Joe, all tubes will do that when you tap on them. Don't do it anymore. It is not good for them. The sound you heard is microphonics and all tubes are subject to various levels of it. This generally is not heard when the source is muted, although if it's bad it may pick up vibrations from walking across the floor or stuff like that. Usually, when microphonics cause a problem, you get a howling feedback noise. If it is just minor, you may get a reverb like effect in the sound. The hum is probably caused by something else, like a bad ground. Some tubes types are more microphonic than others, and preamp tubes are a bad place to have microphonics, since the signal and the microphonics are then amplified many times by the rest of the signal chain. The tubes are probably Russian or Chinese, so if you get the urge to change them, you would be better off getting NOS tubes. They sound better. But they can be microphonic too. Specify to the tube vendor that you need low microphonics for a preamp application. They can usually accommodate this request. If you post the tube numbers on this forum, some knowledgable member can make some good recommendations to you regarding the specific tubes. Again, you should look at other causes for your hum problem.
