The Octopus tester (or VI curve tracer) is a very simple but extremely useful device for repairs of analog circuits. It comes in many varieties... as an add-on to your oscillocope, with a built-in CRT, with LCD displays or selfmade with a few components.
It is so easy to use that we call it "Tubelitester" (Swiss German: Idiot tester). It's so simple that everyone can use it...