This is a Mercury rectifier, a very heavy one. Switch on and off the tester only with a tube inserted. You should heat up the rectifier first. For this, switch off the tester, and remove the plugs marked 500V in the white field. Then with the rectifier inserted, switch on again and heat up until the glass becomes clear again. This can take some minutes. Then switch off, insert both plugs again, and switch on. This is the rectifier test. There is no need to let it run very long. Switch off after 60 seconds, and let the tester cool down 2 minutes before switch on. As long as the glass is hot, the tube needs no pre-heating.

During each test for the 83: NEVER plug or unplug something with the tester on. That means not the 83 tube, and not the plugs. Note, at the plugs metal and some sockets and Anode caps connectrors, can be high voltage!