When I play Auto Harp I have a 1969 Appalachin 15 Bar that I took down to and 8 Bar harp. I use this chord configuration and in most cases there are no sharps except the few needed to create the D and a few minors. The sharped strings I have tuned to natural notes and the chord bars are opened to allow for unison strings. It is a very loud melodic instrument. It is also limited in many ways. I have to create 7ths by picking very creatively. But the incredible ful sound is worth it for me. I have a similar one that plays in D and G and a little of A. But nothing as successfully as the key of C Model that I have described here.
I like the Korg #1 Tuner/trainer best and I have a pick up on the Harp just for tuning.
Bb Dm F Am C Em G Bm D