Posted by John Swensen on June 22, 2000 at 13:18:59:
In Reply to: Questions posted by Educable, but green on June 22, 2000 at 12:36:30:
Some answers:
1) Most piston valves come with holes in the bottom and top of the pistons, to lessen air resistance through them as the piston pumps up and down. This is not venting.
Venting a piston or rotary valve allows a slide to be pulled out without having the valve depressed, and without creating a pressure difference. If you pull your first valve slide out a bit with the valve up, then push the valve down, you will get a "pop" from the sudden change in pressure. This "pop" can upset one's embouchure when playing. Tubist very often need to push and pull slides while playing to make small adjustments to the intonation of individual notes.
Piston valves are vented by drilling a hole in the piston itself, in a spot where it will intersect the ports to the slide with the valve up. Rotary valves are vented by drilling
a hole in the casing, in a spot that intersects with the port on the rotor facing the slide tubes with the valve in the "up" position.
2) The screws on the bottom cap of a rotary valve allow some control of the fit of the (tapered) spindle (or axle) of the rotor in its tapered bearing.
3) The user can do anything he or she is competent to do; of course, a user like Matt Walters or Joe Sellmansberger can do much more than a user like me. Replacing bumpers is a relatively safe operation to perform (they can be replaced again if you mess up), but it can be difficult to get the bumpers forced into their holes without damaging your fingers and/or stop plate. Nitrile rubber (Buna-N) O-ring material with a suitable cross-section (that's the diameter of the rubber, not the ring) works really well. Trim to length and to proper thickness (this is important) with a single-edged razor blade.