I bought my first mandolin recently, after 20 years of playing guitar (and some bass). With the mandolin, having something all-fifths right in front of me and really enjoying the mandolin(!) led me to look into other mando type instruments, and to the question "what would a guitar-style instrument tuned to all 5ths be like?" Well, from there I discovered the various things people have already done in this vein such as NST, but I was already going at it from my own angle rather than having that as a springboard. I purchased an electric Schecter Ultra VI (Bass VI style instrument) about a year back (was thinking in the vein of Charlie Hunter). It has a 30" scale length meant to be tuned to intervals like a guitar, but E1-E3 from a bass guitar's low E up rather than E2-e4 like a standard guitar. I started messing with that particular instrument to see if I could finagle all-5ths. 30" requires a massive string for a Low E1 to sound convincing IMO, but with a .100w low string, a 30" string length is about the scale length for a fretted F# on a standard 4-string 34" scale bass guitar. Tuning a .100w string up to F#1 or G1 is absolutely convincing with a 30" scale length. Though, going THAT low is not necessary IMO. Going to all 5ths with 6 strings is hard. However, from there, I can do something like F#1 C#2 G#2 D#3 a#3 f4 that tuning is the most reasonable way I have to get the 30" scale instrument in all fifths without either a floppy low string or breaking a standard gauge high string. A .010p or .009p string breaks easily tuned much higher than a standard guitar's high e4 or the aforementioned F4 because of it's 30" scale. I have a beat-up old dreadnaught acoustic with a 24.8" scale that I've got in all fifths tuning as well now. On that instrument, I've got a .090 string strung by going through the sound hole (the ball end for bass strings is too large), and string that instrument Bb1 F2 C3 G3 d4 a4. The a4 is holding so far with the short scale length and a .009p string, believe it or not, but that's the weak point. I don't need the high a4, but would prefer to have the low string around C2 instead, but I'd need the octave4plus strings in .009p probably. Haven't tried them for either instrument thus far, but I may. What's interesting to me is, I'd be fine with a 5 string instrument with a standard guitar neck or slimmer, strung in 5ths from C to E (especially if electric solid body or semi-hollow). That'd be like a 5 course mandocello without the double strings. It'd be as high as a guitar, tuned to 5ths, and low enough to get in the realm of the octave that piano players might play a C or D chord. Below that, chording is ugly anyhow. 5 string instruments seem to be either a custom deal, or designed for Keith Richards fans. Mandolin, mandola, octave mandolin and mandocello players, etc have been doing fine with 4 string courses for a while. Guitar necks and bridges are set for 6 strings, most guitar bridges and saddle and headstock tuners are not suited to 5 strings nor the size of the low gauge strings. I've had to unwind some of the wrapping on one or both ends of the low strings in order to get them through the stop piece assembly and/or headstock tuner. I don't expect anyone to start producing 5 string C G D A E "guitars" of standard or baritone scale anytime soon (let alone for a lefty like me!), but it's an entirely different instrument than guitar in terms of it's capabilities, it's voice, and in every way. I'm enjoying playing my 6 string all fifths tuned instruments recreationally and getting to know them (triad and 7th chords are all so intuitive and rich sounding!), and I encourage others to give it a try if they are persistent and a bit handy, but it seems like it is a combination of the market, the cost, and the leap it would take to try "thinking in all 5ths" to get guitarists to explore the possibilities of this kind of instrument. Seems like a shame because I suspect some players could make a 5-string 5th-tuned "guitar" that was properly designed into it's own instrument altogether in a few years time if there were a even just few hanging on every single GC's instrument racks...
This really is an application that seems tailor-made for a multi-scale-length instrument. When I play on short-scale guitars to get an easy high B then the low C is pretty ugly. When I play on longer-scale guitars the C sounds much better but I wind up just doubling the high E and making it essentially a five string instrument. If you could get, say, at least a 26" scale length for that low C and around a 23" scale length for the high B then it would be perfect. I like the voicing even though some of the stretches are pretty tough. The chords seem much more open and spacey than their equivalent "normal" guitar versions.
I think it need not be multi-scale, though the benefit of multi-scale might be using smaller strings than might otherwise be needed and less trouble with breaking high strings compared to an extended scale. Chording and single-notes on all fifths instruments demand a wider stretch than guitar, especially if you want four note/7th chords. So a shorter scale is actually ideal in that regard. Even 24.8" scale can be uncomfortable for me, and I'm fairly versed in the jazz repertoire and 7th chords on guitar. A low C2 that is below the low E2 on a guitar - on a bass, this can be found in 2 places... ~28" with a string ~.080w (3rd fret on the A string) or ~22" with a string ~.100w (8th fret on the E strong). So really, 22" would be fine, like a short scale guitar or in the zone of a regular scale tenor - if you can fit a .100 string. I've got a .090w string on my acoustic tuned to the Bb below it, at 24.8" scale. It works quite well. The problems, IMO, are 1) electric guitar saddles, acoustic guitar bridge holes, etc aren't designed for either large strings, whatever scale they are. 2) 5 strings just makes more sense, but 5 strings on an instrument designed for 6 is another obstacle compared to "ideal" - it feels like a string is missing and/or the neck is too wide, and the empty tuner vibrates obnoxiously. If one has a floating bridge that can be moved a bit, one could better center 5 strings or have a floating bridge for 5 strings made, and then one would just need to have a new nut made (potentially wider string spacing and/or the extra room is more evenly split between the bass string to fretboard edge and the high string to fretboard edge). Or, a custom instrument than can handle bass-size ball end strings through it's hardware, nut, etc, and only has 5 tuners from scratch. I've found a guitar called the "tenorcaster" that could fit the bill, but some of the logistic issues still might come into play, I'm lefty, and I'm not ready to plunk down that much money nor add another instrument.
