Tube testers are a quick and easy way to test large batches of tubes. The good/bad cutoff point is somewhat arbitrary because different tube testers use different operating points for tests and test different things. Tube checkers, emission testers and transconductance testers all do different types of tests. And between testers that do the same test, different models use different standards for operating points and go/no-go conditions. We test the tubes before we put them in kits with a TV-10 B/U, which is a tester that the military used a lot. The bottom line is that how the tube measures and how it sounds in the amp circuit is what matters. If one snuck by us and you got a bad tube that doesn't measure right in the preamp, we will replace it. But you don't know if you have a good or bad tube until you build the kit, test it in circuit and listen to it. Sometimes a tube just needs to run for a while to form the cathode so that it emits electrons at its full capacity. And sometimes a tube can measure perfectly new but have noise problems, or die within the first ten hours of operation. That's just life with vacuum tubes.
