Buy them from a reputable dealer. Try Brent Jesse Recording. He has an ample supply of tubes. He sells new, NOS and used tubes. They are rigorously tested and he guarantees them. He has a very user friendly website.
I will not buy used tubes from a seller that won't stand behind the products they sell. I have been burned by well intentioned sellers on eBay and Audiogon. They sing all kinds of praise about their tubes. Two weeks later one tube fails. you give them a call and they try to blame you or your equipment. BTW, I don't work for and am not related to Brent Jesse Recording. Good Luck.
I will not buy used tubes from a seller that won't stand behind the products they sell. I have been burned by well intentioned sellers on eBay and Audiogon. They sing all kinds of praise about their tubes. Two weeks later one tube fails. you give them a call and they try to blame you or your equipment. BTW, I don't work for and am not related to Brent Jesse Recording. Good Luck.