Tenor/Baritone Uke to Octave Mandolin Conversion

I notice there have been previous discussions on converting large body ukes to octave mandolins but all seem to strive for four-nylon-string OM configurations. I have acquired two well-built tenor/baritone uke bodies that were never finished. I turned one into a baritone uke and for the second I want to make an 8-steel-string octave mandolin. So I get to make the neck any way I like. I used an 18 1/2" scale on the uke and I intend to use the same on the OM. The two big problems I anticipate are 1) the neck joint and 2) the soundboard strength. It appears that the person who built the bodies intended to join the neck with a 1/8" biscuit type thing because he cut a dado of that size in the neck block. I used that technique on the uke and it works fine for four uke strings but 8 steel strings would pop it off pretty quickly. I have some ideas on how to solve this but I am looking for other thoughts. In this picture, the tape shows the line of the internal top bracing. I believe the top would sink under the pressure of eight mandolin strings. I am considering adding a bridge plate under the top approximately where the piece of graph paper is located. I don't want to take the back off the instrument and I believe I can clamp the plate in using magnets. I have never heard of a bridge plate on a mandolin. Any thoughts on this plan.

You could go the Dixie Mitchell route, with a metal rod and the strings running from nut to tailpin, taking the tension away from the neck and body, and have a top with little-to-no down pressure.

I'm guessing you plan a fixed bridge, not a floating one. If so:

1. Your scale length is much shorter than usual for an octave mandolin. This will require much thicker strings, especially on the bass notes, and those strings will likely be too thick to intonate properly. Somewhere between 20.5 and 23.5 seems normal for an octave mandolin, and it will intonate better with a longer scale.

2. I can't see the top surviving more than a few months with that bracing, even if you add a bridge plate. The string tension will be substantially more than twice that of a baritone uke! First, the tension on the fixed bridge will want to belly up the lower bout, and you have almost nothing there to resist it. Have a look at the bracing pattern for a steel-string guitar, and you'll see extra bracing down there to cope. Second, the tension will be trying to fold the body in half around the soundhole, and again there is very little resisting that. I'd want something more like X bracing, with reinforcement between the transverse brace in the upper bout and the neck block to resist the neck and lock folding inwards.

If you want this project to work, I think you have to remove the back and completely re-brace the top.

You might be able to turn this into a GDAE tenor guitar with a floating bridge if you could glue in a near-full-width brace where you plan your bridge patch, though ideally I'd want to arch the top in the lower bout by shaping that brace (so again, back off for re-bracing).

Sorry to be negative, but I think your plan is doomed to failure.

Thanks Professor, In my heart I know you are correct. I am at a loss for what to do with this nice cedar/mahogony baritone uke box. There are too many ukuleles in the world already. A four-nylon-string mandolin is not something I would care to own. Maybe a thumb piano or an incense burner?

Don’t underestimate the uke. I converted a concert (size) uke to fifths, viola tuning a couple of years ago and it’s a really sweet thing, sort of astonishing considering the dimensions. I later got a (cheap) baritone for further experiments, but the instrument was just too crappy. Hoped for a fifths-tuned, smaller scale length easy to pick up version of a guitar voice.

About your mando octave idea, I’m fairly sure that mechanically, there were several intractable issues and many likely subtle ones, so low chance of an interesting build.

Consider 4 nylon strings tuned GDAE. I have a cheap baritone tuned that way and it has a pleasant sound, plus easy to fret! Same tuning as an octave mandolin, and would sound quite different from your GDBE baritone.

If you do, I'd recommend a longer scale length, 21 inches or more. The low G on my uke (19 inch or so) doesn't intonation well, but would at a longer scale.

String tension can change a lot in terms of tone. I’ve often wondered what the ukulele would sound like with a steel support rod inside the sound box.
I’m guessing it wouldn’t be too difficult to make a conversion kit and drill into the end plate to install it.

Too many musical instruments? I don’t think so. :)

I hope you don't mind an interloper chiming in on this. I have ulterior motives, described below.

First, about the splined neck joint. That will work out just fine. Here is a photo of an Andean Colombian tiple, an instrument roughly the size of a baritone uke with 12 (count 'em!) steel strings. They conventionally use a splined neck joint.

Attachment 205835

And the second thing is about the top. The baritone uke string tension is generally about 52lbf. An octave mandolin typically has about 180lbf of string tension. Assuming the instrument will have a conventional fixed bridge it is highly unlikely the top will withstand the static tension without some additional bracing.

My ulterior motive mentioned above is that I have a not-yet-published string set design calculator, and I regularly use proposed instruments such as this to design string sets to test out the calculator. From that exercise, keeping the string tension at about the 180lbf of the conventional octave mandolin and the scale length at the 20" typical of the bari uke yields a string set that exhibits high inharmonicity (skewing sharp of the partials of the vibrating string). It is not terrible, but attempts to design even a slightly lower tension string set would make inharmonicity even higher, and thus more likely to be an issue.

Sorry for the discouraging information.

Thanks Mr Mottola, I wonder how a floating bridge/tailpiece arrangement would alter downward pressure at the bridge, if at all.

Quote:

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Originally Posted by laddyjota

Thanks Mr Mottola, I wonder how a floating bridge/tailpiece arrangement would alter downward pressure at the bridge, if at all.

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A floating bridge/tailpiece setup has less force trying to fold the instrument in half, but a lot more trying to push the bridge through the soundboard. In an earlier post I suggested this would need a ladder brace under the bridge location, and for a mandola I'd guess a fairly stout brace. Can't say how stout, but I'd make it at least 12mm tall by 6mm wide, and that might not be enough.