The valves add tubing to the main tube of the instrument, thus lowering its fundamental pitch. The first valve lowers the pitch by a whole step (two semitones), the second valve by a semitone, and the third valve by three semitones. Used in combination, the valve tubing is too short and the resulting pitch tends to be sharp. For example, a BB tuba becomes (in effect) an A tuba when the first valve is depressed. The third valve is long enough to lower the pitch of a BB tuba by three semitones, but it is not long enough to lower the pitch of an A tuba by three semitones. Thus, the first and third valves used in combination lower the pitch by something just short of five semitones, and the first three valves used in combination are nearly a quarter tone sharp.
The valves add tubing to the main tube of the instrument, thus lowering its fundamental pitch. The first valve lowers the pitch by a whole step (two semitones), the second valve by a semitone, and the third valve by three semitones. Used in combination, the valve tubing is too short and the resulting pitch tends to be sharp. For example, a BB tuba becomes (in effect) an A tuba when the first valve is depressed. The third valve is long enough to lower the pitch of a BB tuba by three semitones, but it is not long enough to lower the pitch of an A tuba by three semitones. Thus, the first and third valves used in combination lower the pitch by something just short of five semitones, and the first three valves used in combination are nearly a quarter tone sharp.