CBOM questions / musings

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sebastiaan56
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CBOM questions / musings

Post by sebastiaan56 » Wed Mar 19, 2008 9:47 am

Hi all,

Ive got this bit of Spittle (I cant really call it Zoot can I?). Tiger Myrtle, verrry nice...... Anyway, Ive bookmatched it and then glued up and cut out a body based on Benedetto's shape without the cutaway. I then did the same with some nice yellow Huon Pine Ive got that goes boing. The middle of the lower bout to the top edge is 232mm.

Im thinking to make a CBOM type object with a flat top and back with Torres style bracing so it will be nylon stringed and a change for me, single course. Im figuring the total string length will be 490mm ish joining the body at the 14th fret. The real questions are; 4 course and tune it in fifths like a Tenor Guitar? or 6 course and tune it like a guitar? In between is five course tuned in fifths maybe even with fanned frets.

Apart from the potential intonation problems of the five course in fifths I cant see issues in terms of strength and bracing in any of the above. Is there a quick and dirty method of working out how much bracing I really need? Is there a quick and dirty method of arriving at the optimal note for each string length / gauge?

Sebastiaan
make mine fifths........

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graham mcdonald
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Post by graham mcdonald » Wed Mar 19, 2008 5:53 pm

'afternoon,

I gather what you are thinking about building is effectively a wooden bodied tenor banjo with nylon strings. A couple of things to think about. 490mm/19" is a funny inbetween scale length, a bit too short for GDae OM tuning, but maybe about right for a long Cgda mandola. My online string calculator can help you with steel strings, but it is of no use for nylon. The D'Addario web site has a downloadable pdf string tension chart, and that should help you work out what possible combination of nylon strings you could use. That could also let you calculate some idea of the total tension, compared to a classical guitar. Then you have to work out how much stiffer or heavier the Huon is going to be and take into account the smaller body size. It is all going to be pretty much guesswork.

Five courses in fifths is always a compromise in getting either to top or bottom courses to work. I don't think it is by accident that guitars have only 2 octaves between highest and lowest open strings, and violin family instruments even less. It is, I suspect, a combination of available string technology and what a wooden box will do. My general rule of thumb is that for any given string length there is a finite range of possible pitches that it is possible to use. A 25" scale guitar can't have it's top string tuned much above the g (3rd fret). The string will either break or pull the bridge off. A lighter string can be used, but there is a practical limit there. At the other end heavier strings can be used to get the bottom E down to C, but going much lower requires that the mass of the string needed to get the tension means that they go plod with out much tonal interest.

An alternative idea might be a baritone ukulele which usually have about that string length and are tuned the same as the top four strings of a guitar (I think, at least the same relative tuning).

Have fun

graham
Graham McDonald
http://www.mcdonaldstrings.com

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Allen
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Post by Allen » Wed Mar 19, 2008 6:34 pm

OK, I've read the above posts, and just when I thought that I might be getting a very small grasp on building guitars, along comes a CBOM. I'll bite. What the hell is that? :?

Hey Hesh, I sure hope it's nothing like a tosser! :lol:
Allen R. McFarlen
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graham mcdonald
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Post by graham mcdonald » Wed Mar 19, 2008 7:40 pm

An unfortunate semi acronym which translates as Citterns, Bouzoukis, Octave Mandolins

A term I refuse to use

graham (grumpy on a wednesday night)
Graham McDonald
http://www.mcdonaldstrings.com

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sebastiaan56
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Post by sebastiaan56 » Wed Mar 19, 2008 8:20 pm

Yes an unfortunate acronym used by as Graham as indicated. Its a pity but this area of lutherie seems to be a riot of termonologies with little standardisation. Not a prob for me as it also gives the compulsive experimenter lots of room without upsetting traditionalists. Imagine if I had called it a mini - OM. Also why I posted here and not a mando type forum.

Thanks Graham, as I suspected Im in the wild blue yonder. No probs, its not a living for me.... The boss told me she would be interested in a Uke about an hour ago so I guess I have clarity now. I'll just use Tenor Banjo tuning so I can stay in my beloved fifths.

I dont have the maths for the Houn top so it will probably be overbuilt, in the end there is only one way to find out. Will be my first rosette as well!
make mine fifths........

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