Why I chose meantone fretting on my Thomas reproduction
I've been meaning to post something on this topic for ages, as a follow-up to the earlier thread in which I was weighing up the pros and cons of equal temperament vs. just intonation vs. meantone. I spent ages trying to wrap my head around the issue and agonising over which way to go, so it would be nice to think my conclusions might help someone else out if they find themselves down this particular rabbit hole!
Just to frame the 'problem', then: I think there are two main reasons why a noter-drone player especially might be tempted away from the modern default option of equal temperament, and towards just intonation. Firstly, sound: with just intonation, every note played on the melody string will sound as consonant as possible against the drones (and especially the tonic), at least in the 'home' mode of Ionian. Secondly, and relatedly, historical authenticity: since pre-revival builders will have worked out their scales by ear, those maximally consonant intervals will have been what they were aiming at when they set their frets.
So far, so good. But there are two down sides with a JI-fretted instrument. One, it's always going to be slightly 'out of tune' compared to most fretted or keyboard instruments, since they'll almost always be in equal temperament. (This one didn't worry me too much, since I was really only concerned with solo play; and in any case, I don't believe players of JI instruments generally find this a problem in real-world jamming situations.) And two - this being the one that bothered me - once you retune to play in modes other than Ionian, the intervals between melody notes and drones start going wonky. Some will still be spot on, but others will be way off, making the scale as a whole worse than its equal temperament equivalent. That was a deal-breaker for me. Yes, I spend most of my time playing in Ionian, but there's no way I want the sound of other modes to be that heavily compromised.
...which is where meantone intonation comes in. Every modal scale is as close as every other to its just-intonation equivalent - and that's pretty damn close. Tune DAA (or whatever) to play in Ionian, and all the important intervals - unison, third, fourth, fifth, sixth and octave - will be either spot on, or within about five cents of where they ought to be (for maximum consonance, as per JI). Tune to DAC, DAG, or DAD (or whatever) to play in other modes, and the same is still true.
Five cents is really very close - well within the 'green' zone of a typical electronic tuner, certainly; and in terms of fret placement, surely about as close as a skilled builder would be likely to get using just a good ear and hand tools. And the fact that the second and (sometimes) seventh scale degrees are a few cents further off on a meantone instrument is neither here nor there, really; those intervals are dissonances anyway.
So to my mind, a meantone-fretted dulcimer ought to be perfectly 'authentic' in terms of consistency with 'by-ear' fretting, and by the same token, as close to a JI instrument in Ionian mode as makes no odds - but with the significant benefit that it should sound equally good played in every mode. You could say it's like an instrument set for just intonation in Ionian by a skilled builder with a good ear and hand tools, but on which just the right frets happen to be 'off' by just the right amount, and in just the right direction (sharp or flat), to allow for retuning to play in all modes.