Bass players are understandably infatuated with the bass itself. Get a group of bass players in a room, and the conversation will eventually circle toward basses. It’s standard-issue shoptalk, and it’s often remarkably well informed.
But then the conversation turns to amps. Maybe because of their relative complexity compared to basses, or the challenge of verbalizing technology’s connection to sound, bass players are generally less fluent amp-wise. The same player who extols the special virtues of ash-body mid-’70s Jazz Basses will often clam up if asked to describe what it is exactly that makes his amp sound good … or bad.
If innovation is a good catalyst for conversation, amps give us much more to talk about. While there have been many stabs at revolutionary change, the bass guitar has basically stayed the same since it was invented. There’s a body. There’s a neck. There are strings and some way of converting the strings’ vibration into an electrical output. Sure, there are many approaches to each facet, but to change any one would essentially reorient the very instrument itself. Amps, on the other hand, are a venue for truly radical change. At the core, an amp needs to make a bass loud. This raw fact has never altered, but the means of achieving this goal are in a constant state of flux, and the last decade has been perhaps the most exciting.
This article will outline the progression of amplifier technology through the years, explain the latest developments, and illuminate some of the technology behind the sound.
In The Beginning
When Fender debuted the first mass-produced bass guitar in 1951, the Precision Bass, it was quickly clear that a purpose-built bass amp would be necessary. After all, one of the chief motives for the development of the bass guitar, besides portability, was its ability to be heard in the increasingly loud world of electrified music, and existing guitar amps weren’t up to the task.
Acoustic 360/361 Ampeg and Fender quickly dominated the burgeoning bass amp industry. The earliest bass amp, 1949’s Ampeg Super 800, was designed to complement Ampeg’s innovative acoustic bass pickup, but the seeds for bass guitar amplification were sown. On the heels of the P-Bass debut was the 1952 release of the Fender Bassman, a 50-watt amp with a single 15" speaker. In 1954, Fender updated the Bassman with a 4x10 speaker configuration and an upgraded circuit. Meanwhile, Ampeg continued to evolve its lineup, culminating in one of the most important amps of all time, 1960’s B-15 Portaflex.
During the mid ’60s, other manufacturers started making their own amps. England’s Marshall and Hiwatt were early pioneers in high-output amplification, and the loud rock bands of the mid to late ’60s quickly took to their 50- and 100-watt heads and large 4x12 cabinets.
The Birth Of Solid-State
Although the transistor had been integrated into radios and other small amplifiers since the early ’50s, it wasn’t until the mid-tolate ’60s that solid-state amps began to steal market-share from the all-tube predecessors mentioned above. Solid-state amplifiers have important advantages over tubes (which in Britain are called valves), particularly their durability and portability. Instead of relatively hot and fragile vacuum tubes, solid-state amps use small, rugged transistors. They’re also generally lighter than tube amps of comparable power, because solid-state amps don’t require an output transformer, often the heaviest component in a tube amp.
Vox released one of the earliest solid-state bass amps, the T.60. The company had engineers on staff experienced with transistor technology because of the Vox Continental Organ, so it was well poised to make the leap to solid-state. The T.60’s 40 watts powered an innovative closed-back 1x15 + 1x12 speaker cabinet, which included a crossover to filter low frequencies out of the 12" speaker’s output. Although the T.60 developed a reputation for unreliability, Vox continued to be an important solid-state brand through the ’60s, in part because of brilliant endorsement deals with major acts of the day, including the Beatles, the Rolling Stones, and James Brown.
By the mid to late ’60s, a few other manufacturers joined the solid-state bass amp fray, including Univox, Kustom, Sears Silvertone, Acoustic Control Corporation, and more. Many of these amps were deemed unreliable, but the technology was developing rapidly. In 1968, RCA—then among the world’s most influential electrical engineering firms—released a paper describing important developments in solid-state amplifiers, helping the technology flourish even further among smaller amp manufacturers.
The ’60s was also the decade when many iconic bass-amp innovators started their careers, including Russ Allee and Roger Smith of AMP and Steve Rabe, founder of SWR. Bob Gallien, the man behind Gallien-Krueger, designed one of the decade’s most powerful solid-state amps, the GMT 226B. When the owner of the local music store where Gallien worked suggested that he focus on bass amps instead of the much more crowded guitar amp field, Gallien saw the opportunity. His innovative concepts and rugged engineering would yield some of the most important amps in history.
Showdown: Tubes Vs. Transistors
As with the music the amps were used for, the 1970s would be a critical decade in the bass’s evolution. The Ampeg SVT, released in 1969, would prove to be the dominant amp of the ’70s, and to its many fans, it continues this dominance today. The SVT (short for Super Valve Technology) was leagues ahead of its all-tube contemporaries. Bill Hughes’ design offered 300 watts of power at a time when most amps struggled to eclipse 100, and its accompanying cabinet featured eight 10" speakers in an innovative sealed and baffled design. It was the first amp to really allow bass players the volume and projection to compete with the screaming electric guitars and thunderous drums of the era’s rock music.
