Leaving the Light On: Vacuum Tubes and their Reemergence

Audio-intensive music enthusiasts, or “audiophiles”, swear by that analog signal produces a warm and smooth sound that is much preferred over the cold and uninviting tones produced by digital signal.
Some audiophiles have gone so far as to return to classic 1940’s electronic device: the vacuum tube. A small device that you might see glowing through the vents of old radios and TV’s, the vacuum tube, due to its vacuum-tight speed, is truly a valuable design. In fact, it is making a comeback to assist national organizations, such as NASA, with an improved computational speed and reduced space radiation.

As Thomas Edison was tampering with light bulbs in 1883, he discovered that an electric current could travel from end to end of a vacuum with as much ease as through a wire. This phenomena has since been named the “Edison Effect” [1]. As revolutionary as this discovery was, it did not enter common use until 1904, when John A. Fleming told Guglielmo Marconi (the inventor of the radio) he had “found a method of rectifying electrical oscillations” [1]. Soon after, Fleming’s vacuum tube diode forced current (flow of electrons) to flow in strictly one direction, thus converting alternating current (AC) to direct current (DC) – a massive achievement [1]. The manipulation of current would not only improve public transit, clean air ventilation, and electroplating, but also drive steel conveyor motors to better manufacture steel and aluminum parts during World War II [2].

In addition to industrial applications, folks at home found alternative uses for vacuum tube amplification – radio, television, telephones, guitar amps [Fig. 1], and anything else that needed an amplified signal became household staples. Such tube-based amplification led to the crucial role of vacuum tubes playing a crucial role in the origin, reinvention, and broadcasting of music genres such as blues, jazz, country, and rock n’ roll [3]. However, despite the vacuum tube’s popularity they remained difficult to manufacture and consumed energy inefficiently. An alternative was imminent, and came in the form of the solid-state transistor, which managed to replace many, if not all, vacuum tube applications.

By 1950, growing technological developments became increasingly demanding in both performance and volume; vacuum tubes could not sustain high frequencies and could not be manufactured quickly. This provoked scientists to revisit an invention first created in 1874: Ferdinand Braun’s “solid-state rectifier”. Through expansive research and implementation, this rectifier eventually led to today’s solid-state transistor [5]. This transistor proved to be a deadly foe for the vacuum tube because of its lower price and increased bandwidth. However, the vacuum tube had a chance at revival through its unique benefits. Current rectification is being reimagined to implement not only the compact size of the transistor, but also the vacuum-tight speed of its vacuum predecessor, which could contribute major benefits to all fields of engineering.

As previously mentioned, the vacuum tube stems off of Edison’s discovery of electrons’ ability to traverse void chambers at fast speeds. So what makes those electrons want to traverse? Commonly, at the starting end of the vacuum tube there is a cathode – a negatively charged electrical conductor that electrons emit from when heated (also known as “thermionic emission” [6]). On the other end is an anode – a device with a positive charge that attracts and catches these negatively-charged electrons. If this anode is hooked up to an electrical device, like a motor, and the neighboring cathode is hooked up to a battery, then the vacuum tube acts as a wall between the device and its power source until the tube’s cathode is heated by a separate power supply. By heating the cathode, the electrons jump from the cathode, zoom through the vacuum, reach the anode, and ultimately connect the battery to its respective device [2].

Essentially, heating the cathode of the vacuum tube is like lowering a bridge at rush hour – the cathode is work, the anode is home, the electrons are the cars, and heating the tube is like lowering the bridge. As a result, the current going from the battery to the motor strictly moves in one direction – nothing feeds back to the battery, thus making a direct current (DC). This merely explains vacuum tube rectification of a signal; however, tube amplification is what this device is best known for today (radio, music, etc.). In order to make the vacuum tube amplify a signal, a positively charged molybdenum grid is placed between the cathode and anode [Fig. 2]. This grid is connected to a weak voltage, like a signal from a radio tower. When the signal is negative, the flow of electrons is lessened, when it is positive, the flow is increased [8]. By combining the positive charges from the anode and this grid, the electrons are encouraged, even more so, to exit the cathode and head for the anode when heated. This results in faster electrons as well as an amplified grid signal. [Papers regarding more technical descriptions of the inner workings of the vacuum tube can be found in electrical engineering university library textbooks dating back to 1940, such as Fundamentals of Vacuum Tubes (1941) by A.V. Eastman.]

When the solid-state transistor came into the picture, it offered to perform these same tasks by manipulating and exciting silicon, a semiconducting material. These transistors can accomplish the same tasks for a cheaper price, less power, as well as at higher frequencies [9]. However, with this smaller price tag came less speed. The electrons in these solid-state transistors risk collision with other particles, thus making the electrons’ paths not as clear or fast as they could be. NASA plans to clear up these paths by using the vacuum tube.

An article posted on the NASA – Ames Research Center (ARC) website, “Nanoscale Vacuum Electronics: Back to the Future?”, accepts vacuum tubes’ decade-old fate at the hands of the solid-state transistor, but touts the device’s relevance. Meyya Meyyappan, the principle investigator for this vacuum tube revamp, aims to “combine the best vacuum tubes and conventional silicon semiconductor processing to create nanoscale vacuum tubes” [9]. In the end, ARC aims to combine the best of both worlds – the “vacuum-channel transistor” [9] that combines the high speeds of a vacuum tube and the lower power consumption of a solid-state transistor on the nanoscale [9]. Through a smaller, more efficient, vacuum-based design, the device may even operateat terahertz frequencies, sustain high heat and radiation, and compete with semiconductor prices [6]. All in all, this device removes the biggest drawbacks of the traditional vacuum tube by shrinking its size and eliminating warmup time (the biggest power hog).

To accomplish this, NASA’s vacuum-channel transistor eliminates the cathode (the electron-emitting component). If the tube is small enough, the electric field across the tube is enough to draw electrons from the attached power source without warming up the cathode. As per traditional solid-state transistors [Fig. 3], the gap between the source and drain is bridged by a silicon dioxide gate which, when electrified, allows current to flow about the gate. By applying this same gate concept, adding a thin oxide layer to insulate the gate, then having the tips of the source and drain separated by nothing but a vacuum, the movement of the current from the source to the drain speeds up. This method eliminates cathode heating and non-vacuum barriers, thus making the device more efficient.

Once such a device is functioning, it will be able to operate in the terahertz region. Such a frequency range would allow for vast amounts of research and development to be done in the areas of high-speed communication and hazardous materials sensing [6].

Clearly, NASA is pushing for scientific revolution and innovation for the vacuum tube, but how relevant will the vacuum tube be in the years to come? Will it be common again? Can it gain popularity on its own without NASA-level development? Jerry C. Whitaker of the Society of Motion Picture & Television Engineers Committee [11] has shared insight on the realism of a tube comeback:

“​Although receiving tubes have more-or-less disappeared from the scene, power tubes are alive and well and are performing vital functions in thousands of divergent applications…. Despite the inroads made by solid-state technology, the power vacuum tube occupies – and will continue to occupy – an important role in the generation of high-power radio frequency energy in the high-frequency regions and above. No other device can do the job as well [12].Jerry C. Whitaker

Even if the hardware may be outdated, the analog signal tubes make is still relevant to many. Jon Lloyd of Juno Records, for instance, has commented on the tube’s comforting nostalgia, saying, “People will buy a Kindle for convenience, but people will still want to have a bookshelf” [13]. Even if NASA’s designs are not successful, nostalgia may keep vacuum tubes relevant for decades to come.