Posted by Matt Walters on January 17, 2002 at 17:10:00:
Here is an interesting topic. If airflow is restricted by the largest obstruction in the airstream, then what good is a big bore valve section that has large blisters in the ports of the pistons?
York made some short chubby 4/4 CC tubas that had about .720 bore tubing, but the ports of the pistons were only .690. The tubing blisters in the ports were very small compared to the pistons of other horns with a similar bore.
Conn had an interesting valve in the .693 bore of the original Wonder Eb's, 10J BBb's and 26K sousaphones. The size of the piston is identical to the .734 bore pistons of the famous 14K and 36K sousaphones (standard action) that many people like myself and Joe S. think is great. So anyway, those .693 bore pistons had very little blisters in the ports. Now, will it play like the larger .734 bore does with more obstruction? Maybe not if you use the smaller tubing.
We've already done that experiment and oversize tubing with an adapter ferrule on each side of the piston, will indeed blow more freely. That most likely has something to do with resistance at solder joints and smaller bend crooks. Anyway, that explains why the Conn 52/56J has a nice open 4th valve.
Now, will the .693 valve section play as freely as the .734 bore if both use the same size tubing? I keep thinking I should take two .693 bore valve sections and a .734 to make two, 4 valve sections. Use 18 mm (.709) tubing in the 1st three valves and 18.5 mm (.728) bore tubing in the 4th. Well that's three or four projects away.
Matt