[ Ref. Deane Shepard in 187024 MMDigest ]

If you don't finger any valves, you can't play all the notes on a tuba.
You can play only some notes -- those not requiring any fingering.

Otherwise, the sound you hear is not produced by the tuba itself, but
it's like putting a tuba in front of a loudspeaker, that will play
anything you send to it (even the sound of a piano...).  The tuba will
simply alter the sound of the speaker, like the tone controls in a
stereo amplifier.

Leonardo Forti
Rome, Italy