Say Good Bye to the Last Vacuum Tube Product

by Lou Frenzel
Oct 20, 2014

Now another major vacuum tube product seems poised to fade away: the microwave oven.

Vacuum tubes disappeared from electronic products years ago.  Yet there have been some lingering vacuum tube-based products in production.  The last major vacuum tube retirement was the long-lasting cathode ray tube (CRT).  In the late 1990s and early 2000s, the CRT finally gave way to the LCD/LED and other flat panel displays in video monitors and TV sets.  Now another major vacuum tube product seems poised to fade away:  the microwave oven.

The main component in a microwave oven is a magnetron, a high power microwave vacuum tube oscillator.  It operates at 2.45 GHz. It radiates the food heating and cooking it.  The magnetron was invented during World War II for radar.  Magnetrons can easily generate thousands of watts of power.  Electrons from a filament-heated cathode are attracted by a high voltage anode but a surrounding magnet modifies the electron paths thereby exciting cavity resonators tuned to the desired frequency producing a continuous wave (CW) signal.  That signal is coupled to an antenna that directs the energy according to the application.

The magnetron was the component of choice for radar but also RF heating applications.  Industrial uses came first but in the 1970s, several companies offered “radar ranges” for consumer cooking.  They rapidly became popular because they reduced cooking time and made cooking more convenient.  Today virtually every kitchen has a built in microwave as well as a conventional oven.  If not, a counter top microwave oven is usually present.  It is a must-have consumer appliance that we all take for granted today.

Despite the wide microwave acceptance, they do have their problems.  They often cook unevenly and typically lose power over time as the magnetron filament and cathode deteriorate.  At last it is possible to replace the magnetron with solid state devices.

Up until now there were few if any transistors that could crank out hundreds and certainly not thousands of watts or power at microwave frequencies.  That has changed in the past years with advances in GaN technology and advanced silicon LDMOS.  Today Freescale Semiconductor is offering devices that will let appliance makers produce even better 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%.  Another device targeting RF heating applications is the MHT1002N for 915 MHz that can deliver 350 watts at 63% PAE.  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.

Freescale is also offering an RF Power Tool System, a development platform of hardware, software and documentation that gives inexperienced RF appliance developers all the tools to create a product fast and easy.  For more details on all this, go to: www.freescale.com/RFheating.

Maybe the magnetron is really not the last vacuum tube product, but close to it.  There are still some other microwave tubes around like the klystrons in some satellite ground stations and  the traveling wave tubes (TWTs) in satellites.  The new GaN transistors will probably replace some TWTs in future satellites but klystrons will be tough to replace.

It will be a while before all the magnetron microwave ovens disappear just as it is taking years for CRT monitors and TV sets to fade away.  But you can look forward to the future where your popcorn will be more precisely popped in a solid state oven.

Discuss this Blog Entry 10

on Oct 22, 2014

Vacuum tubes are alive and well in audio. Tube guitar amps are popular, as are home amps for audiophiles. Tubes also remain common in high power RF amplifiers, although solid state versions are also available.

Perhaps this was more about applications where vacuum tubes were the only option. That would be more accurate.

on Oct 22, 2014

I, for one, do not miss working on vacuum tube electronics - especially the scraped knuckles from the jerk when inadvertently touching something with plate voltage on it. And, of course, the "g...d...mn son of a b....ch!" that inevitably followed.

on Oct 22, 2014

Old song we'll be hearing for a long time yet. The Freescale and NXP product do have real advantages for some applications, but remember you can buy a complete 1KW microwave oven at retail for $50; we are a long way from matching that with LDMOS. The TWT market is still in the hundreds of millions of dollars, though GaN is gradually breaking into that business too. Vacuum tubes will eventually go away from most applications just because there are so few designers who understand them, but for very high voltage/high power applications they are still surprisingly cost effective.

on Oct 22, 2014

There are also some SCARY SERIOUS tubes in the power grid ...
http://en.wikipedia.org/wiki/Mercury-arc_valve
"Solids" are eating their domain slowly away but I think we are with them for some time to come...

on Oct 22, 2014

Vacuum tubes are still common in dental and medical practice in X-ray machines. Of course, it is possible to make X-rays using a femto-second LASER, but it will be a while before the price comes down to vacuum-tube levels. In the meanwhile, field-emission carbon nanotube cathode vacuum tubes are being developed now for high radiation environments, terahertz amplification and other uses, e.g. http://escholarship.org/uc/item/3rd0g181.
It ain't over yet for da tube. ;-)

on Oct 22, 2014

Nothing thus far can reproduce the transfer characteristics of a well designed vacuum tube audio system. Improvements in capacitor and resistor component construction have moved tube design very close to the solid state measured signal to noise curves. Regulated power supply designs have extended tube life.
Hearing tests are a wonderful benefit to the recording industry assisting in the restoration of quality audio recording for an entirely new generation before subsonic energy of the generic bass 'thump' in audio systems can destroy another young person's auditory system, never hearing real transient audio.

on Oct 23, 2014

While it's true that great advances have been made in products that do things that tubes do well, many of the top offerings in musical amplification utilize vacuum tubes. These are in demand by all age groups.

on Oct 27, 2014

High Power vacuum tubes are alive and well in the particle accelerator world. LANL's LANCE machine uses Megawatt klystrons and they're working with Thales on the Diacrode - https://www.thalesgroup.com/en/canada/security/diacrodesr-particle-accel...

on Oct 29, 2014

Let's not forget that a couple of decades ago some people laughed when they saw that a MIG flown in by a defecting pilot still had vacuum tubes in it, until someone remembered that tubes survive an EMP better than solid state devices. When the Big One comes it may be only the really old-time Hams that still have working rigs. Anyone know how to bypass the ECU in your car to get it running again? Is it even possible?

on Jan 30, 2015

I have a very small computer desk that also doubles as my working and listening area, and it is constantly full of all kind of gears and never enough in space. Early last week after receiving the iTube, I had the whole iFi chain setup on the desk for the iTube review and thought that I would have to disassemble them quickly in the next few days in order to reclaim the space back. The whole setup, as I am about to finish writing this review, is still sitting on the desk after almost 2 weeks. It could very well be staying there for a while longer, given how much music I have been enjoying recently over my Creative speaker - and to think that I am predominately an on-the-go IEM user. I guess this really sum up the iTube magic thingy. Sometime it is the seamlessly little stuffs that add up to make the whole experience worthwhile.

Please log in or register to post comments.