The valves — again — do not function all that differently.
The differences between "pistons and rotors" are much more a factor of how the horns are
shaped.
In the past, it would have been safe to say that rotary tubas used a "Germanic" design language (not "German"), with rotary valves being what could be purchased via the local cottage industry.
To use Rotary valves, the maker needs to have space to stack a series of them in a long, skinny row. Rotor casings, being made individually and joined by a ferrule using soft solder, are not only much "fatter" than a piston, but are also spaced out much more than pistons, because that ferrule must have adequate length on each knuckle for the solder to make a strong bond.
Pistons, on the other hand, are very tall and thin, and the traditional way of joining them into a set is to braze them together permanently. Each piston has a different internal layout. Usually, there are two sets of measurements, and the 3rd is the 1st that has been flipped around. (The knuckles are rotated to the needed positions at the time of assembly.)
The reason for this is
strength, as the distance between the valves has to be tight. You cannot create a strong bond between two tubes that add up to the length of the knuckle between two pistons using a ferrule and soft solder.
Here is why this has to be: Pistons are operated by your fingers DIRECTLY. Rotary valves are actuated indirectly. THEREFORE…
Rotary valves can have a super-fat OD and can be spaced apart as far as needed, BECAUSE THE PLAYER DOES NOT HAVE TO PUT HIS FINGERS ON THE VALVES.
Look at a rotary tuba. Notice that the linkage system works to compress the space between the rotor stems to something closer to the human hand in span, then the levers do this again. You end up with a middle schooler's hand being able to easily manipulate four valves that are about eight inches apart.
OKAY… SO WHAT DOES ALL THIS HAVE TO DO WITH THE TOPIC AT HAND?
Rotary tuba bugles developed as they did for a few reasons.
• A tuba must fit in an average-sized adult male's lap (the builder's main demographic) while giving easy access to the mouthpiece and the valves.
• A tuba's bugle must have adequate physical space to install a block of valves that also minimizes any damage to the acoustics of the straight bugle.
• The bugle must have bends at locations where nodes and antinodes are least affected by 180º bends. (Tight bends in tubing can be seen by the sound wave as a slight expansion of the tube's bore size and can drive pitches that have nodal points inside that curve somewhat flat.)
Because of this, piston and rotary tubas' bugles are shaped differently to accommodate their valves.
Next…
Because front/side piston valves take up very little room horizontally and vertically when compared with rotors — but require a great deal of DEPTH to install them (with access to the rear caps) — they end up in the center of the wrap where there is a big donut hole for them to live in. FA piston tubas tend to be rounder in shape (shorter/wider) when compared with Germanic rotary tubas.
Rotary tubas require a long, narrow "slot" in the bugle to install a stack of rotors (with access to the rear caps). Therefore, most older rotary horns are designed with bugles that are tall and narrow.
FA piston tubas have the entrance to the valves very close to the player's face, with the 1st port being angled toward the player, so they naturally have a
very short leadpipe (anywhere between 14" and 20" being pretty common). If you have a natural-ish taper rate, this gives you middle-zized valves, the two most common sizes for FA tubas (with notable exceptions) being .687"/.689" and .750", which are not large at all.
Rotary tubas usually have the valves set over to the far side of the bugle with the leadpipe entering from straight above the valves or (on many very old BBb tubas) from 45º
away from the player, with the leadpipe being close to three feet in length! If you follow a sensible taper rate, this sets the valves much farther along that taper and requires the builder to use LARGE valves.
Because (for many years) rotary valve makers were limited in the upper end of bore size by then-available tooling, they could not make rotors above a certain bore size. So the taper between the receiver and the first rotor in the stack had to be pretty moderate, so when you got out to the bell, the throat was fairly narrow, and a narrow flare was used.
FA piston tubas, with very little room between the receiver and 1st valve, ended up with slightly faster tapers, and had another foot and a half for the taper to develop after exiting the valves. These horns ended up both more round AND volumetrically "fatter" with sometimes stupidly large sousaphone flares on the end.
Add to all this (something so important to design that I am mentioning AGAIN) the fact that
pistons are operated directly and rotors have a transmission system, and you can see that the valves themselves have little to nothing to do with how a horn plays. It is all these other MUCH MORE IMPORTANT factors that create the differences some of you think you are feeling due to "pistons or rotors". It is not the different valves you are sensing. It is that these two different types of valves REQUIRE two very different types of tubas.
IT IS THE TUBAS THEMSELVES THAT DIFFER.
Even when you have horns available with piston and rotor versions (PT-6, Neptune, etc.), it is more due to the compromises that must be made to effect that massive change than the type of valves themselves. There are some key differences between the bugles of the PT6 and the PT6-P that no one seems prepared to acknowledge. Nope. It is the VALVES causing all those "piston versus rotors" differences, baby; the valves.
Yep. That must be it.
