Those who ought to know say that many youngsters today cannot name the first three U.S. presidents, and so they condemn both the lack of interest on the part of the kids and the inability of their teachers to excite their interest.

In this same vein, I wonder about the knowledge of young engineers about the history of our own profession.

Consider the once ubiquitous vacuum tube. How many under-40 engineers are aware of the origins of the thermionic vacuum tube, or of its importance during its halcyon years? Some may even believe that aside from a few special-purpose tubes (magnetrons, klystrons, photomultipliers, and CRTs for example), tubes are no longer manufactured and are found only in museums.

Fortunately, “tubescence” (an interest in the study of vacuum tubes, their history, and the circuits in which they were used) is still with us. Tubes, of course, were at the heart of the circuitry that launched the electronics field — amplifiers, oscillators, detectors, phase inverters, multivibrators, waveform generators, and relay circuits among them.
Even today, vacuum tubes are being manufactured in several countries, including China, Russia, Slovakia, Yugoslavia, the Czech Republic, and Ukraine. Among the most popular types still made are the 12AX7 and the 6L6. By some estimates, audio tubes may be a $500 million industry. And there are several manufacturers of tube amplifiers, many of them guitar amplifiers. While the usual measurements of frequency response, noise and distortion show no differences between solid state and tube amplifiers, many audio engineers, audiophiles, and musicians insist that tube amplifiers sound better, possibly due to the way they handle transients.

To meet the demand from audiophiles, in 1998 Westrex Corporation reissued the 300B triode, an audio amplifier that Western Electric had produced from 1938 to 1988. Westrex made the reissues in the same Kansas City plant where the originals had been manufactured. One could purchase a mahogany-boxed pair of the new 300Bs, branded “Western Electric, Made in USA,” for $800!

The Famous Five

It may well be that the longest-lived consumer electronic product was the five-tube AC/DC transformerless superheterodyne radio receiver. It was well known to the radio technicians of the day (as well as to today’s tubescent crowd) as the All American Five, or AA5, so named because it used only five tubes. Their heaters were connected in series and drew 150 mA when connected directly to line voltage. The AA5’s manufacture by all major radio set makers began in 1941 and continued well after World War II. Radio repairmen were familiar with the octal-based tube lineup: 12SA7 (mixer/oscillator), 12SK7 (i-f amplifier), 12SQ7 (detector and first audio amplifier), 50L6 (audio output), and 35Z5 (rectifier). Customers learned the lineup, too. On those rare occasions when their set failed to play, they put all five in a paper bag and sequenced them through the local drug store’s do-it-yourself tube checker to isolate the culprit. A seven-pin miniature glass tube version was later designed, and the AA5 principle went on to be used in several black-and-white and some inexpensive color TV sets, the latter as recently as the 1980s.

An Achilles’ Heel

Ironically, while heat is the needed requirement for tube operation, it is also its primary nemesis. Electrons are emitted from a tungsten or thoriated-tungsten filament, or from a barium oxide/strontium oxide coated cathode. The tungsten filament must be heated to about 2,000^o C; the heated cathode surface to almost 1,000^o C. Aside from serving its purpose of generating free electrons, heat is of no further value and can only be detrimental to tube life and costly to dissipate. It is no surprise that only one ENIAC computer was built (it had 18,000 tubes that needed to be cooled!). Transmitting tubes create so much heat that the larger ones require sophisticated water or forced-air cooling.

When tubes would experience low emission, “depleted” cathodes were often blamed, but the failure was more likely due to a poisoning of the cathode surface caused by migration of impurities from other parts of the tube structure. Tubes are also subject to burnout (open filaments or heaters), a failure mechanism aggravated by switching them on and off. To alleviate this problem, the 2,000-tube Colossus computer was never switched off.

Everyone in the Act

History buffs take delight in cataloguing the brand names of vacuum tubes. Tube museum curator Patrick Dowd once compiled a list of 406 brand names of the popular 201/201A tube. Some of the names might not have inspired great customer confidence, as, for example, Wizard, Double Life, Unitron-No-Bee, By Heck, and Good Luck. In truth, even at the height of the vacuum tube era, there were relatively few large-volume manufacturers of receiving tubes. Small job shops could make a few tubes on simple equipment provided they could obtain the necessary tube parts, but serious manufacturers like RCA would require expensive automatic equipment to seal and exhaust tubes at a high rate of production. Many tube marketers would purchase tubes in quantity from major makers having specified they be branded with their own trade names. Others would acquire the rejects (mostly low emission tubes) of major makers and reclaim them, usually by “hotshotting” them (operating the heater briefly at about 150 percent of its rated voltage while drawing about twice the rated current from the cathode), then rebrand and sell them to dealers and radio repair shops. Final testing of the reclaimed tubes was a simple matter. In what was popularly called the “light and play” test, the tube was plugged into an ac/dc set. If the tube lit and the set played, the tube was good.

Long Live Tubescence

The days when the tube reigned supreme are long gone. Yet many of us whose careers bridged the years during which semiconductors nudged tubes to the side occasionally enjoy immersing ourselves in the history and lore of those exciting times. And, sometimes, just sometimes, we imagine ourselves relaxing, cigar and cognac in hand, basking in the warm glow of a pair of 6L6s that are pushing and pulling to the strains of Beethoven or the beat of Basie.

Resources

• K. R. Spangenberg, Vacuum Tubes, McGraw-Hill, 1948.

• K. Henney, Radio Engineering Handbook, 5th Edition, McGraw-Hill, 1959.

• W. H. Kohl, Materials and Techniques for Electron Tubes, Reinhold, 1960.

• R. Tomer, Vacuum Tubes, Howard Sams, 1960.

• R. G. Kloeffler, Electron Tubes, John Wiley, 1966.

• R. O. Hamm, “Tubes vs. Transistors: Is There an Audible Difference?” Journal of the Audio Engineering Society, May 1973.

• G. F. J. Tyne, Saga of the Vacuum Tube, Howard Sams, 1977.

• The OTB (The Old Timer’s Bulletin) Official Journal of the Antique Wireless Association. See the column “The Vacuum Tube” in each issue.

Collections and Exhibits

• The Antique Wireless Association Historical Museum, East Bloomfield, NY.

• Vacuum Tube History Exhibit, the Engineering Library, Manhattan College, New York City.

• L. A. Sibley, “Visiting a Tube Collection,” The OTB, May 2004, The Antique Wireless Association.

 

E-mail this page to a friend Tell us what you thought of this article