This has happened to most of us at some point or another. You come upon a tube with no markings whatsoever mixed in with a bunch of miscellaneous tubes. Or, like I just did, you get a “lot” of tubes from a reseller and most are marked but a few are wiped clean. But how does one go about positively identifying the mystery tube? Especially with the numerous types of 7 and 9 pin miniature tubes in existence this can seem to be a rather daunting task. However, just like a game of “20 Questions”, it is actually simpler than one might think.