It does what it says - it tells you the cathode current of your power tubes, allowing you to set your amp's bias properly. You still have to know how to set your amp's bias! The Bias King Pro just gives you a handy, safe way to read each tube's current draw. Well made, seems very robust. The coiled cords are old telephone hand-set cords, so they'll last forever. The octal sockets are very sturdy. If you're paying someone to reset your bias you'll save a bunch of money in the long run with one of these units. If you haven't been resetting your bias at all then you're probably not getting the best your amp can give. Highly recommended piece of gear for the guitar amp lover.