by Bob Osmun
We've all heard it: the annoying clatter of a set of noisy valves. The sound can range from merely annoying in a casual playing situation to downright disastrous, in a recital or close-miked recording session. What causes this noise, and what can be done to prevent or eliminate it?
Causes of noise in valves
There are several possible sources for the noise you hear. Loose stop plate or stop arm retaining screws are one common source. Badly mis-aligned valves caused by worn, broken, or missing bumpers that allow the stop arm to hit the stop plate or bent linkages that hit the stop arm are among the possible culprits, as are worn mechanical linkages. Another very-hard-to-diagnose source of noise is unsoldered indexing pins on Paxman and King horns. All these are possible, but the most common source of noisy valves is end play.
End Play: What is it?
End play is excess clearance in a valve assembly that allows the rotor to move vertically in its casing. When you press a valve lever it puts pressure on the rotor shaft, which responds by turning. When it has turned as far as it can it still needs to respond to the pressure and it does so by moving up in the casing. When the rotor moves upward as far as it can it smacks into the thrust bearing on the bearing plate, producing a click. If you have a horn with noisy valves you can demonstrate this by simply pushing up on the end of the rotor shaft.
Where does it come from?
At one time, especially here in the US, makers were incredibly casual about end play. Many horns arrived brand new with significant vertical movement in the valves and no one thought much about it. As better made European instruments started to become more widely available people started to realize that noisy valves were not something they had to put up with and American makers responded with much better fit valves. Another source of end play, obviously, is wear. Especially when valves are not properly cared for mechanical wear can cause significant vertical play in only a few years. The longer this goes uncorrected the more the problem is exacerbated. One or two thousandths of an inch can cause problems; older instruments with ten or twelve thousandths are not uncommon. End play is adjusted by hand in the assembly process. It is a ticklish adjustment that requires skill. Some manufacturers (including some very good ones) build instruments with too little end play. The bearing plate, if seated all the way down into the casing, will cause the valve to bind. When the assembler puts the valves together he will tap the end of the rotor shaft to push the bearing plate out enough so that the valve will turn. Someone skilled and careful can do this so that the valves remain tight but in the hands of someone less skilled and well trained, like many of the repairmen out there, it can be a recipe for disaster. To me, one of the most annoying sources of end play is caused by careless manufacture of the top cap. As we heard earlier, when the rotor reaches the end of its rotation it tends to push up in the casing. This upward force, repeated thousands of times, tends to push the bearing plate up in the casing, causing end play. The purpose of the top cap is to hold down the bearing plate. If the threads are too long the cap will rest on the outside of the casing rather than the plate. This creates a gap that allows the bearing plate to move up.
How can it be corrected?
The first thing to do it try some oil on it. Many horns are allowed to run completely dry. The best oil to use is a bearing oil specifically made for musical instruments. (At Osmun, we use Hetman lubes.) If oiling doesn't help, or doesn't help for more than a day or two, sterner measures are required End play is corrected by machining off the underside of the bearing plate where it sits on the casing. This moves the thrust bearing lower in the casing and takes up the excess play. If the cap doesn't hold down the plate the threads are shortened till the cap sits on the bearing plate instead of the outside of the casing. Some people will try to adjust the end play by inserting shims. We find this method unsatisfactory because it's only an approximate adjustment and shims can fall off, get lost, etc. We believe firmly that the instrument should be set up so that it can be taken apart and put together by someone with no special skill without any problems. Once properly adjusted a set of valves that is reasonably well maintained should retain their adjustment for years. So, if your valves have any problems get them fixed and then keep them oiled. We correct end play as part of a Complete Valve Service or a Rotary Valve Rebuild.
(Thanks to Gebr. Alexander for the fantastic valve pictures)