What is this thing?

[Reece]

This set has the filaments connected in series. In a dark room if you can see the dim glow of any of the 1-volt tubes, then all the filaments are OK, which is where to start. Another check would be to do a continuity test on the filament pins of the tubes that won't go in the tube tester.

[Jeffhs]

One note of caution: If you use an ohmmeter to check continuity of tube filaments in battery radios, many of which used 1- or three-volt tubes, you are taking a very real risk of burning out the filaments in the blink of an eye. Many older VOMs/VTVMs (vacuum tube voltmeters) had one or more batteries used only on the resistance ranges; if there is more than one such battery in the meter you are using, it may well blow the filaments long before you realize any damage has been done. I don't know whether or not today's digital multimeters use these batteries, but if they do, the same thing can happen if the meter is used to test tube filaments. Six- and 12-volt tubes in AC-powered radios, however, can be tested using a modern DMM if you don't have a tube tester.

I'm not sure why the battery radio tubes are so sensitive to burnout if tested using a standard ohmmeter. My best guess, however, is that these filaments are made of very thin wire which can be burned open before you know it if subjected to higher than normal voltages; this could go at least double for battery radio tubes that used the filament as the cathode, for almost instantaneous operation as soon as the radio is switched on.