It’s the heart of King’s superiority. The fastest and smoothest action of them all!  The main purpose of adding an “E” valve to the “F-Attachment has been to make possible a complete chromatic scale without having to retune the instrument in order to play “low-B.”
The new King double valve bass trombone does this in a simple, easy-maneuverable manner which is unique.  Plus offering exclusive features such as: an optional “D” crook making possible to scale down to the lowest “B” without ever reaching as far a 6th position, fast and quiet short stroke valve action, and Matching Bore throughout the entire horn. –King Trombone Masterpieces Catalog, 1971.
 
This vintage King Duo Gravis Bass Trombone is a piece of history as well as a beast of a player. Featuring a dependent valve system with the original D crook, this bass trombone has an impressive low register with easy low C's and B's. The bell is a rare King rose bell and the original engraving is still visible. The horn has seen heavy use and is missing about 50% of the original lacquer. The rotors function fast and independent of each other and the hand slide is in excellent shape with a smooth feel through all positions. 
 
Comes with original King case