Illustration by Laura Kottke.

There are two main types of bass amplifiers on the market today: tube (or “valve” for us Britons) and so-called “solid-state” designs. But despite the seeming simplicity here, the everyday reality of finding the right bass amp for your needs is much trickier than you’d expect. For starters, although tube circuits have been around for decades, many bassists still don’t really understand them. But recent advances in solid-state technology have even greater potential to be confusing. In this article we’re going to demystify one of the most important areas of amp design that even many experienced bassists don’t adequately grasp—power-amp types and their associated power ratings.

The reason we’re focusing on these is because amplifying beefy bass frequencies—moving all that air and thumping listeners’ chests—requires far more power than a guitarist needs to wail on a 6-string. And once you’ve got a deeper understanding of how to ascertain the true power capabilities of an amp, the better you’ll be able to navigate the sea of marketing-speak and tech talk surrounding amps large and small.

In order to dive deep on these two topics, though, we need to discuss a few aural and electrical basics first. To some our conversation might feel pretty complicated in parts, but I promise that if you stick with me through the tech talk, I’ll tie it all together so you’ll walk away with newfound clarity and practical knowledge that’ll save you a lot of money, time, and hassle in the long run. In fact, you might find yourself asking, “How come no one ever explained all this before?”

The Science of Sonics

A bass note is sounded when a string anchored at two fixed points, the fret (or nut) and the bridge, is plucked. The note produced by the resulting vibrations is called the fundamental frequency (see Fig. 1). If it were possible to pluck exactly in the center of the string, the wave produced would be a sine wave (Fig. 2). Practically speaking, it’s virtually impossible to pluck the string exactly in the middle, and as a result harmonics—predominantly second, but also fourth and sixth—are also produced (Fig. 3). This is a wonderful thing, though, because these harmonics are responsible for the rich, unique tones we all love. Each instrument and player in a band produces a differing set of tones via the unique harmonic properties inherent in their instrument, and the resulting sine wave we hear is a combination of those harmonics that looks similar to Fig. 4.

AC Meets DC, and Watts Are Born
Now let’s briefly look at some power fundamentals. A battery such as the 9V units used in effects pedals produces direct current (DC). The electricity from DC sources is under pressure to flow continuously like water from a tap. The unit of measure for this pressure is volts (V), and the rate of the flow is measured in amperes (A). When we multiply V x A, we get Watts (W)the unit of measure for the power being used.

However, things aren’t quite so simple when the current flows back and forth like the alternating current (AC) from a wall socket. The waveform for AC current should look like that produced by the vibrating string in Fig. 2. Another equation calculates the power contained within this alternating waveform—averaging it over time—in order to relate it directly to DC current. The equation involves a bunch of dual-axis chart graphing and square-root stuff that most players have no interest in. The resulting values from this more complicated math are designated RMS (for “root-mean square”), and with these RMS values the same V x A = W equation we used for DC current now holds true for AC: V (RMS) x A (RMS) = Watts (Avg).

Watts (Avg) is often incorrectly referred to as “RMS power,” “RMS watts,” or “watts RMS.” However, the RMS value of the power waveform is different from the average power value (22 percent higher for a sine-wave signal), and therefore should not be used. The correct term specified by the Federal Trade Commission is continuous average power. Continuous average power ratings are a staple of performance specifications for audio amplifiers and, sometimes, loudspeakers.

Basic Amp Operation
Now let’s look at how amplifiers work. Amps need the pressure (volts) of the AC power from the wall socket adjusted in order to make it suitable for use with your bass and speakers. This is usually done by means of a transformer or a switch-mode power supply (SMPS). The transformer and SMPS also convert alternating current to a smooth, steady direct current. This steady stream of power must then be regulated by an electronic “tap.” In a tube amp, the tubes function as the tap regulating the flow. In a solid-state amp, it’s transistors.

As your bass string vibrates over the pickup (which is, in fact, a tiny generator), an electrical waveform is produced and then fed to the amplifier, where it is made larger and used to turn the transistors or tubes on and off in a matching way so as to produce a larger version of the bass-note waveform. In order to respond quickly to notes you might play at any moment, the tubes or transistors in your amp are always turned on slightly—like a car engine idling, ready to take off when you press the accelerator. The rate of this idling state is known as the bias setting in tube amps. Transistor amps also have bias, though it usually isn’t adjustable. We’ll talk more about bias later.

You’ll recall that we said amps send DC to power the electronic taps. But if the stream of current is steady regardless of how much gets used up amplifying a signal at any given moment, what happens to the power not being used? The answer, my low-end-loving friends, is why moderate- and high-powered bass amps these days have cooling fans and/or heat sinks: The excess current is turned into heat that must be dissipated in order to prevent damage to the circuit. This is how conventional linear amplifiers operate, and there are two basic types of these: class A and class B. (There are also sub classes such as class AB.)