Questions about Tube Testers
The tube is good, because the amplifier plays well on it.
Initially this may perhaps seem a simplified approach, but the answer is correct. Please let me explain why:
It's like when going to the doctor, and have your blood pressure tested, and expect from him, the more accurate his tester is, the more accurate he can tell how your health is. What is the use, knowing digitally tested, the number is 174.5, while the only thing that counts is, you are much too far over 140.
The same is with Tube Quality Testers. If a reliable, well build tester says, this tube is "just ok", you know a lot more, than when getting a numeric result, without saying if that is a good or a bad result. That would quickly unveil the quality of the tester as well, when so called good tubes don't work well, or vice versa. The lowest level with this, I have ever seen, is the Amplitrex AT1000, but this is not the subject here.
My favorite tube Quality Tester is the Funke W19. In the blue book, which belongs to it, are several pages written by Max Funke, about how he thinks quality testing should be done. He defines the test results in three areas. Good: Meaning this tube will work in every application for an expected long time. Bad: Meaning it should be replaced. And this curious "?" area, which he defines as follows: This tube shows signs of wear, but it does not necessarily need to be replaced yet. As long as the amplifier plays well on it, it can be considered a good tube. These are words by May Funke, who spend his entire life, developing Quality Testers, ending with the crown on his work: The W19S.