The amplifier itself is the best tube-tester. If a tube fails to bias correctly in situ in the amplifier, it should be replaced.
Unless you want to spend thousands of bucks on a separate tube tester that can actually test power tubes at their typical voltage and current, you don't need a tube tester at all. This was mentioned earlier. The vintage tube testers, many of which are collected by strange people who collect tube testers, were generally NOT able to test power tubes at meaningful levels of voltage and current. Of those vintage testers, Hickok is the brand worth having, because Hickok testers along with only a few other brands can test transconductance. Testers ARE nice to have around for small signal tubes, used in the front end of amplifiers and in preamplifiers/phono stages. But even that is a bit frivolous. I do own a Hickok Model 533A, one of the older models probably from the post-WW2 era. I do use it to check transconductance (Gm), which is a sign of the life stage of a tube. If Gm is below the accepted range for that tube, you can ditch it with a clear conscience. I paid $75 for the 533A. The grandson of the 533 is the 539, one of the most sought after. A fully reconditioned 539 in top operating condition will cost you more than $1000, easy. 539s come a little closer to being able to test power tubes properly but not really close enough. I would bet almost no one here besides me owns any tube tester. I bought it for my own interest in DIY.
Unless you want to spend thousands of bucks on a separate tube tester that can actually test power tubes at their typical voltage and current, you don't need a tube tester at all. This was mentioned earlier. The vintage tube testers, many of which are collected by strange people who collect tube testers, were generally NOT able to test power tubes at meaningful levels of voltage and current. Of those vintage testers, Hickok is the brand worth having, because Hickok testers along with only a few other brands can test transconductance. Testers ARE nice to have around for small signal tubes, used in the front end of amplifiers and in preamplifiers/phono stages. But even that is a bit frivolous. I do own a Hickok Model 533A, one of the older models probably from the post-WW2 era. I do use it to check transconductance (Gm), which is a sign of the life stage of a tube. If Gm is below the accepted range for that tube, you can ditch it with a clear conscience. I paid $75 for the 533A. The grandson of the 533 is the 539, one of the most sought after. A fully reconditioned 539 in top operating condition will cost you more than $1000, easy. 539s come a little closer to being able to test power tubes properly but not really close enough. I would bet almost no one here besides me owns any tube tester. I bought it for my own interest in DIY.