For most electrical engineers, the vacuum tube or, more formally, the vacuum electron device (VED), is a quaint curiosity and artifact of the past, and with good reason: The world of solid-state transistors and ICs is where most electronics is these days. Advanced versions and extensions of Lee DeForest's 1906 Audion triode -- the first electronic analog amplifier -- were critical to making our industry the powerhouse it became. Look at the products of the first half of the 20th century: It's astonishing what skilled scientists, engineers, and manufacturers accomplished using these devices. Even the classic five-tube AM radio, which brought wireless to millions, is an outstanding example of design, cost-effectiveness, and performance.
Various 12AU7 (ECC82) Vacuum Tubes / Valves (Courtesy of Diy Audio Projects.com)
Though no one today is going to build a VED-based computer or cellphone, there are still plenty of VEDs in widespread use. Start with the ubiquitous microwave oven in almost every home: It has a 2.4 GHz, 1 kW magnetron (a typical unit's maximum power) to excite the water molecules in the food.
But the use of VEDs goes well beyond that single mass-market application. I was surprised to read a summary of a report “Microwave and Millimeter Wave High-Power Vacuum Electron Devices: Changes Are Looming on the Horizon” from ABI Research, which claims the annual market for specialized VEDs for military/aerospace markets is over $1 billion annually. Depending on where you sit with respect to the active-electronics components business, that's either a big chunk of money or too small to really think about. The key sentence from the summary is this: "These tubes remain essential elements in specialized military, scientific/medical, and space communications applications."
Reality is that if you need tens of RF watts, at frequencies in the GHz range and higher, or kilowatts for your TV broadcast transmitter in the 100-to-500 MHz range, you're looking at a VED solution. If you are working with microwave and millimeter waves and need high power, you don’t have much choice -- at least, not yet.
But what about the future of VEDs? Predictions about the future are always risky, as any look back at predictions made in the past shows. Still, the report notes that devices based on gallium nitride (GaN) may make significant inroads over the next few years, judging from products that are available now from many vendors or in the immediate pipeline. Perhaps that magnetron in your microwave oven will go GaN solid state before too long?
Nonetheless, for the foreseeable future (which is an unknowable period in the high-tech world), VEDs are here to stay in one form or another, with their wonderful names such as klystron, magnetron, traveling wave tube (TWT), crossed-field amplifier, to cite just a few of their many versions. I just hope that as their use declines, we don’t lose the manufacturing "secret sauce" it takes to make these complex marvels of glass, metal, and ceramic, which can calmly produce and dissipate kilowatts and more.
Are there any other products you know of that we might naïvely assume are "obsolete" (such as the electromechanical relay) but which are actually still going strong?