TubeNet

Here is a hypothetical question. How much would it cost to add a 4th piston valve to an old King or Conn tuba that currently has only 3?

Hank74

Here is a hypothetical question as part of a hypothetical answer; why would you even care what kind of valve it was as long as it was the proper bore and worked OK?

I did that with a big old Conn a couple of years ago:
http://www.galvanizedjazz.com/tuba/BigConn.html

It took a lot of time and patience and a well-stocked junk parts bin. The result was playable but rather Rube-Goldbergish.

This past year I put a rotary 4th valve onto a small Conn:


It also took a lot of patience but I like the way it plays.
Details:
http://www.galvanizedjazz.com/tuba/frugalhorn.html

This kind of project really has to be a labor of love, like building your own boat. You could never afford to pay someone to do it.

If you are prepared to spend big bucks on a 4th valve project I recommend buying an entire 4-valve section new, from Mirafone or Meinl-Weston. They make valves now that are better in every way than any of the old ones. I have done that with three different tubas and am very happy with all of them. (Those horns are not for sale!) Example: