Can we all agree? tubes are cool, no? i mean, even if you know nothing of what they do, there’s something objectively appealing about glass bottles that get warm, glow in a soothing amber, and (at least anecdotally) seem to contribute positively to tone in ways that engender lust and long-winded geekery. They’ve long since been rendered obsolete in pure function by other devices, yet they persist. Ever wondered why?
The vacuum tube is a miraculous device, and was an essential component of almost every piece of electronics in the first half of the 20th century. Invented in 1904, the vacuum tube is basically a device that allows for the control of current between electrodes in an evacuated container, typically made of glass (but not always). While tubes found early application in a dizzying array of devices, for our purposes, it’s a tube’s ability to amplify an audio signal that is of concern.
In the most basic tube used for amplification, a filament or “heater” (sort of like the filament in an incandescent bulb) gets hot when current is applied. This heat causes one of the tube’s electrodes, the cathode, to begin emitting electrons. Since electrons have a negative charge, a positive charge applied to another of the tube’s electrodes (the anode or “plate”) attracts these electrons, creating a one-way flow of current from the cathode to the anode. If the voltage applied to the anode is high, a substantial amount of current will flow.
Now comes the fun part. Another component can be placed between the cathode and anode, called a “grid.” If the tube’s circuit is designed such that the grid is negative with respect to the cathode, the electrons will be blocked from reaching the anode. In this condition, the tube is “off.” But, if the grid’s voltage is allowed to fluctuate such that it allows some of the electrons to pass through and on to the anode, we now have a means of controlling the flow of current through the tube. The magic is in the fact that the amount of voltage necessary to control the grid can be very small in comparison to the current it’s controlling. In this sense, it’s exactly like the valve on a firehose—think about how little physical effort it requires to control the massive amount of water on tap (this is why the Brits call tubes “valves”). Since the fluctuating grid voltage is a direct analog of the much larger current flowing in the tube, the fluctuation in current at the anode is basically a much more “powerful” version of the same signal tied to the grid. We’ve created an amplifier: a small signal controls the creation of a much larger version of itself. In the case of a tube amp, that small fluctuating signal applied to the grid is the output of our instrument.
Tubes get hot, are relatively large, are often made of glass, require high voltages, and can have a short lifespan in many applications. For all these reasons and more, by the mid-20th century, the “solid-state” transistor, which shares none of the above shortcomings, essentially rendered tubes obsolete for all but a small handful of specialized applications. It just so happens that one of those important applications is audio. The reason is that when tubes are pushed to the point of “saturation,” meaning the condition in the tube is such that is no longer able to amplify in a linear, high-fidelity fashion, the resulting distortion is pleasing to the human ear. The reason? As tubes are pushed into saturation, the distortion tends to be mostly composed of “even-order” harmonics, new overtones that have a consonant relationship to the input signal. Tubes also move into distortion more gradually, softly compressing the signal as it reaches overdrive. Solid-state devices, by contrast, tend to distort more abruptly, yielding a harsher sound characterized by the presence of odd-order harmonics.
Nowadays, there are many clever ways that solid-state and digital devices can emulate the singular sound of tube amps. Regardless, they do have a unique sound, so if you have the chance, adding a tube amp to your collection can add a new, often warmer color to your sonic palette.
Bass Player Senior Contributing Editor Jonathan Herrera is the magazine’s former Editor-in-Chief. An accomplished player, Jonathan is now a full-time musician and producer. His latest endeavor is Bay Area recording studio Airship Laboratories. Catch up with him at jonherrera.comand at airshiplaboratories.com.