Posted by John Swensen on February 10, 2000 at 19:14:33:
In Reply to: Is the St.Pete bore the biggest?????? posted by UGO BOSCAIN on February 10, 2000 at 09:16:23:
My own measurements suggest that the St. Pete valve bore is closer to .830" than .850".
That said, remember that rotary-valved tubas generally have the valve section further along the (tapered) bore than piston valve tubas, so they, naturally, have a larger bore than similar-sized piston tubas.
In terms of resistance, blowing air (no buzz, no sound, other than rushing air) through a St. Pete with four .8??" valves down has far more backpressure than a 6/4 Nirschl, with all five of its .750" valves down. Much of this resistance can be attributed to turbulence through the St. Pete valve ports; typically, there is a large burr of metal in one or more ports of each valve on a St. Pete. I removed the burrs from one St. Pete valve set in the process of a complete valve rebuild, but the horn is not, yet reassembled. When I finish, I'll report the differences between stock and rebuilt valves.