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It's not the size of the ship, it's how loud you can play The Ride in the elephant room.

bloke wrote: ↑Wed Oct 06, 2021 7:31 am I actually have a tuba for which I have four bells that all fit it, and are all different.

Are these equipped with ordinary detachable-bell tenons and collars, like a normal detachable bell tuba but at the joint (as pjv was saying) at the top of the bottom bow? or do you resolder the bell as you go back and forth between bells? Bolted on leadpipe? It's kind of cool idea, but I bet you're one of very few who ever go there.

They are just run-of-the-mill detachable bells.
When examining almost all tuba bells, though, the expansion-and-flare part really doesn’t start until quite late.

Those that begin earlier – such as those eye-popping Russian helicons - tend to really stand out, because they are so unusual.
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An exception could be said to be the non-convertible PT-6 versus PT-7, whereby the expansion is different beginning at the small end of the bottom bow, and the entire remaining portions of the instruments are the same.

Still…If someone to have both bottom-bow-and-bell assemblies (that could be interchangeable) on those two models, I’d wager that their owners would pick one of them over the other, and only use that one.