This is a deeper discussion for those that want to learn more about how tubes should be tested. If you are new to tubes, don’t read more into this than there is to read. In other words don’t worry about it. If you are buying your tubes from Upscale Audio, you don’t even need to read this because we've already taken care of it for you... Go listen to music and relax.
First, let's get one thing out of the way: All small signal tubes (12AT7, 12AU7, 12AX7, 6922, etc) are microphonic. The amount of microphony varies by tube, and how that microphony affects the performance of your gear depends on how that tube is being used in the circuit.
This is especially true in guitar amps that are called "combo amps," where the speaker is mounted within inches of the tubes. In this situation, an overly-microphonic tube will pick up the vibrations from the speaker. This vibration is then amplified and the end result can sound like distortion and other unpleasant sounds.
Tubes can have easy or hard jobs. If the tube is used to amplify a signal, then it is expected to make noise if tapped on. If it is used in other positions, such as an output buffer stage, then it may make no noise no matter how hard you tap on it. That’s why you should never tap on tubes. It can damage them, and you learn nothing about the tube's performance by doing so.
Let us turn to page 107 of the Bible for tube junkies: The RCA Radiotron Designers Handbook: Fourth Edition.
"There is no published standard test for microphony outside of Service or manufacturers' specifications."
It goes on to say:
"In radio receiver factories the valves are usually tested in the chassis in which they are to operate, the set being tuned to a signal generator with internal modulation, and the volume control set at maximum. Any microphonic sounds which quickly die away are not considered as cause for removal of the valve, but a sustained howl is so considered. Such a valve may not be microphonic in another position in the chassis or in another model of receiver."
"Not withstanding the considerations above, all valves cannot be expected to be non-microphonic when placed directly in the acoustic field of, and close to, the loudspeaker in a receiver, particularly in a small cabinet."
It’s like speed ratings on tires. A more basic tire will have a rating of S, a tire designed for higher speed and performance will be H or V, and the most expensive and highest performance rating being Z.
So it is with tubes. The message here is that at Upscale Audio, we test most small signal tubes one by one in circuit using our own process. We then grade and sort them based on their level of microphonics. This makes it easy to find the right tube to fit the job necessary for your gear.
Can a tube be too low in microphony? Sometimes a little bit of microphony can be a good thing! In my opinion, tubes that have a smidge of microphony can lead to a sense of “air” or depth in soundstage. Using tube dampers can decrease, or even eliminate microphonics, but it can also remove that air and depth. Some folks don't mind that, others do. If you have tube dampers, try using your tubes with them, then listen to the same tracks without, and make a judgement call... there's no right or wrong answer here.