I'm looking to buy a tube tester and would like to make it a bit future proof to my buying habits.  Before researching the testers, I never knew that 12 pin tubes existed.  I'm talking about 6KD6 compactron type tubes.  Does anybody here use them, design with them, etc.?  Has anybody even seen a piece of audio equipment that does use them?  I'm guessing that I won't ever need to test them, but would like to know more about them before I dismiss them.