| When it comes to getting a PVA (precision valve alignment) for your piston-style horns, there are a variety of technicians to choose from, each with his own approach. Different washer and shim materials are used and some even modify the instrument by machining key elements. The approach I prefer is to use synthetic washers that are unaffected by moisture and oil and hold their thickness over time. Also, my preference is NOT to alter or modify the horn by any machining. In this way, the horn can always be easily returned to a former setting by re-installing the original washers. It is my opinion that the single most important factor in obtaining a quality PVA lies with the technician performing it and how meticulous he tends to be. As long as he has the necessary equipment to see properly and uses quality materials, the result depends on how much of a perfectionist he is. Obviously, if he is busy or rushed, the result will probably be compromised. Finally, I think it’s safe to say that PVA’s are not “rocket science”. When it comes down to it, you are basically trying to align two surfaces and that can be done with visual observation and using the appropriate washers to match ports down to just a few thousandths of an inch. It’s not at all unusual to find even new horns with misalignments of 0.015 to 0.025” and, in some cases, even more. That much error will definitely affect response and reducing that error to just a few thousandths can noticeably affect intonation and response. Anyone who doubts this, just try slightly depressing all three valves and see for yourself. Horns have been designed with specific port sizes to optimize playing characteristics. It just makes good sense to align those ports to make sure you’re getting the maximum result from the intended design. |