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I just finished up this mandocello which is the first one I've built. It was a bit of a challenge in that there is almost no information out there that I could find on the finer points of building one of these, so I winged it on many aspects.

Mandocellos are a bit of the Wild West in that their design is not really standardized other than having four pairs of strings that are tuned in fifths to the same pitches as a cello: C G D A, low to high. They can have classic mandolin style carved top bodies, archtop guitar bodies, or flattop guitar bodies. There has also been a steady stream of people converting old archtop or flattop 6-string guitars into mandocellos. Scale lengths range from 24" to 27" with associated tradeoffs between playability and sound. Nut widths and string spacings are not standardized and again there's no information on it that I could find. String spacing within courses is important, particularly for the low ones. If the strings in a course are too close together, they will buzz from hitting each other when picked and if they are too far apart, they are hard to fret properly.

A small number of mandocellos have an added fifth course on the high end and I decided to build one like that. An interesting thing about having the five courses is that the lower four are a standard mandocello while the upper four correspond to a standard octave mandolin (with an extra lower course). In addition, if you capo at the fifth fret, the upper four courses correspond to a mandola (with an extra lower course).

Another challenge was that the total string tension is about 295 lbs so that required adjustments to the brace heights using the cube rule. I also scaled up the bridge size from what I use on 6-string guitars to increase the footprint.

This one has a red spruce top, Indian rosewood back and sides, bridge and fretboard, a Khaya neck, and koa binding, rosette, end graft, and headplate. The lower bout is 14.5". The scale length is 25". The string gauges range from 0.013 to 0.070.

I'm happy with how it turned out. It has a full, lush sound and it has a deep bass sound from the C-course which is C below the low E on a guitar. Now I'm figuring out what I can play on it.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Yikes! Cellos are very lightly strung at 100-130 pounds. I can play some on a guitar, nothing on a violin or viola. Surprisingly, the cello seems like it is much easier to play than the violin or viola. It's the under the chin thing. The long stretches with a 690mm ~27" scale doesn't seem too terrible. One consolation; without frets; is that intonation is easier than with a 13" violin scale. You just have to get used to moving you hand as much as your fingers. I just found Segovia 2-3 octave scales online, and I think practicing those would help my guitar playing.
Bowing and vibrato are other things altogether, so I'll stick with playing guitars.

Nice design. It looks like a cross between an archtop and a flattop.

That thing isn't that big. The binding is very nice, and koa; well koa doesn't need anything does it.

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Why be normal?

Beautiful instrument, Jay.

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Craig