Tubas are normally in the key of F, Eb, CC, or BBb and can have 3 to 6 valves. Because they are so big, some tubas have a compensating system. This means a fourth, fifth or sixth valve is used to make the instrument sound more in tune - especially the lowest notes.
It takes a lot of breath to play the tuba. It is one of the loudest instruments in the orchestra but can also play very quietly.