There are a multitude of testers out there. 1st you want to buy one that tests for dynamic mutual conductance vs emission. The mutaul conductance is said to be more accurate. 2ndly make sure that the tester will test the tubes that you are working with. The older testers don't do nine pin tubes and some newer ones won't have 4 pin sockets. Make sure that the tube setting charts and manual are included. The B&K 700, 707 and 747 are some of the easiest to use. 3rd, even when the add sais that the tester works, it may not work correctly. I just picked up a B&K 747 that will not calibrate. The seller will not respond, so it's of the shop for repairs I guess. You may want to do a search in the Tube Asylum over at AudioAsylum. A lot of good advice is archived there. Ebay seems to have an endless supply of testers up for auction. Do an advanced search for "tube testers". There's always about a 100 on the list. Prices range from 10's of $ to over a 1000. I'm starting to wonder if it boils down to you get what you pay for. Take your time, and try to know what you want. Good luck, Bob D