So I've built a few guitars and now I'm looking at building a MandoCello.
My plan is a guitar body shape based on a Gibson L-O with a pin bridge X bracing and a shorter 24.25 scale length.. Normally in bracing the top I would thickness it until it was the appropriate "floppyness" and then brace it and carve the braces till I got to tap tone I wanted out of the top... But that is for 6 strings, How do I insure the bracing is strong enough for the heavy 8 strings of a Mandocello but still keep a good tone??????
One thing to sort out first is string gauge. Here is what I would do: calculate the total tension of your typical guitar strings with that scale length. That is your baseline; this should give you an idea of what your previous work dealt with.
Now, figure the total string tension of the eight string set-up, and compare the two. Depending on your string gauges and tuning, you could end up with more, or possibly even less, total tension on the top.
Get an idea of how different the total will be: if it is about 8% more total tension, then try to add a corresponding 8% stiffness in the bracing, as compared to your previous work. I don't mean mathematically make every dimension that much larger; more just to use your instinct to judge the stiffness.
Duh good point, not sure why that did not come to mind.. It would certainly be a starting point..
I'm just concerned that as I increase the stiffness I'm hurting the sound (at least after a certain point) but of course if the thing collapses I've really hurt the sound :-0 it is really all about getting as close as you can isn't it ..
I'll do some calculations and post results here...
Kerry
Josh Humphrey wrote:One thing to sort out first is string gauge. Here is what I would do: calculate the total tension of your typical guitar strings with that scale length. That is your baseline; this should give you an idea of what your previous work dealt with.
Now, figure the total string tension of the eight string set-up, and compare the two. Depending on your string gauges and tuning, you could end up with more, or possibly even less, total tension on the top.
Get an idea of how different the total will be: if it is about 8% more total tension, then try to add a corresponding 8% stiffness in the bracing, as compared to your previous work. I don't mean mathematically make every dimension that much larger; more just to use your instinct to judge the stiffness.
So by my calculations a guitar with light strings is about 120lbs and a MandoCello with D'addario's Mandocello strings is about 270lbs tension.
So that's a bit worrying to me
I realize to be twice as stiff the braces don't need to be twice as thick but still ... I plan on doing a thin carbon fibre lamination in the braces but the whole thing is going to be a guess.. Advice from anyone who has been there would be great
Those numbers seem strange. According to the D'Addario string tension guide (http://www.daddario.com/DAstringtension ... iveID=2871 and http://www.daddario.com/upload/tension_chart_13934.pdf), I would expect a set of lights to come in around 150 lbs on a 25.5" scale guitar. The equation that D'Addario has gives me the same numbers as they have in the charts when I have tested so I have relied on it when what I need isn't in the charts. 270 lbs would worry me. But when I made a 12 string (long ago), I designed for 250 lbs using a 3/8" X-brace instead of 5/16" on a jumbo body and adding a third tone bar. That guitar worked very well.
Well, for a tailpiece/floating bridge instrument, that amount of tension would make more sense- the break angle could be designed to mitigate the tension onto the soundboard by a desired amount.
However, with a fixed bridge system, you are stuck with all of the string tension at the bridge, with no tailpiece to bear the burden.
One option is to experiment with different (smaller) string gauges. I have tuned a .056 PB guitar low E down to C (at 24.75") with acceptable sound, albeit a bit floppy. Especially if it is doubled courses, you may be able to go a few gauges lighter to get an overall lighter-built instrument. I suppose it all depends on the tone you're after!
Wow! Just looked at D'addarios Mandocello strings- .074 for the low C!! Holy smokes. I am willing to bet that these are designed for tailpiece instruments. I think for fixed bridge you could try something a little lighter
Yep .074, and yes they are loop end strings but to tune to the proper tuning for a MandoCello I don't know how much lighter you can go..
KLW
Josh Humphrey wrote:Wow! Just looked at D'addarios Mandocello strings- .074 for the low C!! Holy smokes. I am willing to bet that these are designed for tailpiece instruments. I think for fixed bridge you could try something a little lighter
Kerry, we require full names first and last here. So can you please provide one of the Staff with your full name and we'll change your information for you, thanks.
Mark Swanson wrote:Kerry, we require full names first and last here. So can you please provide one of the Staff with your full name and we'll change your information for you, thanks.
This will probably not be much help but the mandocellos I made were based on archtop guitar bodies, used tail piece and movable bridge design. For structural consideration I put a strut between the top of the neck block and the top of the tail block to support the compression of the heavy strings. Also added an "e" course to the treble side, making it 5 courses.
Anything worth doing is worth doing well, so make it sturdy but make it for sound production. I think you will be wasting your time if using lighter strings than in the D'Addario set, which is designed for a 25" scale.
Well I figure a MandoCello is not too much more tension than a 12 string so I'm going to do some 12 string bracing research and see what I can come up with..
Someone with more knowledge will come along and confirm or correct this but. . . My understanding (based on casual readings of discussions on 12 string bracing design) is that a 25% increase in top thickness and brace height will provide double the stiffness. Essentially adding 25% more weight can provide sufficient stiffness for doubling the string tension; so you double the input horsepower and only add 1/4 extra weight!
Somewhere in this section you can see pictures of a flat top mandolin I recently made with a pin-less fixed bridge. It is doable, but not without design problems. Most notably keeping the bridge footprint down while allowing space for the loop-end windings. Make sure you get some strings before you design your bridge and draw it out full scale.
PMoMC
Take care of your feet and your feet will take care of you.
Yes I've heard that relationship as well or at least something close.. My only worry is the old say "in theory, theory and practice are equal, in practice they are not"
Had a wild thought the other night what would happen if I used Cello strings, 1/2 or 3/4 size cello strings would be about right from my recollection no idea how they would pluck, I know cello players do pizzicato now and then but not 100 %
Use a string gauge calculator and put in what scale you are going to use and the pitches of each string. The calculator will tell you what gauge strings to use for those specs. Shorter scale needs a thicker string, longer scale needs a thinner string, but the calculator will let you know specifically which gauges you need.
Just so you know, I started out playing mandolin and tend to tune all my instruments in fifths. I have an old Gibson archtop (24.75" scale length) tuned to C-G-D-A-E-E. That low C is awfully quiet at that scale length. It's there, but I wish it were more powerful. I'm gearing up to try an archtop build soon motivated by this experience using multiple scale lengths and I'm aiming for 26"-23.5" (to try to get a high B string in, too).
Just to revive this topic... The Mandocello in question is now getting close to finished.. I ended up doing a Gibson L-O shape with a slightly deeper body and a pretty standard if slightly taller X brace system... found some ball end strings and went slightly lighter than the standard for a Mandocello..
Also ended up a Fanned-Fret .. 24.75" to 26.25"
I'm looking forward with some nervousness to see how it sounds...
A few build photos added.. at the point of carving the neck now...
.. 
