TubeNet

What bloke says is true, but it also is a factor that very few tuba models are optimised for the extreme upper range, as there is very little purpose in doing so.

Then it must be said that some of the most beautiful tuba playing I ever heard was Jørn Arnsted, retired of Odense Symphony, playing Danish hymns in the tenor range on unaccompanied F tuba.

Klaus

If you are blowing into your horn with a strong enough buzz and not curling your lips under your teeth or closing the back of your throat... you should be able to get every not up to the c above blue balls f for trombones to sound like a normal note and resonate just fine.

It also has to do with the fact that the terminal node on any brass instrument becomes unstable above @ the 12th harmonic, or maybe somewhat higher, depending on the bell geometry and the horn starts functioning as a megaphone rather than a resonator.