Official Luthiers Forum!

When building a 12 fret design I know the most common changes that is made to the string geometry is that the scale length is shortened along with the bridge being moved closer to the center of the top.

I'm curious to know, if you wanted to build something like an OM with a 12 fret neck and a longer scale such as 25 or 25.5. Would it need to have the bracing geometry altered or would shifting the bridge plate be the only thing that's needed?

I'm thinking of guitars like the 000-28NB and the older Roy Smeck Radio Grande's. Both of which had longer scales but with 12 fret necks.

Dan,

Scale length is independent of the location of the body join.

The Martin Norman Blake OM/000 is a short(er) scale with a 12 fret neck. It could easily have had the longer Martin scale.

In many cases, the bracing does need to be moved to properly engage the bridge, usually at the wings, unless the body shape is changed. If the bridge changes location when using a 12-fret neck on a body originally designed for a 14-fret neck, the bracing will have to move along with the bridge.

Below is a composite I put together to demostrate that the bridge often is in the same place whether on a 12- or 14-fret neck, at least in most of the Martin universe.



You can see that the two guitars, a 14 fret OM and a 12-fret 000, have the same scale length. To compensate for the different body join, the OM body is flattened and the bridge is in essentially the same place. The change in bridge location is mostyl a myth in such cases.

_________________
formerly known around here as burbank
_________________

http://www.patfosterguitars.com

I'm doing a 13 fret 000 on a 14 fret body, this will be the second one I've done. I just moved the bridge down the same amount as the distance from the 13th to 14th frets or 0.673". I closed the X about 1 degree to make sure I still overlapped the ends of the bridge. For a 12 fret you would have to move the bridge down about 1.386", assuming a 25.4" scale. I would not want to close the X up any more than I already did so I would move the X down some along with closing it up just slightly. I haven't done a 12 fret 000 on a 14 fret body so that would just be my approach; I'd be curious to know how the factory did it.

_________________
Steve Smith
"Music is what feelings sound like"

_________________
Tony Thatcher
Bozeman, Montana

It's a best practice to have the lower legs of the X-Brace cross under the bridge wings. I make sure the the brace crosses the lower corners of the bridge or at least center through the middle of the outside tips. So if you have to move the bridge plate chances are you might need to move the X or open or close it a bit more.

Pat pretty much laid it out and Steve's follow-up is basically what I was about to type. I'll add that I recently used my 14 fret single O-ish sized body for a 12 fret guitar and did what you suggest (move the bridge back). I did what Steve proposed he would do and altered the x and position to make sure I caught the ends of the bridge. I drew it all out and moved stuff around until it looked right to me. I didn't want the x angle to be too narrow. It ended up sounding pretty darned good.

At the same time I made a 12 fret parlor sized guitar with the same scale (martin short scale) but the body was even shorter. IIRC the body length was 18.25" or 18.125". That one took a bit more head scratching to get what I thought I wanted. The X ended up being pretty far forward of the bridge location. The upper arms of the x hit just above the waist which worried me a bit at the time. It sounds pretty good and has more bass than I expected from such a narrow box. Even though the body was narrow, the bridge moving back put it closer to the middle and I think the extra space between the x and bridge helped too. There is a thread about it here somewhere that has a video with a sound clip. It was originally part of one of the OLF challenges that I didn't finish so the build thread may have pictures of the bracing (I don't really remember).

_________________
Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Pat,
That is a great graphic explanation. It is easy to see that the primary change is the lengthening of the upper bout to meet the neck at the 12th fret. I learned something today. Thank you!

_________________
Joe Beaver
Maker of Sawdust

Pat, thanks for the graphic. Although I do have to admit I feel like it's slightly deceiving. I understand that if 2 instruments have the same scale length that the scales will line up if you just line up the frets/bridge location. What I see as slightly different though is that the bottom line of the lower bouts do not line up. What this would tell me (and I could be wrong) is that if the lower bout is shorter in this regard (smaller top) then that would in fact change where the bridge sits in relation to the top.

Here's an example of what I'm referring to with the Norman Blake. According to Martin these have a 24.9 scale. They have a 14 fret body but a 12 fret neck.


Compared to a 000 with the same scale length but a 14 fret neck on a 14 fret body


You can see just how much lower the bridge sits just by referencing it with the curves of the sides.

And the Roy Smeck. The Smecks were built on the same body as a J35/Jumbo/J45 but with a 12 fret neck and a longer (25" and 25.125" occasionally) scale. This example is a 1941.


And a 1941 J35. Same body, shorter scale but a 14 fret neck


On another instrument note, my main player that I love is my 12 fret L-00. The body shape is identical to a 14 fret L-00 but it simply has a 12 fret neck crammed on it. The tone is so different from a regular L-00 and in inspecting the bracing all they did was shift the bridge plate down a bit to sit under the bridge.

