Below is an explanation by Dave Werden for the 4th valve side placement on compensating euphoniums (from
http://www.tubaeuph.com/eu-articles-comp.cfm). It amounts to more than simply a weak pinky issue, although I admit I am just plain out more comfortable with the side mounted valve, an easy ergonomic fit. We could also debate the straight shift versus automatic transmission in the same light.
Besides the more solid sound and chromatic flexibility, I find compensating euphoniums to have a much more solid response. I find sixteenth note slurs to be a lot cleaner on a compensating euphoniums.
Ryan
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4th-Valve Placement :a "Side Issue"
While many manufacturers place the 4th valve immediately next to the 3rd, Hirsbrunner and Sterling-Perantucci place the 4th valve halfway down the right side of the instrument, intended to be played with the left hand. While this can be justified by simply noting the relative weakness of the 4th finger of the right hand, there is also a consideration relating to the compensating system.
Euphoniums are essentially conical-bore instruments. That is, their tubing is almost constantly expanding from the mouthpiece to the bell. The most notable exception to this is found within the 1st, 2nd & 3rd valves, where the bore size is constant. However, the separation between the 3rd and 4th valves allows the connecting tubing to expand gradually as it approaches and passes through the 4th valve's tubing, maintaining a more constant taper. For example, in the case of the Sterling-Perantucci euphonium the main bore (measured at the 2nd valve) is.592". At the compensating loops it has expanded to .630", and it expands to .670" within the remainder of the 4th valve tubing. By preserving a more conical bore through this area, freedom of response and consistency of tone are enhanced.