This tube supposedly can not be tested with this tester, becuase the UX4 sockets has no way to connect bothh anodes of a rectifier. Yet we found a way for that :) An unuseual way, but you just need to have this ideay. If the Russians would have known, they sure would have made a card for the 5C3, as this is "the" tube that was rebased by Svetlana and sold as 5U4G rectifier. This card has no adjustable settings, complete set up is by the plugs only. So you can make no error, while testing this tube.