Posted by Rob P-M on May 15, 2002 at 23:14:58:
In Reply to: Bore Size and piston valve stroke posted by Dave on May 15, 2002 at 17:24:28:
Take a look at the thread on Short Action.
Assuming the air pathway holes in the piston are circular, and given the need to avoid one pathway impinging on another, the larger the bore size, the larger the holes in the piston must be. The larger the holes in the piston, the longer the valve stroke must be to move from the open to the closed position, lining up the different pathways. Simple, obvious.
The only way to shorten the stroke is to make the verticle size of the hole smaller. Reducing bore size is one way. Other ways have included oval ports, the method used in the Conn short action valve, and using dual smaller bore tubes, the method on some Holton sousaphones. The methods shorten the stroke, almost to cornet length, but there have been objections as to stuffiness and intonation effects. I can't speak as to the Holton solution (which does not seem to have caught on), but my experience with Conn short action valves, in the 2XK sousaphones and the 2XJ tubas has been positive. The intonation issues for the 2XJ tubas are lessened (for most of us, but not for everyone) by using a sousaphone bit.