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This is a simple question, but I will entertain Freak Jury asides.

The YFB-621 F tuba bore size is listed as "17.5-18.5mm (0.689-0.728")."

Does that mean the rotary valve is 17.5mm and the four piston valves are 18.5mm? Or are the rotary/first three pistons 17.5mm and the fourth valve is 18.5mm?

i.e., is the piston valve section on these tubas a straight bore, or is the fourth larger?

Thanks. Idly wondering and poking around tuba specs while at work.

Blake

Look up the CC and BBb versions, without the 5th valve. If it lists and same bore specs, that answers your question

LeMark wrote: ↑Tue Jan 31, 2023 8:34 am Look up the CC and BBb versions, without the 5th valve. If it lists and same bore specs, that answers your question

Smart. The YCB-621 has a graduated bore size that is the same as the F tuba's, so it seems like the rotary + first three valves are 17.5mm and the fourth valve is 18.5mm.

Thank you for the idea, Mark!

I'm about as sure as I can be (??) that the rotor is a .689" rotor, since it's before the pistons and in the mouthpipe tube.
The 4th piston is the larger bore one.

Interestingly, the YBB-621 offers all four pistons in .689", while the YCB and the YFB have a graduated .689"-.728" bore. But yes, it seems like the rotor is the same size as the first three pistons.

Question answered. Thanks!

arpthark wrote: ↑Tue Jan 31, 2023 8:45 am Interestingly, the YBB-621 offers all four pistons in .689", while the YCB and the YFB have a graduated .689"-.728" bore. But yes, it seems like the rotor is the same size as the first three pistons.

Question answered. Thanks!

A reasonable explanation is that a BB-flat tuba bugle expansion is more gradual than that with F and C built on the same (for lack of a better term) "frame".

5, 1, 2, and 3 are all the same. 4th is larger and the slide will not interchange with any other valve. (I played one for twelve years.)