...A banjo and a dulcimer have similarly sized vibrating surfaces, the same scale length, similar tunings...
The big difference there is the thickness of the vibrating surface - the banjo being skin, the dulcimer wood - smaller strength vibrations move more air on tensioned skin leading to amplification.
Robin, Is there much difference in the basic construction design for stringing between a gut strung Spanish guitar and a wire strung English guitar/cittern? I don't know much about guitar construction, but if there isn't then why would a dulcimer need a different construction for the same type of stringing?
There is significant difference in construction. Spanish guitar are very lightly build - particularly the soundboards. A "western" guitar is strongly braced to take either the downpressure of a floating bridge or the twisting created by a fixed bridge. The Spanish guitar top will collaps from the downpressure of steel strings strung from a tailpiece and the bridge will either rip out or the top bow if steel strings are added to a fixed bridge on a Spanish guitar. I know that from personal experience of playing around with string conversions on Spanish guitars. Additionally, the vibrations from the bridge on both the banjo and the Spanish guitar are directed into the heart of the soundboard ( or skin). On a dulcimer the bridge sits at the edge of the soundboard.
These are the reasons why I'm particularly interested in finding out if there are any examples of old gut strung mountain dulcimers - as to make something that was effective as an instrument you'd probably want to make a few design adjustments compaired to the constructions familier to us? Something simple like a floating bridge - violin style - could make all the difference between the instrument working or not with gut strings (like the design of Dave Lynch's performace models where the bridge cotacts the soundboard directly and some way inboard of the instrument's tail). I don't know for sure if that design change would work for gut strings as I've not tried it myself. Ken Blooms suggestion of a very, very light build over ribs could work - although I'm not sure if the mountain folk would have the luthier skills for that? IF gut strung dulcimers did exist than the construction would have to be pretty simple I guess - perhaps just a bigger sound box and a floating bridge a little more "inboard" than usual?
Also, when Howard Brockway in that 1917 article talks about different "dulcimores" some gut strung and some wire strung we have no idea what he was looking at? His interest was in the tunes rather than the instruments. If you think about the gut strung mountain banjo - lay that on your lap like a dulci banjo, add a few staples, and it should work (I've not tried it to find out). Howard Brockway describes how some players swear by quills for strumming and others swear by leather - it is a pitty he didn't go into such deatail about the instrument design!!!!!
Anyway - the existance of gut strung mountain dulcimers is all supposition until someone actually posts that they have seen one!