Memories...

If you can identify the device at right, you're (a) at least as old as I am, and (b) probably a geek.

It's a “tube tester” – a device used in prehistoric times to test vacuum tubes. Don't know what vacuum tubes are? Back in ancient times, before transistors were invented, vacuum tubes (“valves” in the U.K.) were the active devices in electronic equipment. There were audio amplifiers, radios, televisions, and even computers that depended on vacuum tubes to function.

Unlike transistors, vacuum tubes were not very reliable. They'd break (usually because the “heater” – an electric heating element – would fail). They'd also change their characteristics for various reasons, causing the equipment they were in to malfunction. Because of this propensity to fail, vacuum tubes were nearly always mounted in sockets; the glass bulb of the vacuum tube had pins sticking out that would plug into these sockets.

But how would you know if you had a bad tube? If the heater was broken, you'd think it would be easy: the tube wouldn't get hot. Unfortunately, manufacturers often used a trick (to save money) that made this not work: all the tubes in a device would have their heaters wired in series, so if one of them broke, all the tubes would go cold (much like some Christmas light sets, where if one light goes out, they all go out). If something else was wrong with the tube, though, there wasn't any possibility of an easy diagnosis. The only thing you could do in such a situation was to pull all the tubes out of your radio or TV, and traipse down to the nearest store that had a vacuum tube tester.

In my early days as an electronics hobbyist (in the '60s), I used to collect broken TVs from the TV repair shops. Those shops were common – TVs broke a lot, and they were expensive; repairing them actually made economic sense. But frequently one of the older TVs would be beyond repair, and the repair shops would give them to me for free. They were a gold mine of valuable parts for me – I'd carefully tear them apart, removing all the individual components, straightening our their leads, and testing them. Except for the vacuum tubes, as I couldn't afford a vacuum tube tester. So every few weeks I'd head down to a local electronics shop that had a vacuum tube tester, with a bushel basket full of vacuum tubes I'd pulled out of dead TVs. I'd stand in front of the tester for a few hours, trying one tube after another, sorting them into “good” and “bad” piles. I got very familiar with these beasts!

Later, in the U.S. Navy, I worked on several older pieces of equipment that had lots and lots of vacuum tubes – and of course they had a vacuum tube tester on board the ship. Most of my fellow technicians had never even seen one of these things, so my expertise came in handy.

The vacuum tube testers were marvels of ingenuity, able to set up the proper heater voltages, bias voltages, and signal voltages for thousands of different types of tubes – all with switches and individual components. The designers of these machines had to be puzzle solvers of the first order to figure out how to do all this with so few components to work with (they were constrained both by the space required and the cost)...