Here's my problem...
Instruments- discuss specific features, luthiers, instrument problems & questions
Derik, this is not a problem, at least not in the sense that there is anything wrong. What you have is a traditional diatonic fretboard. I have not seen the video by Brett Ridgway to which you refer, but he was surely playing a dulcimer to which a 6-1/2 (or 6+) fret had been added. There is nothing wrong with your dulcimer. That is how traditional dulcimers were fretted.
To find the major scale, tune DAA and start on the third fret of the melody string. Going up the fretboard, it will be obvious. Don't skip any frets.
Alternatively, you could tune DAd and start on the open string (which is what I assume you were doing). But instead of playing the 6th fret, play the 9th fret on the middle string. Then complete the scale on the 7th fret of the melody string.
The traditional method of playing the dulcimer involves leaving the bass and middle strings to drone and playing the melody on the (you guessed it!) melody string. In that style of play, you re-tune the melody string to get the right mode for the song you are playing. So a song that uses the major scale (ionian mode) you would tune 1-5-5 (or DAA in the key of D). But for other songs, such as Old Joe Clark or Going to Boston, which use the lowered 7th (mixolydian mode), you would tune 1-5-8 (or DAd in the key of D), and for others that sound kind of "minor" such as Shady Grove you would tune 1-5-7 (aeolian mode or DAC in the key of D). And so forth.
A lot of modern players (such as myself) play dulcimers with the extra fret added, but many traditional players prefer the original diatonic fretboard such as the one you have. Theoretically, you could have someone add that extra fret to your dulcimer, but please contemplate not doing that. What you have looks to be a stunningly beautiful instrument (I love the purfling and the ebony overlay!), and you may want to keep it in its original form.
Even if you decide not to limit yourself to traditional modal music and the traditional melody/drone style of play, you do not need to add that extra fret. As I've suggested above, the note you get with the 6+ fret on the melody string is found elsewhere, so there are always workarounds.
updated by @dusty: 03/14/21 01:56:35PM