A simple good test to see if any tubes are microphonic in tube preamps or amps, is to turn the volume up to normal, have someone listen close at each speaker, and for you to gently flick each tube with your finger nail. The person listening at the speaker will tell you if they can hear that flick, bad ones will really stand out."
George. I would rather think Kevin Deal has the true skinny on tube microphony...
From Kevin’s website...
Microphony, tube dampers, and tapping on tubes -
Okay...listen closely class, because I will say this only once: DO NOT TAP ON TUBES! You can permanently damage them! Repeatedly tapping on a tube’s glass can cause a perfectly good tube to become too microphonic for use.
ALL tubes are microphonic to a greater or lesser degree. Whether or not that microphonics is audible will depend more on the tube’s function in the product, rather than the microphony of the tube itself. In some positions, you will never hear anything, even with the worst tube. In other positions you will hear something even with the best tubes. At that point you make a judgement call: does this level of microphony interfere with my listening enjoyment?
Tubes are supposed to make noise when tap you tap on them! If a tube is truly microphonic, and in a position where it is critical, you will know it. It will feedback as you play music and be so objectionable you’ll turn it off.
For those interested in a more in-depth discussion on microphony, click here.