HEy, i just got my new seven stringer today, but for the last few years i've been playing only flats. does any company (particularly rotosound) make flats in 7 string flavoring?
Assuming you are tuning B-E-A-D-G-C-F, you could buy a set of your favourite six string flats and use a plain steel F string from Conklinguitars.com (the SIT Powersteel string is the plain one, while the Snakeskin string is roundwound). It may not have the exact sound of the other six strings, but it's as close to a flatwound high F string as you're going to get.
