Return of the Vacuum Tube

sciencehabit writes "Peer inside an antique radio and you'll find what look like small light bulbs. They're actually vacuum tubes — the predecessors of the silicon transistor. Vacuum tubes went the way of the dinosaurs in the 1960s, but researchers have now brought them back to life, creating a nano-sized version that's faster and hardier than the transistor (abstract). It's even able to survive the harsh radiation of outer space."

Comments

Amps

[Aeros]

These are still widely used in some of the best amps out there.

Re:Amps

[jenningsthecat]

Yes, transistor audio amplifiers are "way more linear" with gobs of negative feedback applied, if THD, (Total Harmonic Distortion), is your measurement criterion. ANY amplifier is more 'linear' with correctly applied negative feedback. The basic premise is that added harmonics are bad - if you feed a pure sine wave into an amplifier, you want a pure sine wave at the output. The problem is that in audio, THD is a fundamentally flawed measurement with very poor correlation between lab measurements and listening tests.

THD measurements are taken as the ratio of the total power of all harmonics to the power of the fundamental, with no weighting of any kind applied. The trouble is, human hearing doesn't respond to harmonic distortions in this linear fashion - our ears find higher order harmonic distortions much more apparent and objectionable.. This deficiency was noted by prominent BBC engineers D.E.L. Shorter and Norman Crowhurst in the 40's and 50's, when they proposed weighting harmonics by the square or the cube of the order; but their voices were drowned out by market forces that wanted a simple, flattering figure of merit that made the newer, more powerful pentode-based amps, (with lots of negative feedback), look better on paper than their lower-powered triode predecessors. The market won out over scientific and technical accuracy, (it usually does), and today engineers the world over, ignorant of this history, mistakenly believe that low THD is the gold standard for measuring and defining audio amplifier quality. (For a good technical analysis of distortion and the sound of an amplifier, see Lynn Olson's excellent investigation.

By the way, in the 'tubes vs transistors' debate, good triodes have the advantage of being more intrinsically linear than transistors. This means that they require less negative feedback to tame their distortion, and often sound wonderful with NO negative feedback. The THD figures of amps built this way are often quite poor, but look at their spectra and you'll see predominantly second- and third-order, with a smooth and rapid falloff of higher order harmonics. Occasionally solid-state amps can give this kind of performance, but tubes have an easier time of it. Designing a good-sounding, (as opposed to good-measuring), audio amp, requires a lot of skill, and a lot of knowledge about distortion mechanisms and how to counter them. Unfortunately the prevailing practice in HiFi is to add more gain, throw most of it away with additional NFB, get a nice low THD figure, and call the job done. Amps designed this way generally sound like shit, if not initially, then after 20 minutes or so of listening, at which time listening fatigue sets in.

Re:Vacuum tubes have never left!

[Anonymous Coward]

Uh... Microwave ovens use a magnetron http://en.wikipedia.org/wiki/Microwave_oven#Design I've repaired many a Microwave ovens and I have never seen any vacuum tubes.

Too smart for your own good. A magnetron is a vacuum tube. Not all vacuum tubes are transparent. Hell, the "vacuum tube" in the article has neither a tube or a vacuum!

Re:Ahistoric Hyperbole Rant Warning

[scharkalvin]

Actually an open heater was NOT the way most tubes died. The coating on the cathode that emits electrons when heated gradually decays and emission drops off to the point that the tubes transconductance is too low for it to operate. But the heater rarely burns out, at least not in indirectly heated tubes. Another way they die is that air gradually leaks in and the vacuum becomes too poor. The silver flashing on the side of the tube will then turn a milky white as the chemical "getter" that absorbs air has absorbed all that it can. Once the getter coating is depleted the tube will become gassy. A tube can also die from shorts when closely spaced elements break loose from vibration and touch. Over heating will soften the elements and cause the same effect. Tubes can handle a much higher percent of overload than solid state devices however. Tubes computers were never faster than solid state ones even if the tubes themselves were faster. Because of their size the total wiring in a tube computer is much longer than in a solid state system. In transistors it is the "holes" in the crystal structure that "move" and the speed of light in silicon is lower than in a vacuum for electromagnetic waves. Still these waves have less distance to propergate in an IC than a bunch of interconnnected tubes. Finally note the description of this new tube technology, it is really a "vacuum state" IC. I always wondered when nanotechnology would be applied to thermionic "valves" (as they say across the "pond").