I have a few questions to ask. Number one- what is the difference between rotary and piston valves. Number two- I have only played tubas with three valves why are there tubas with different numbers of valves and does it change the sound of the piece. Number three- what is the meaning of different keys on tubas.

I'm not a tuba player - - - but - - -
One- rotary valves are round a work much like a ball valve in hydraulic lines; they make a 1/4 turn to open or close the valve.Piston valves, like on most trumpets go up and down (like a piston on a car). My dear close Tuba player friends intimate that many professional tuba players will use rotary valves on their small tubas and piston valves on their larger tubas - it has to do with the mass you have to move to get the valve open or closed.

Two- Understand that valves are only placing the instrument into a certain length. Usually the 2nd valve adds a half step, 1st one whole step, and 3rd a step and a half. A fourth valve sort of acts like a trigger on a horn or a trombone and 'technically' puts the tuba in a different key - but thats what any valve does. There are also valves called "ascending" valves which raise the pitch rather than lower the pitch. No, it doesn't change the sound of the piece it does make some fingerings easier and some notes better in tune.

Three- "what is the meaning of different keys on tubas?" Just that, different tubas are in different keys. It can be very difficult to explain it here without your having the knowledge and understanding of the basic physics of brass instruments. But I am sure you come to understand it down the road. (Or maybe somenone else here can explain it simply).

if an instrument is in a different key doesn't that just refer to the legnth of tubing on an instrument and what your tuning note corraspondes to on a piano? i am not sure but that is what comes to mind.

Mismell you are correct yes, but it does not afect just your tuning note.

Fact: the length of the instrument determines the key it is.
So a tuba that is 16 feet long would have a fundamental note/key of Bb. A 17 foot long Tuba would be in the key of C.(These numbers are not accurate but for explanation purposes only).

So, to tune to a concert A the 17foot tuba needs to play an A and the 16foot tuba needs to play B natural. Simple enough. Much Tuba music is written in bass clef, but some like Brass band music is written in treble clef. Further once you have tuned you have to now play the music. Many Tubas are in Bb, but the music is written as though the instrument is in C. So from the begining players are taught transposed fingerings (but they aren't told this - trombones are taught the same way.).

Back to tubas - The Bb Tuba player is going to play the music as is. The guy with a C tuba is going to have transpose everything by sight by 2 semitones. If the player is using his F tuba of Eb tuba, then of course he has to transpose accordingly.

The adbantage to the different tuba keys is two fold. Sometimes it is size and weight, and sometimes it is a tone color the tuba player wants in order to blend with a specific group.

... that all, unless you live in the UK. Understand that elsewhere in the world tubas ARE transposing instruments. BBb tuba happens to transpose the same as a BBb contrabass clarinet - that is, they read treble clef just like a trumpet or baritone/euphonium treble clef. Then there are the CC, F, and Eb tubas. I believe the UK uses mostly the CC and F variety, while in the US we use BBb and Eb tubas.

Outside of brass bands, our tubas are treated as being "in C", even though they actually aren't. If you played "C" on an Eb tuba and didn't know you were playing an Eb tuba, you were actually playing Eb because the tuba is in Eb. HOWEVER, we are taught that on the Eb tuba that the fingering that corresponds to A is actually the C. We are taught to transpose at sight, in effect, by learning where those open notes are.