im about to buy a tube, maybe second hand for my mic and outboard. wondering what questions do I ask the seller? they also say they will supply a test result of the tube. what am i looking for in these test results and what tests must i ask for to know if the tube is good. there seems to be tubes of the same name like telefunken that are made in different countries. what brand and what country do i look for in buying a good one and what models and country do i avoid. how long do tubes last for unused? where do these second hand sellers, specially ebay find these tubes if they are not being manufactured anymore?