.

I've ordered the Kinkead book. An actual book immediately feels like something more useful than dozens of scattered YouTube videos. Maybe books will become the new thing.

This timber appears to be growing in popularity and nice reports on tonewood - Maton have done it, which is not surprising. It bends well with little springback apparently. But to your point they didn't use it for the neck. So point of further research on whether to replace that timber. I'm used to laminations with harps because they have about a tonne of pressure and you can't avoid tension across the grain without laminations.

Brett

[56nortondomy]

I've used sass for one neck but wouldn't use it again, it's prone to movement. I use CF in all my necks and I'd highly recommend that, it's easy enough to install. Maybe get some QLD maple, NGRW or mahogany for the neck I think they would be a better choice. Wayne

[Abstraction]

Thanks Wayne, that's quite definitive. Especially with a 12-string with its extra load.

[Mark McLean]

Wow - I just did some reading about string tension in a harp! A 12 string guitar hasn’t got much to worry about compared to that. But, nevertheless, the recommendations that Wayne gave sound good. I also like Martin’s comment about using a 6 pin bridge with 2 strings per pin. A traditional 12 pin bridge is massive and unnecessary, and drills a dozen holes through the soundboard which can’t be good for it. And lightweight tuners is also a really good tip. Scarf joint at somewhere around 12-15 degrees. You choose whether to put the headstock part on top if the neck, or under it. If you do it on top the joint line will be on the headstock and you can cover it with a faceplate, and maybe also a backstrap. If you put it underneath, the joint line will be on the neck and is sometimes visible around the first fret area. At the other end of the neck most of us just stack blocks to make a heel ( typically three pieces).

[Taffy Evans]

Hi, I have included a few photos that may give food for thought, they have worked well for me.
Neck grain orientation when not on the true quarter sawn. Three pieces with laminations.

Neck reinforcement using adjustable rod and carbon Fiber.

And carbon fiber reinforcement at the neck block to stop the neck from collapsing into the body, as some twelve-string guitars can often do over time.

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My experience of bridges using one pinhole per pair of strings. I have changed the bridge for a guy who built a twelve-stringer using this method. Due to the closeness of the strings behind the saddle he had to slot the saddle under the strings to keep them apart. Due to his bridge design, the pin holes were too close to the saddle, so there was very little, and a very tight break angle over the saddle. This put extra tension on the bridge, and it cracked.
The idea may be satisfactory with a suitably designed bridge. I replaced the original bridge with one the same but with 12 holes.
Cheers Taff.

[Mark McLean]

Great photos Taffy.
I am a fan of neck block reinforcement too.

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Regarding 2 strings in one pin. You are right, it makes it necessary to cut two slots from the pinhole to direct the strings in parallel paths to the saddle. They don’t need to be very big slots, but you need the pins to be far enough back from the saddle to have a decent break angle. That must have been a really narrow bridge if the pins were crammed up so close to the saddle slot?
Here is the bridge on that mandolin/bouzouki shaped instrument in my other photo. 8 strings in 4 pins. Disregard the odd looking shape and angle of the bridge as it is a multi scale thing (also an experiment with decorative laminating of multiple timbers in the bridge, which is something I probably won’t do again).

[Abstraction]

Thanks Mark and Taffy. Very helpful options to think through and the pics really help. I'm encountering different folk who have used the Sassafras neck and not encountered problems. My piece is long and closer inspection suggests the more radical curve can be avoided - I'll cut it to size to check. Then carbon fibre would be a definite and laminations I can also consider. The opposed grain makes sense.
Neck block reinforcement is a new concept to me. Interesting. No impact on sound? How common is it?
I have an Irish mandola so love your bouzouki.

[Mark McLean]

Brett, Taffy and I are leading you down a rabbit hole with the neck block reinforcement, aka flying buttresses, aka “tone tubes”. This is certainly not a common thing and is not found, to my knowledge, in any factory made guitars. It was an idea propagated by some prominent luthier innovators such as Rick Turner, Mike Doolin and Hanns Brentrup over the last few decades and it has had quite a bit of discussion on this forum over the years (ANZLF has tended to been rather innovation focused in the past). It makes a lot of sense to control the rigidity of the box and resist the tendency of the string tension to want to fold the instrument in half - and to not just rely on the soundboard bracing for that function. It certainly does not harm the sound of the instrument, and can allow you to lighten up the soundboard bracing a bit to optimize power and tone.
But it is definitely “non-standard” so perhaps we shouldn't be leading you into such things with your first acoustic build!

[Abstraction]

It's a good alert to what the tensions are doing and role of the neck block. I spent ages thinking through tension in harps.

[Darrel Friesen]

I've only built one 12 string. Used the 000 plan from Stewmacs and based on advice from the Mimf way back when, increased the thickness of the top by about 20% or so as the top is Western red cedar. I also increased the height of the braces by 20% or so. 13 years later at standard pitch and it's still working very well. Neck is mahogany with a maple lamination. 12 frets to the body and bolt on neck.