Sorlowski, what you are experiencing is simply part of life with tubes. As you've discovered, the microphonics is due to the tube, not the equipment. And, it can be variable from tube to tube. The cause is the internal construction of the tube: the parts inside the tube can vibrate and when they vibrate, noise will pass into the audio circuit of your equipment (depending on where the tube is in the circuit). The noise is the result of mechanical vibration, it's not electrical. And it's not necessarily a construction quality issue. But it can certainly be variable from tube to tube coming off the same production line depending on the tolerances of the contruction process (which some would argue IS a quality issue, but of a different sort).
It is possible to test for tubes that will be less microphonic, but it requires specialized equipment.
Generally, the answer is DON'T TAP ON THE TUBE! ;-)
The fact that a given tube is microphonic does not, of itself, mean that there is something wrong with the tube. If you don't hear noise during normal operation of your equipment, it's not a problem.
Also, there are all sorts of after-market tweaks for trying to reduce the internal mechanical noise of tubes. That becomes a whole additional universe of things to try to see if any of it makes an audible difference in your particular system. Some will.
Hope this summary helps a bit.
.
It is possible to test for tubes that will be less microphonic, but it requires specialized equipment.
Generally, the answer is DON'T TAP ON THE TUBE! ;-)
The fact that a given tube is microphonic does not, of itself, mean that there is something wrong with the tube. If you don't hear noise during normal operation of your equipment, it's not a problem.
Also, there are all sorts of after-market tweaks for trying to reduce the internal mechanical noise of tubes. That becomes a whole additional universe of things to try to see if any of it makes an audible difference in your particular system. Some will.
Hope this summary helps a bit.
.