Surprisingly, transistors have not made vacuum tubes obsolete in all applications. Here's a look at the jobs that only tubes can do.

{Scientific American cover}

Where Tubes Rule

by Michael J. Riezenman

In a solid-state world, vacuum tubes are still secure within some very interesting strongholds.

"Passion" and "cult" aren't words one normally associates with electrical engineering, but they do come into play the minute tubes are mentioned.

Vacuum tubes, that is to say. Audio tubes to be precise.

For the uninitiated, which is you almost certainly if you're younger than 40 years of age, vacuum tubes were the active electronic devices used by primitive peoples before transistors and integrated circuits were invented. In fact, in the early part of this century, the very word "electronics" referred to a branch of physics connected with the behavior of electrons in a vacuum.

Devised in 1904 by John Ambrose Fleming, the first tubes (or valves, as the British call them) were simple diodes, which permitted electric current to flow in only one direction. Electronics really took off around 1912, when Edwin Howard Armstrong figured out how to build useful amplifier and oscillator circuits with the audion tube, invented six years earlier by Lee De Forest. By inserting an electrode known as a grid between the diode's other two electrodes, known as the cathode and anode, De Forest created a controllable device in which small changes in the voltage on the grid resulted in larger changes in the current flowing between the cathode and the anode. Such a three-electrode tube is called a triode.

Although the evidence today seems to suggest that De Forest had only a slight appreciation of what he had wrought, after much experimentation, Armstrong did. In a seminal moment in electronics history, he coupled the tube's output circuit back to its input to boost its feeble gain, thereby inventing the positive feedback circuit.

Over time, thousands of different tubes were developed, from subminiature devices the size of a cigarette filter to the hefty units still used in high-power radio transmitter, radar, and industrial heating equipment. In addition to triodes, engineers came up with tetrodes, pentodes and other tubes with multiple-grid electrodes.

Small receiving tubes, of the kind found in tabletop radios by the milions between about 1920 and 1960, have now been almost completely replaced by transistors, which seem to last forever. They require neither high voltages nor warm-up time, lend themselves to real miniaturization and use far less power.

Pleasure and Passion

So pervasive have transistors become that few people today even think about tubes in the context of home audio equipment. There exists, however, a small but passionate minority that believes that the best transistor-based amplifiers cannot re-create a piece of music as pleasingly as can a properly designed amplifier built around vacuum triodes. "Pleasing," of course, is a subjective word, and that is where the passion comes in.

As explained by Kevin M. Hayes, founder and president of Valve Amplification Company in Durham, N.C., a manufacturer of tube based audio amplifiers, the case for tubes begins with the realization that industry standard laboratory measurements of amplifier performance do not adequately answer fundamentally subjective questions such as, "Is this amplifier better than that one?" The problem, he says, is that the workings of the ear and brain are not understood well enough to identify the necessary and sufficient set of measurements for answering the question.

Back in the 1930s and 1940s, tota1 harmonic distortion became a widely accepted parameter for describing amplifier imperfections. All amplifiers create spurious, ostensibly unwanted signals at frequencies that are some whole-number multiple of the signal being amplified. Thus, a second-order harmonic distortion consists of stray signals at exactly twice the frequency of the amplified signal. Because all amplifiers of that era were based on tubes with similar kinds of nonlinearities, they all tended to generate harmonic distortion of the same kind, and a single number representing the total harmonic distortion was a valid tool for comparing them. It correlated well with the subjective listening experience.

Those tube amplifiers generated mainly second-order harmonics plus small amounts of other low-order even harmonics (fourth, sixth and so on). Second-order harmonic distortion, for that matter, is difficult for a human being to detect. Moreover, what can be heard tends to sound pleasant.

Transistor amplifiers, in contrast, generate higher-order harmonics (ninth, tenth, eleventh and so on), which are much easier to hear. Worse, the odd-order ones sound bad. So it is possible to have a transistor amplifier whose total harmonic distortion - as measured by laboratory instruments - is significantly lower than that of a comparable tube amplifier but that nonetheless sounds worse. To make a long story short, total harmonic distortion is not a particularly good way to compare amplifiers based on fundamentally different technology, and it is not clear what is - other than listelling to them, of course.

The debate can get quite heated - and not a little confusing - because the performance of an amplifier depends as much on the details of its circuit design as on its principal active devices (tubes or transistors). For example, using the feedback configuration in an amplifier circuit can reduce total distortion levels, but at a price: an increased percentage of those easily perceived, higher-order harmonics. According to Hayes, transistor amplifiers need more feedback than amplifiers based on vacuum triodes, which he believes to be the most optimal audio-amplifying devices. (Hayes's favorite triode is the 300B, developed at Western Electric in 1935.)

Tube Strongholds

Then there are the cultists who not only prefer tube-based audio equipment but insist that single-ended amplifiers are superior to push-pull units. In the latter, pairs of output tubes are arranged in a circuit that tends to cancel even-order distortion. Single-ended outputs, lacking that cancellation, can have as much as 15 percent total harmonic distortion, mostly second order. Though readily detectable, the effect is not unpleasant, tending to add richness and fullness to the reproduced sound. According to Hayes, it was used deliberately by manufacturers to improve the tinny sound quality of 1940s radios.

Fraught as it is with human and technical interest, the controversial audio market is a relatively tiny part of the tube business, which is far larger than most people imagine. Tubes are still playing a major role in high-frequency, high-power applications. In general, at almost any frequency there is a power level above which it makes more sense to use a tube rather than an array of transistors as the final power amplifier.

Microwave ovens area good case in point. They need to put out a few hundred watts at two or three gigahertz, a requirement easily satisfied by a kind of tube known as a magnetron, which costs about $18 to $25 in quantity. These microwave oven magnetrons descended from those used since the earliest days of radar, during World War II. (Remarking on the importance of radar during the war, Winston Churchill once described an early magnetron as the most valuable cargo ever to cross the Atlantic Ocean.)

Radio transmitters are another point of interest in tube country. In building power amplifiers for AM radio transmitters, where the goal is to generate 25 or 50 kilowatts at one or two megahertz, the tendency today is to go solid-state. For ultrahigh-frequency transmitters, which operate above 300 megahertz, tubes still reign supreme.

State-of-the-art communications satellites, too, are typically tube equipped. Intelsat - the international consortium that operates about half the world's commercial communications satellites - employs both solid-state and tube-based power amplifiers in its Series VIII satellites, which are just now going into service. Their Ku-band transmitters, which work at around 12 gigahertz and generate fairly narrow "spot" beams of ground coverage, use amplifiers based on so-called traveling wave tubes. The lower-frequency C-band transmitters operate at about five gigahertz and use both technologies, depending on how much power they are intended to deliver. Below about 40 watts, they use arrays of gallium arsenide field-effect transistors. Above that level, it's traveling wave tubes.

Although predicting the future is a notoriously tricky business, especially when it comes to electrotechnology, it seems safe to say that tubes will be with us for a long time. Undoubtedly, the frequency and power levels that can be handled by solid-state amplifiers will keep climbing. But the infinite space above them will almost certainly remain tube territory.

Michael J. Riezenman is senior engineering editor of IEEE Spectrum magazine. During the 1960s, as an electrical engineer with ITT Defense Communications, he designed control circuitry for use in communications satellites.

Reprinted from Scientific American, Special Issue/1997, all rights reserved.

Note: Red text emphasis and bold type fonts added by VAC to highlight key points.