Standard


Red Spruce 12 fret



In cases of instruments like these where you have a body that was intentionally designed for 14 frets and you cram a 12 fret neck on there, the bridge simply *has* to move down the body to compensate for the scale length.

When this bridge moves I know the plate has to move with it to keep the top from folding. And like others have said the bracing is altered so that the bridge sits on the X. (thanks Steve Pat Bryan and jfmckenna for your input). What I'm curious about is how that affects the response.

So I guess to take my questioning a little further. When you take a 12 fret neck with a longer scale and put it on a 14 fret body. Besides making sure that the bridge wings sit on the "X" and altering the angle of the X a bit, what else needs to be altered if anything to make it a good instrument?

Dan

There a couple of things happening there. First with the Martins. They adapted their well established 12 fret bodies to 14 fret when the demand forced them to. All they did was squash down the upper bout to expose 2 more frets. My second picture shows 2 tripe-O's from the 30's and you can see the only body difference is that upper bout. Between the 000 and the 0M they changed the scaled length, but made 2 different models out of it. The Norman Blake model is unique in that it has a 12 fret neck on their 14 fret body - probably a few customs have been made like that, but the Blake is the only time it was offered over the counter I believe.

Gibson was a whole different thing - they were newer to guitars and built mostly arch tops. When they went to flat tops they did a lot of experimentation. The early or later L-0 body could have 12, 13, or 14 frets, the body switch was 1929. My first picture is of Dylan playing a 1929 13 fret Nick that has been stripped and made natural.

My third picture is of 3 Nick Lucas Specials which were the L-0 body but deeper - '28, '30, and '34 (IIRC). The picture is showing how they changed the body shape, moved the soundhole, changed its size, changed scale lengths, changed length of fretboard, changed bridges, added a pick guard, some were ladder braced, H-braced, A-braced, or X-braced, and this is the time when they moved to sprayed sunbursts. Even the head shape was fluid.

So hard to compare what Martin did with what Gibson did.

I built a mahogany 000 12 fret with a deeper body and the sound is a dead ringer for my 1974 000-28, 14 fret in Rosewood. I can barely tell them apart. And when I place them face to face, they are identically shaped except for that upper bout.

Ed

When you make changes to a design that you know works well, it pays to spend extra time thinking through the effects of those changes. I built a 13 fret guitar recently, implementing my own body design, and there were lots of things to juggle. I wound up moving the waist and soundhole a bit, in addition to the bridge location. I’m very happy with how it came out, but it could have been a hot mess if I hadn’t spent the time to run through everything and make sure it all worked out. I find G Thang very helpful in that process. Just drawing everything out (either via design software or by hand) is also essential.

My most recent build was a 12-fret neck on a 14-fret body. I'd built two already with 14-fret necks on the same body outline as the 12-fret. I needed to move the bridge and bridge plate down, close up the 'x' a bit, shorten the fingerboard a couple of frets to allow the soundhole to look 'normal', and used one tone bar. It sounds wonderful, very different from the 14-fretters. A fun project.

_________________
Peter Havriluk

Joe Beaver wrote:
"That is a great graphic explanation. It is easy to see that the primary change is the lengthening of the upper bout to meet the neck at the 12th fret."

Historically the 12-fret neck was the norm. As ruby50 pointed out, they shortened the upper bout to make the 14-fret model, so the evolution of the design was just the opposite.

Legend has it the change was made to accomodate banjo players who were changing to guitars as the banjo craze of the 1920s slowed down.

Seems that the tone difference when going from the 14-fret OM/000 body to the 12-fret 000 body might be attributed to the extra body length and the extra air volume that comes with it.

_________________
formerly known around here as burbank
_________________

http://www.patfosterguitars.com

The Martin 14 fret bodies had a wider upper bout which partly compensated for the lost air volume. I do think the shortened body did have a negative effect on the tone.

If I read the question correctly, this is more about taking bodies used with 14 fret-to-neck guitars and the changes needed to make them 12 fret-to-neck instruments. We've done Norman Blake-style 000 and 00 instruments, as well as 12 fret-to-neck SJ's...really a case of deciding on scale length, number of frets (19 pushes the soundhole down just a bit, and 20 a bit more). With a cutaway and short scale length, the instrument is much kinder to those with some physical limitations...the twist in the left wrist is reduced, and there is less reduction in upper register access than one might think with an intelligently design cutaway. I prefer the 00 version with the slotted headstock, but in terms of practically, the cutaway NB-style 000 is tough to beat for balance, ergonomics, and volume (with just a minor depth adjustment to 4-5/16" at the tail block).

