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This vacuum tube module was used in an IBM programmable calculator from 1948. This module, called a "trigger", stored one bit of information. Let's take a closer look and see the module operate.

This module was part of IBM's 604 Electronic Calculating Punch. It read 100 punch cards a minute, performed calculations that were specified by a wiring panel, and punched the results on cards. In the lower left, you can see rows of tube modules with their handles sticking out.

Before transistors, vacuum tubes were widely used as switches and ampifiers. A heater heats up the cathode, which emits electrons. They are attracted to the plate, causing current to flow. A negative voltage on the grid blocks the electrons, controlling the current flow.

The tube module uses a "dual triode" tube, two amplifiers in one glass tube. In the first photo, the two vertical dark rectangles are the two plates. The second photo shows the tube powered up, with the heaters glowing orange at the top and bottom.

I've powered up the trigger module in this video. It is switched on or off by two inputs and remembers its state until switched again. I push two buttons to turn it on or off, and its state is displayed by two vintage neon bulbs. This may seem trivial, but it is a key component of computers.

The IBM 604 calculator used about 1300 tubes and rented for $550 a month. It was a very popular system with over 5600 units produced. IBM advertised it as equivalent to 150 engineers.

For more information on the tube module, including circuit diagrams, see my latest article: www.righto.com/2026/07/ibm-...