All 12AX7 type tubes are microphonic. Some are just more so than others. You can check your pre-amp tubes by turning the amp on, gaining it up and tapping lightly on each tube with the end of a pencil or a chop stick (my favorite). You will be able to hear the tapping through your speakers, which is normal. It is not normal for a tube to ring like a bell after itís tapped. If it does ring then itís what is called microphonic and should be replaced.
In the case of very high gain amps like the Peavey 5150 and Ultra or the Mesaís and Marshalls you will find that by simply swapping the pre-amp tubes around you can find a combination which will yield the least amount of noise.
Power tubes on the other hand are rarely microphonic. EL84ís seem to be the most susceptible to being microphonic and having filament rattle. The worst combination of course is a combo amp using EL84ís especially some of the high powered oneís. The speaker vibration can rattle those little tubes to death in short order. All power tubes can be checked for being microphonic just like pre-amp tubes.This text was copied, with permission, from www.eurotubes.com.
What does potting do to my pickup anyway? The term "potting" refers to the sealing of the coils in a solid material. Potting stabilizes the components of the pickup so that they cannot move relative to each other. This elminates vibration-induced signals that make a pickup microphonic. Wax is used because it works well, is inexpensive, and it makes it possible to work on the pickup later.
The bottom line? If a pickup is microphonic it needs to be potted. Of course, we can't recommend potting the pickups on a valuable vintage guitar because doing so will decrease its collector value.
For more information check www.guitarnuts.com. Parts of the their text was copied.