TubeNet

What is the average cost of adding a fourth valve to a horn?

Almost 20 years ago, I had a 5th valve + tubing made for my Eb tuba. It is a detachable 5th valve + tubing assembly. I think it cost about $1,500 since it was custom work. I'd expect a 4th valve + tubing custom added to a tuba would cost at least that much today

Adding a 4th piston to a 3 valve piston block doesn't sound like much fun. It CAN be done, but it typically seems like the hobbyist/repairmen take on these jobs for themselves, instead of doing or offering this work for customers. I could be wrong, though.

You didn't say whether you are thinking about pistons or rotary. In my opinion it is easiest to add a rotary valve because its input and output are co-linear. There are quite a few piston-valve CC tubas on the market with a rotary 5th valve placed downstream from the main tuning slide. Since the bore there is larger that is an ideal place to put a 4th valve onto a 3-piston tuba. I have done it to several tubas, but only as a ham-fisted shade-tree tuba mechanic. I don't even try to make them look good, I just make them play reasonably well. Here's a pretty clear description of my first one:

http://www.galvanizedjazz.com/tuba/frugalhorn.html

It didn't cost me much because I used junk parts and my own unskilled labor. To have it done properly (by a skilled person who is trying to earn a living) with a new valve, new tubing and a custom linkage would run into big bucks. There are several well-qualified people on the Tubenet who could do it right, but I doubt if any of them would quote even a ballpark estimate before seeing the instrument.

probably better to just transplant entire 4v set. If a .656 or .687 bore will work then there's a remote chance it is worth doing. If larger bore required it's just going to be hard and expensive to find. (well unless Chynese or Taiwanese or top action or rotary)