Mention vacuum tubes in consumer products and it sounds like a time-machine statement; didn’t they go out with disco and bell-bottom pants? Hard to believe but true: you most likely do have a vacuum tube in your home. No, I’m not talking about that now-obsolete glass CRT TV you may have tucked away somewhere even as you replaced it with a flat-screen TV. Nor do I mean those high-end audio amplifiers for audiophiles who feel that vacuum tubes give better sound, as they are only a very small segment of the population.
Instead, I am referring to the resonant cavity magnetron in your microwave oven (see a “how it works” here), Figure 1 . Yes, it’s true, there’s one of these ancient devices in that oven, generating the moderate-power energy at 2.45 GHz that excites the water molecules though resonance and absorption, to heat them and the food.
Why stay with a magnetron, when a large fraction of commercial and industrial RF energy systems of comparable power have gone solid state via LDMOS and GaN technologies. An interesting article “RF Energy Systems: Realizing New Applications” in the December 2015 issue of Microwave Journal (free, but registration required) explains the situation in detail. The gating items are no surprise: performance and price. According to the author, the initial goal is to have a 2.45-GHz solid-state amplifier with output power of 300 W and efficiency of at least 70% and at a cost of $12; the next goal is higher power in the 600-to-1000 W range.
Changing over from the magnetron to a solid-state design is not simple drop-in form/fit/function replacement of one component with another. There are many electromechanical and RF considerations to be understood and resolved, as well as issues of control, reliability, and other factors. Even if you don’t care whether you low-cost home microwave oven has a magnetron or a solid-state source, it’s interesting to see what such a switchover really entails along so many perspectives. Recently, Ampleon (the 2015 spinoff of NXP Semiconductors) and Midea (a Chinese consumer-products company) announced a 200-W oven, the result of a year-long collaboration into the use of solid-state technology for compact oven design (see here for press release), but details are vague.
Should we miss the magnetron, when it's time to go finally arrives? Probably not, this is the nature of progress. But perhaps we should at acknowledge its history and role. The 10-cm (what we now usually refer to as 3 GHz) cavity magnetron was initially developed in the UK during WWII, and hand-carried as a “top secret” to the US for design refinement, system integration, and volume production. (If you are interested in the history of the magnetron and radar's role in WWII, check out this online IEEE article “The Cavity Magnetron: Not Just a British Invention” or Robert Buderi's very readable book “The Invention that Changed the World” (Figure 2 ); of course, there are many other histories available.) It was this highly guarded key component which enabled the construction of high-resolution compact radar, which made such a huge difference to the war effort—especially radar sets which weighed “only” a few hundred pounds and thus could be fitted into aircraft.
When the magnetron is replaced, that will pretty much eliminate the last vacuum tube from the typical home, but it doesn't mean that vacuum tubes are totally gone from our industry. They are still used in high-power broadcast stations, of course, although solid-state devices are now standard for transmitters into the tens of kW. A specialized tube called the traveling wave tube (TWT) is still the dominant amplifier used in communications satellites due to its combination of electrical and mechanical performance and reliability/ruggedness characteristics (thermal and radiation). Of course, vacuum tubes are still the only source of many of the specialized particle and energy beams used in high-energy physics.
There's still some serious vacuum tube work being done according to some recent articles in IEEE Spectrum. “The Quest for the Ultimate Vacuum Tube,” explored the fascinating R&D work being done now to enhance the TWT and leverage some of the latest solid-state techniques to improve it further, while “Introducing the Vacuum Transistor: A Device Made of Nothing” looked at merging the vacuum tube and the MOSFET.
Have you ever worked with vacuum tubes? Do you miss their ethereal, primitive warm glow or inherent ruggedness? What do you most like or dislike about them?