The mechanics of a 14 to 12 fret conversion:

With the length of the extension known from fret count and scale length, the soundhole is located, which also fixes the UTB location. With saddle located referenced to the body edge, the clearance around the edge of the soundhole and the degree of bridge wing overlap fixes the X brace locations, with the number, location, and relative of the tone bars and fingers decided and sketched in on the planview. The bridge plate is fixed by the front and back edges of the bridge, and the last thing to do is decide on UTG, trapezoid patch, or no reinforcement under the extension (we use a trapezoid for most work).

For a 000 14 fret to 000 12 fret conversion, Martin rearranged things a bit, and we tweaked things just a bit more based on the first NB-like instrument out of the shop. Unlike Mr. Foster's graphic emphasizing the lack of any real shift in the bridge location to overall body layout on the 12 and 14 fret variants of the Martins, as well as Mr. Minch's photos and discussion, the NB treatment does push the bridge down towards the tail block by a significant distance. The soundhole movement towards the tail block echos the bridge, although Mr. Blake minimized that shift - perhaps for acoustic reasons - by reducing the number of frets to 19 from the 20 used on the 000 and the OM/000 (same body, but the OM is a sub-class of the 000).

The first illustration shows the movement of soundhole and bridge on the 000-28NB, but the change to a paddlehead on the 000-18NB did not alter that the general arrangement.



For a 00 14 to 12 fret conversion, the X angle is nearly identical, with the X and tonebars/fingers shifting towards the soundhole a bit.

The second photo shows the x-ray style plan view of the NB top (the brace face shown is toward the soundhole; the angles noted are the angle measured closet to the soundhole, and length measurements are as noted).

X-brace angle: 94.5 deg
Distance from edge of body on centerline to center of soundhole: 6.62"
Dist to bridge plate : 12.1"; width of plate: 1.75"
X-to-tone-bar angle: 72 deg; Distance along x from intersection to TB#1: 4.72"; TB#1 to TB#2 distance along X: 1.66"
X-to-finger-brace angle: 93 deg; Distance along X to FB#1: 2.58"; FB#1 to FB#2 distance along X: 2.20"

_________________
We have become a civilization that elevates idiots, prostitutes, and clowns. Am I still to defend it? Yes, for its principles. Yes, for what it was. Yes, for what it still may be.

-Mark Helprin, The Oceans and the Stars: A Sea Story, A War Story, A Love Story (A Novel)

With something as complex as the guitar small changes can add up. If you use the body plan of a 14 fret instrument and swap in a 12-fret neck you will have to move the bridge and the sound hole, as shown. That naturally leads to other changes. Some, such as the bracing, are obvious, but others are less so. The 'normal' sound hole location, centered slightly above the waist, seems to have an effect on some of the internal 'air' modes, which can alter the tone in certain ranges. It's complex, involving the way the top and back vibrate as well, and depends to some extent on how pronounced the waist is. The point is that, as has often been pointed out on these lists, with good guitars everything can matter in the sound. It's part of what gives the guitar it's range of 'tone color', and it why we love them. It's also why it's so hard to sort it all out.

Martin at various times had 18, 19 and more recently 20 fret fretboards and moved the soundhole around accordingly. Using an 18 fret fret board might help put the soundhole in the more modern position on a 12 fret neck/14 fret body combination.
Personally, I like the 12 fret neck on the 12 fret body. Contrary to what a 'dozer operator once told me, an inch can make a difference in more than just one place.

When I did the two 12 fret joins on 14 fret bodies I wanted to move the soundhole towards the heel (hoping to help with bass response). to do that, I had to make them 17 fret fretboards. I don't think many people would try to play the 18th - 20th fret position on a 12 fret non cutaway anyway. At least no one has noticed yet. . .

_________________
Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

That is exactly what I was looking for information on. Thanks Woodie for that great post, really answered alot of my questions.

Thanks, Mr. Kirkland - glad to help.

Mr. Minch's shot of the early style Nick Lucas 12 fretter reminded me that we saw one come back in for the usual checks which we had reset, refretted, and converted from a poorly done replacement rectangular bridge to the correct belly-pyramid. There are drawings somewhere in the shop of this narrow-waisted jumbo-shape instrument with shorter scale length and overall size just a bit bigger than a Martin Size 0...very nice little guitar, but I see few Gibson-style guitars built to the pattern.

_________________
We have become a civilization that elevates idiots, prostitutes, and clowns. Am I still to defend it? Yes, for its principles. Yes, for what it was. Yes, for what it still may be.

-Mark Helprin, The Oceans and the Stars: A Sea Story, A War Story, A Love Story (A Novel)