You may remember the days when you tested your own vacuum tubes at the local department store tube tester. Reminds me of my first part-time job.
Okay, so I will be the first to admit this article may pose some cultural challenges for younger readers. Many questions, like: What is a department store? What is a tube and why would I want to test one? All good, historical questions.
Until the mid 1970’s, you see, most home entertainment equipment ran on vacuum tubes. These were precursors to transistors with some major differences. Namely size ( they were much larger), voltage (much higher) and, as mechanical devices, much less reliable. Compared with other components such as resistors, coils, capacitors and wires, tubes were the most likely source of failure.
So, back in the day, when your television, home radio or hi-fi stopped working, your first suspect was a bad tube. Compared to modern transistors and integrated circuits, tubes sat in pin sockets on a chassis. They were quite easy to remove and replace.
In high school, I worked at the customer service desk at the radio and television section of Eaton’s Department Store in Winnipeg. Next to our counter was the department store tube tester, similar to the B&K Dyna-Jet Model 707 shown above. Every day, dozens of customers would come in with tubes to be tested and ask for my help.
Of course, we stocked replacement tubes, which typically sold for around two dollars. At the time, a tube tester like this needed to test may 2,000 different tubes of different sizes and capabilities. We stocked maybe the most popular 100 tubes, like the 12AV7 double triode.
Department Store Tube Tester – How It Worked
First, you looked the tube up on a list, which told you several things. Which socket to use. And how to set up the configuration switches on the tube tester. After that, you just needed to push one or two test buttons. Properly configured, the big meter on the case told you the condition of the tube.
Since most tubes are basically amplifiers, you tested mutual conductance. In other words, how much grid signal voltage caused how much change in plate current across a load. Old time hams will know what I mean. There were other tests for gas, leakage and shorts, as well.
Amazingly, devices like the B&K could test 99% of tubes on the North American market. Take a look at one in action