The vacuum tube (or thermionic valve in British usage) is probably the greatest invention of the 20th century. It heralded the beginning of the information age, an event as significant as the invention of the steam engine that brought humankind into the industrial age (with all its benefits and pitfalls). It made possible for the first time the amplification of electrical signals, which is the foundation for telecommunications, mass entertainment and information technology. Copper readers are probably most interested in the role these devices played in the development of sound recording and reproduction. Even though vacuum tubes have been replaced by semiconductors in most sectors of the electronic industry, they remain stubbornly present in our hobby. Is this a sign of anachronism, or do these ancient devices still have unique advantages despite the amazing technological advances made during the past century?
The phenomenon of thermionic emission, the emission of electrons from a heated element, was reported by Thomas Edison and others independently in the 1870s and 1880s. The first practical vacuum tube device, the diode, was invented by John Ambrose Fleming at the Marconi company. The fact that electrons only travel in one direction, from the cathode (negative voltage potential) to the anode (positive voltage potential), was useful for rectifying an alternating current into a direct current. In 1907, inventor Lee de Forest placed a metal grid between the anode and the cathode of a diode, and noted that by altering the voltage potential of the grid relative to the cathode, he was able to control the current flow. A small change in the voltage of the grid can effect a much greater voltage change at the anode. This became the basis for a voltage amplifier, which he called an Audion, and more generally known as a triode. In early versions of the triode tube, the cathode was directly heated by an electric current. Subsequent developments used heaters that were separate from the cathode, insulating the signal from the noise of the heater supply.
The capacitance (called Miller capacitance or the Miller effect) between the grid and the anode limits the frequency bandwidth at which a triode can operate. By placing a separate grid (called a screen grid) between the control grid and the anode to create a tetrode, the Miller capacitance is significantly reduced, allowing the tube to operate at higher frequencies. This also significantly increases the amplification factor. Since the screen grid is at a positive voltage potential, it captures the secondary electron emission coming from the anode. As the secondary emission increases with the anode voltage, it reaches a point where the anode current starts to decline, and the screen grid current increases. This is known as the “tetrode kink.” An additional grid (called a suppressor grid) placed between the screen grid and the anode, and electrically connected to the cathode (and thus remaining at a negative potential), repels these electrons back to the anode and solves this problem. This type of tube with five elements is called a pentode. Another way to solve the problem is by aligning the electron emission from the cathode with the apertures of the control grid. These beams of electrons alter the space charge between the screen grid and the anode, limiting the transfer of electrons from the anode to the screen grid. These so called beam tetrodes have the advantages of higher transconductance and better linearity than pentodes.
What are the advantages and disadvantages of vacuum tubes? Tubes are transconductance amplifiers, meaning the anode current is controlled by the grid voltage. As long as the control grid remains at negative potential relative to the cathode, very little current flows through the grid. This means the device has a very high input resistance (i.e. it draws very little current). This is advantageous, since the capacitance C of the coupling capacitor in parallel with the input resistance R of the tube forms a high-pass filter with a cutoff frequency of 1/(2πRC). A low RC value will result in the early rolloff of the low frequencies. With a large R, only a small C is needed. In a typical tube circuit, coupling capacitors with values less than 1µF would often suffice, allowing the use of high-quality film capacitors.
Transistors, on the other hand, are current amplifiers, which means the collector-emitter current is influenced by the base-emitter current. These devices therefore have a low input resistance, requiring large value coupling capacitors, usually of the electrolytic type. Electrolytic capacitors are inferior to film capacitors in a variety of performance parameters for reasons that are beyond the scope of this discussion.
On the other hand, tubes tend to have rather high output resistance. There are different ways to address this issue. In situations where very low output resistance is needed, such as when driving typical dynamic loudspeakers with load impedance of 16 ohms or less, transformers are often used to lower the output impedance. High-quality output transformers are bulky and expensive, and even the best ones have a sonic signature. It is possible to design tube amplifiers without output transformers, usually by using a large number of devices in parallel, but this solution presents its own challenges. Transistor amplifiers generally do not need output transformers (even though some manufacturers such as McIntosh opt to use them as “seasoning”), since power transistors can output large amounts of current. Output impedance can be further reduced by using parallel devices and negative feedback, where a portion of the output signal is fed back into the input in order to reduce noise, cancel distortion, increase stability and provide other benefits. In situations where higher output impedance can be tolerated, such as in preamplifiers, special tube circuit topologies with low output impedance, such as the cathode follower, can be employed. However, some designers believe the cathode follower is in the same league as followers of Satan.
Negative feedback (NFB) is a sensitive subject in the high-end audio field. Some designers, usually those who favor single-ended amplifiers with directly-heated triodes, consider NFB to be anathema. These are usually the same people who consider the cathode follower as akin to Satan, since the cathode follower employs 100 percent negative feedback (albeit only locally). But Satan can be rather enticing; otherwise nobody would worship him. Aside from the aforementioned lowering of output impedance, negative feedback also extends the frequency response and reduces total harmonic distortion. So, what is not to like?
The early transistor amplifiers of the 1970s tended to have very high levels of NFB in order to win the “measurements wars” of the era, where specs were everything (well, to some people). However, the semiconductor devices of the era, with their limited open-loop gain, did not do well with high levels of negative feedback, producing distortions that the designers of the day were ignorant of. Consumers who relied on their ears rather than on reading equipment reviews knew something was wrong, but it took the industry a lot longer to figure it out. This experience put a generation of audiophiles off solid-state, and kept the manufacturers of tube equipment alive, but solid-state amplifiers today bear little resemblance to those of that era. Most solid-state equipment employs some NFB, whereas many tube designs eschew the use of negative feedback entirely. This is possible due to the fact that vacuum tubes are inherently linear devices. Moreover, the distortion produced by a triode is mostly second- and third-order harmonic distortion, which is only noticeable at very high levels. It is therefore unnecessary to reduce the distortion by using NFB. Tube amplifiers with output transformers also do not require NFB to achieve a low output impedance. Therefore, for those listeners allergic to negative feedback, tube equipment is the way to go. NFB is often accused of robbing the music of color and purity, but again, it all comes down to implementation.
Even though I own tube equipment, I am agnostic as far as the tube vs. solid-state debate goes. Both technologies are capable of state-of-the-art performance, and it is all in the details. For me, tubes have several attractions. Tube equipment can often be built using the point-to-point construction method, where the components are directly wired to each other without the use of printed circuit boards (another option is to connect the components to turret boards). Point-to-point is useful for an amateur like me with little competence in designing printed circuit boards. And why fuss around with, say, 30 volts in a solid-state circuit when you can live the excitement of working with a vacuum-tube amp with internal voltages of 500 or even 1,000 volts DC? There is also the fun of discovering the flavors of different tubes, the so called tube-rolling experience. This is not something one can normally do with solid-state consumer audio equipment.
In the next few installments, I will discuss commonly-used tube types and circuit designs, as well as some tips on tube collecting.
Header image: vacuum tubes in a vintage McIntosh MC240 amplifier. Courtesy of Wikimedia Commons/Sebastian Nizan.