A friend was at a yard sale and bought a facsimile antique wall telephone that was really an AM tube radio. The radio did not work. I used a simple way to test the tubes and fixed the problem.
Step 1: Tube Circuitry
Until the late 1970s many drug stores and electronics shops sold electron tubes. Those outside the USA know them as valves. Each store selling tubes usually had a tube testing machine with several dozen tube sockets and various charts to find the right data for your tube.
Below is the electrical diagram for a typical tube. The filament heats up when current flows through it. When heated by the filament, the cathode emits electrons that are attracted by charge to the anode. Various grids insert signal charges, often fluctuating, to restrict or speed the flow of electrons to the anode.
Although there are a number of things that can go wrong in an electron tube, one of the most common is that the filament burns out.
Step 2: Testing for a Burned Out Filament
As you look at the bottom of a tube, you see a pattern of pins. There is a wider gap between two of the pins. My friend's radio used a couple of different types of tubes, but one was a 50C5, which is a very common tube. The first two pins (red circles) go to the filament. I used my ohmmeter to check all of the tubes for two pins with a few ohms resistance between them, as would be expected in a filament. In one tube, there was no resistance between any two pins, only open circuits. That suggested this was a faulty tube. I happened to have a couple of old tube radios I was using for parts. I scavenged a 50C5 tube from one of them and my friend's radio worked again. She was very pleased, whether she ever plays that radio, or not.