Tuesday, October 28, 2014

The Last Vacuum Tubes Just Got Put on the Endangered List

If you're "of a certain age", say above 45 as a rough guess, you probably know what vacuum tubes are even if you're not an electronics geek.  Chances are you remember the TV or radio warming up for 20 seconds before the program started.  If you were a child in the 60s, you might have even taken vacuum tubes from a TV set that wasn't working up to the store to test those tubes on a drugstore tube tester.  I actually still have two tube-based radios in the ham shack, and run them from time to time, although my "everyday" operation is on modern, feature-rich, solid state radios. Mrs. Graybeard has a prized collection of several tube-based Zenith Transoceanic radios. 

Solid state: transistors, silicon diodes, FETs, integrated circuits, and other devices, have been replacing vacuum tubes in electronics since at least the time of that first transistor radio I wrote about last week.  Today, the only places they're used is in industrial and military electronics that are both operating at extremely high frequency and extremely high powers.  While there are a couple of niche markets where vacuum tubes are still strongly used: high end audio and instrument (like guitar) amplifiers, and there's a very active vintage radio market, where they're referred to as "boatanchors".
If you're not active in those areas, I'd bet that the only vacuum tube in your house is the power amplifier in your microwave oven.  The only other one you're most likely to have is a CRT. 

Microwave ovens use a tube called a magnetron, an invention that dates back to WWII.  The ovens work in a frequency band close to the WiFI band at 2.45 GHz, and generate thousands of watts of power; your microwave is probably around 1000 Watts output.  On the other hand, aircraft have used solid state transmitters for Collision Avoidance Systems just below that frequency, around 1000 MHz, since the 1980s.  The last TCAS system I worked on generated 1000W and used just three power transistors.    

You probably have gathered where I'm going: semiconductor maker Freescale (formerly Motorola Semiconductor) has announced a line of transistors designed to produce solid state microwave ovens.
The product of interest is Freescale’s new MHT1003N, a 250 watt LDMOS transistor for 2.45 GHz that provides a power-added efficiency (PAE) of 58%.  ...  Using the MHT1003N, manufacturers can use from one to eight of these 250 W units to build a microwave oven with the desired power level.  And the magnetron’s 4 kV power supply goes away in place of a supply of 28 to 50 volts.  Furthermore, the crude on-off control of the magnetron can be replaced with full variable power control.  Using multiple antennas, one per amplifier, provides better coverage of the cooking chamber.  This allows food to be cooked more precisely while the unit operates more efficiently.  And the product lifetime is significantly greater.
LDMOS - Laterally Diffused Metal Oxide Semiconductors - are a type of silicon transistor construction first developed in the 1990s.  They're falling out of favor in the last couple of years, being replaced by GaN - Gallium Nitride - which offers better efficiencies and better microwave performance.   Changing the power supply from that 4000V supply to a 28 or 50V supply will take the largest weight in a microwave oven out; its power transformer.  Going solid state with several amplifiers, each running its own antenna promises better, more even cooking, better power control, fewer issues with hot spots and power loss as the oven ages as benefits to the user. 

You can be sure these will be quite a bit more expensive for the early adopters, but historically, it looks like the handwriting is on the wall for vacuum tube microwave ovens. 

I'm not sure it's actually on the market, but a company called Midea has previewed the first consumer microwave oven based on the Freescale transistors, back in June.    
Cutaway view of a magnetron - old RF guys call them a Maggie.  Wikipedia photo.


  1. I don't own any tube gear, but I do have a Zenith Transoceanic that doesn't work. I got it at an auction for $2, though I had to take some other crap along with it. :-)

    Do you know anywhere that might repair it?

  2. As long as vacuum tubes resist EMP etc. better than solid state circuitry they will still be around.

  3. Maggie isn't an amplifier, she is an oscillator. The difference:

    Amplifier: RF in, RF out
    Oscillator: DC in, RF out

    Maggie was a top secret device during WWII, protected against capture by explosive destruct charges. Some WWII surplus got released to the civilian market with destruct charges still in place.

  4. Since I am a radio know nothing, can you advise on a reliable portable ac and battery powered unit to receive multiple bands in the event of a major disruption? A SHTF radio, that will pick up shortwave , ham, etc?
    It would be nice if it could run off 12 dc too. I would love to see a post on this subject.....

  5. Anon, I have a little Techsun PL-660 that works just great.

    SiGB - I have a couple of the Zenith sets, too. They work great, but one of the tubes is going for a small fortune these day!

  6. Anon 9:56 raises an important point and I'm embarassed to have posted that slipup. Yes, Maggies are oscillators; in fact, a power oscillator.

    Power oscillators are a vanishing technology. Current circuit design doesn't use them for a variety of reasons, mostly regulatory.

  7. Anon 1257: you said, can you advise on a reliable portable ac and battery powered unit to receive multiple bands in the event of a major disruption? A SHTF radio, that will pick up shortwave , ham, etc? I don't know that radio that Dr. Jim mentioned (Techsun PL-660), so I'll look into it a little.

    If you've got $20 to play with, I have an AM/FM SW radio called a Kaito WRX911 which is extremely good. The battery life on a set of alkalines is incredible. Months and months on two AAs. Amazon has them

  8. I looked at that Techsun 660- it too is a Kaito. Sounds like a good unit- I have a couple hundred to invest in this- I just hate being out of contact when the power goes out- and If it ever goes out due to EMP, solar flare, domestic or foreign attack, likely the internet will go too.

  9. You can find a lot of reviews of receivers over at the eHam website:


  10. Thanks drjim. Great info.
    anon 1257

  11. Appreciate your blog a lot. FYI, the bigger microwave ovens, in the 100 kW class, are surprisingly efficient. Operators may expect about 85% efficiency in converting the HVDC into RF, at about 915 MHz. Naturally, in the midst of the 33 cm ham band. The big magnetrons are about 1 1/2 feet long and weigh about 40 lbs. The heater needs 12.6V at 115 amps to warm up. Anode requires 18 kV at 5 - 6 amps. RF flows through large waveguide equipped directional couplers to the cooking chambers, fed by conveyer belts. All beastly but effective.

  12. Anon 1807 10/31 - Thanks a lot!

    I've never heard of those systems. Beastly is a good word.

    What are they used for? Do they cook at 915?