Hi Tom..yup give it a go..not sure on ur TV-7DU unless the tube you tried to check cudda been a dud or one side gonzo??..here BTW are two more from the 8CJ family..6386 and 7861..b/4 you do check them please take a moment and compare the two basings 407A and 8CJ just so you will see the almost identical diagrams of these two..it'll assure you I think..the only oddball thing on the 407A is that one tube manual lists the fil voltage as 20v AND 40v..??..I'm thinking misprint or maybe two together in a series string??
Anyhoo the other 6 I have say 20v filament/heater..let us know if and how it works and if ur tester is like mine in any respect at least the test switch to check gain is fused and spring loaded..to eliminate AH OH!! 73 de DAVE
The filament connections are a little odd. When you power the filament between pin 1 and 9, it needs 40 Volts. When you tie 1 and 9 together and power between these pins and pin 5 (CT of the filament), you need 20 Volts