I owned a Holton Superbone TR-395 and used it fairly extensively for a number of years. It is a very small bore since it basically uses trumpet valves. Since the valve block is spliced into the trombone between the main slide and the bell section, it takes up some of the front to back space that normally would be dedicated to the slide. Ergo, the slide has just 6 positions. If you want to play 7th position, you use 6th and hit the 2nd valve. Also, the valves are played with the left hand. The balance is good since the valve block is very tightly wrapped, but the horn is HEAVY for a tenor trombone.
Doug
Adams E3 0.60 Sterling bell - Prototype top sprung valves
Concord Band
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Townsend Military Band