If you must buy a tube tester first learn about the features and capabilities of all the various Hickok testers. Then buy the best you can afford that has been refurbished, calibrated, and guaranteed by the seller. And keep in mind that most cannot properly test a power tube. For small signal tubes, they’re fine. I stipulate Hickok because they were the premier company and are still revered. Hence there are guys who can repair them, and parts are available. Also, they were simply the best. So far as I can tell, Hickok made the BK testers too. That’s if you’re going to buy vintage. If you want to spend big bucks and buy new, see Amplitron and others.