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Dear repair Gurus: How difficult and expensive would it be to turn the above tuba into a 4 valve? 3+1 or 4 valve front action? I realize a 3+1 would not be compensating or that a 4 valve front action would require a new or harvested valve and slide set. Could that be done with a purchased removable set? Thanks.

Of your options, it would be easier to add a large 4th valve after the main tuning slide. Unless you have a very responsible brass tech, you may be better off selling your current horn to buy an Eb that already has 4 valves. If you're attached to your 3 valve King, it would cost anywhere from $500 to $1,000 to add a valve. This is including finding a valve, acquiring appropriate length tubing, and putting it in a good spot with good bracing. Already having a valve helps with cost. Where are you located?

Wouldn't be inconceivable to put rotors after the MTS...might as well add 2 if you do it, because 4 valves won't get you many notes. maybe it would have to be progressive to catch up with expanding bore using common sizes like .748/.787/.795/.826

now how you link them up to actually be reachable, would take some wizardry.