Complex sentence: It has been incorrectly noted that the tuba is a conical brass instrument and the sousaphone is a cylindrical brass instrument; actually both instruments are semi-conical--no valved brass instrument can be entirely conical, since the middle section with the valves must be cylindrical.
Split into simpler sentences: It has been incorrectly noted that the tuba is a conical brass instrument and the sousaphone is a cylindrical brass instrument. No valved, brass instrument can be entirely conical, since the middle section containing the valves must be cylindrical.