gionvil wrote:here is the picture of my 5th valve lever... Any suggestion?
This is how I would do it.
5th-valve-linkage-holton.jpg
It will provide an arc of movement on the button that will match your thumb, and the shorter the crank length between the button and the hinge with respect to the crank length between the hinge and the ball joint, the less the throw. And the the ball link needs to be in line with the movement of the stop arm to keep it light. It is the hinge points that determine the mechanics, so bend the bits any way you need to clear your hands.
With the rotary valve as configured, however, I think the hinge must be below the valve and not above it to work as you want with the minimum of complexity.
This approach will happily use the same ball link and hinge as what you already have. All you'll need is some brass square stock and a bit of soldering. It will be easier to construct by taking the leg from the hinge to the ball link, and instead of butt-brazing it to the hinge, use a piece of square stock bent at 90 degrees, so that it runs along the rod from the button to the hinge. Then you can just solder it to that rod. That would be very strong and stiff.
Bob could have made this easier by clocking the stop arm and bumper plate 90 degrees clockwise from its current position, but that would require fabricating a new stop arm and drilling/tapping new holes on the top of the rotary valve casing. What complicates this is that the path of the stop-arm connection is at right angles to the path of the button.
Rick "who would require a thumb ring for this tuba" Denney