I continue to be puzzled by repeated complaints (regarding various instruments) concerning "valve noise". First, this should not be terribly difficult to diagnose. And then, second, it should not be difficult to fix -- unless there's something really fundamentally wrong with the instrument that's causing it, and then you need to know that.
Piston valve noise is almost always caused in some way or another by (1) Felts (wrong size/thickness, or worn, or possibly "too hard"), (2) Valve guides or channels (worn, poorly made, or dirty), or (3) springs. Otherwise, something that SEEMS to be valve noise may not be valve noise at all -- e.g., a broken or partly broken solder joint somewhere in the instrument, or a loose "solder blob" inside the instrument. But these things can be found and addressed.
In the case of springs, the Chinese instruments in my experience just don't have very good springs. Hey, if you can't see it, who cares, right? And you may wonder by what magic a genuine Yamaha euphonium has so much quieter springs than your Chinese clone. You think it might be because the Yamaha springs are plastic coated? Then get some Yamaha springs and throw them into your euph, eh?
If the problem is something like valve guides and channels and it turns out that this is because the channels were machined a bit too wide and the result is valve clatter, then yeah, you're probably stuck -- because while this is theoretically fixable, it's not worth going there. However, I haven't seen that in my Chinese instruments.
One problem with serious valve noise I had was on my (duh!) 1924 Buescher tuba that still had the original (brass) valve guides. Clatter, clatter, clatter ...! But it didn't take much effort to take those (almost 100 year old) guides out, true up the channel tops a bit, drill and tap the valve guide holes, and put plastic guides in. The result was silent valves. Actually, with the plastic valve guides and the Yamaha springs in it, the valves on that ancient horn are quieter than those on either my Mack Brass euph or Wessex tuba (both of which are very quiet).
The other serious problem I have is with the rotary valves on my Amati oval euph. Terrible clattering. This is because the linkages are worn, and the only solution to that short of replacing the entire linkage (not going to happen!) is to drill out the bushings and valve arms and put in slightly larger pivot screws. Sometime ... maybe this coming winter.
But my point is that USUALLY valve noise is eminently fixable, and MOST OFTEN in the case of PISTON valves it simply involves first FINDING the problem and then applying the very simple fix -- like buying plastic coated valve springs (or maybe some felts), which will typically cost you around $20.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)