[chriswood1900]

Rich
David's advice is spot on for a quick check, what you have is a basic emissions tester very much like the Heathkit series. By the time the 600C was introduced I believe the parts were from Japan. There is nothing wrong with emissions testers as long as you remember that they were designed for servicemen to quickly screen out bad valves, the readings as you have discovered are relative between good and bad valves but are not comparable with readings from AVO or Hickock types testers and as such you don't need to worry about anything other than line adjust and filament voltages.
Good luck and have fun checking your valves.