@trumpetb
The Bb Civil War era cornets I’ve played over the years tend to be pretty dull sounding, especially with a period mouthpiece. I suppose you could call it mellow, but it’s not the same “mellow” as a good modern cornet. So, yes, it was difficult for me to make mine sing, but I don’t think the 3rd slide design had much to do with it . As far as the fixed length 3rd slide goes, yes, certain notes required lipping, and not just the ones using the 3rd valve. The Henry Lehnert cornet I own, while one of the better cornets I’ve played from that era, still has intonation problems and requires a lot of lipping to play in tune. Especially troublesome is the bottom space F, which is extremely flat. Lipping it into tune results in a really thin sound. There’s no compensating with a slide for that problem, especially since the D and F in the staff are a little sharp.