Fender, Sunn, and Acoustic were the decade’s other dominant bass amp manufacturers, some with designs originally released in the late ’60s. The Fender Dual Showman, while not technically a purpose-built bass amp, found much favor for its sweet tone, large 2x15 cabinet, and durability. Sunn’s Coliseum head, an all-tube monstrosity that rivaled the SVT for girth and complexity, could be seen on many loud stages, especially among the era’s top British bands. On the other side of the technological spectrum was Acoustic’s 360 stack, consisting of the Model 360 preamp and 361 powered cabinet. Released in late 1968, the solid-state Acoustic stack was one of the most coveted rigs among period bass players. While not as powerful as the Ampeg and Sunn amps, the Acoustic’s clever design, solid-state immediacy, and unusual tone-shaping circuit helped it stand out. The 361 cabinet was at least as important as the 360 preamp to the rig’s unique sound: It included a rear-firing horn and single 18" speaker, and was legendary for its projection into large rooms. It would become a key part of the tone of bass greats like John Paul Jones, Larry Graham, and Jaco Pastorius.
The Rise Of Hi-Fi
By the end of the ’70s and into the ’80s, solid-state technology had come a long way from its unreliable beginnings. Engineers were extracting unprecedented levels of power from amps, with much of the early effort focused on PA-style power amps. PA (public address) amps must be powerful enough to drive full-bandwidth sound through large arrays of speakers, and solid-state amps offered enough power, plus substantially increased reliability and portability compared to tube amps. Meanwhile, the bass’s role in music changed substantially; the advent of synthesizers, the emergence of electronic dance music, and the dawn of digital recording all had a huge impact on the instrument’s tone as well as players’ techniques. Many bassists who had been content with the rich and thick sound of their SVTs, B-15s, and Acoustic 360s now craved greater clarity, speed, and power to better suit the era’s music. The same fidelity that marked the period’s popular hi-fi home stereo gear became an increasing priority for bass players.
Many manufacturers flourished in the ’80s, contributing essential amps to the industry. Former Acoustic Control Corporation engineer Russ Allee formed AMP (Amplified Musical Products) in 1981, soon hiring fellow Acoustic cohort Steve Rabe to help design the AMP 420 bass head. While not a huge seller at the time, AMP’s modern approach to solid-state design would be hugely influential, and Allee would continue to be a major figure in bass amp design for years to come.
Mesa/Boogie Bass 400+ Steve Rabe founded SWR in 1984. His mission was for a bass amp that mimicked the full-range, detailed sound of studio monitors. Building on the solid-state experience he gained at Acoustic and AMP, Rabe’s first amplifier, the PB-200, could be described as one of the earliest truly modern bass heads, with many features that continue to be commonplace in today’s heads. A “hybrid” amp, the PB-200 employed a single 12AX7 tube in its preamp section, for buffering and driving the input. It incorporated an “Aural Enhancer” circuit, a variable EQ contour controlled by a single knob that boosts lows and highs and cuts mids as it’s turned up. It also boasted a semi-parametric EQ section, a bi-amp function (allowing highs and lows to be amplified separately), and a balanced xlr output.
Realizing that SWR needed a bass cabinet to sell alongside its hot new head, Rabe turned to David Nordschow of Eden Electronics for assistance. Then better known for PA-style cabinets, the first Eden design for SWR would radically change cabinet design through the decade. The Goliath I cabinet, with its 4x10 + horn configuration, was capable of high volume—but it was also articulate, quick, and blessed with extended high-frequency response thanks to its built-in tweeter. Subsequent Goliath speaker designs would move in-house, but Nordschow’s early influence was critical to their development.
Nordschow saw an opportunity for Eden to expand into amplifier design. An early partnership with famed audio engineer James Demeter yielded the VT-40 head, a kind of hybrid of Demeter’s VTB-201s preamp and a 400-watt solid-state power amp. Eden continues to be a major force in bass amps today, as does DNA (David Nordschow Amplification), which Nordschow launched after his split from Eden.
After its early success and innovation with the 226B, Gallien- Krueger had more tricks up its sleeve. In 1982 it debuted the 800RB, an iconic head renowned for its durability and clever circuit, which included a built in DI and bi-amp capabilities. It also introduced the essential feature set that would characterize GK’s later, more powerful designs.
Mesa/Boogie was also an emergent powerhouse during the ’80s, first gaining fame with its complex and flexible line of alltube guitar combo amps. It also released one of the few truly innovative all-tube bass heads of the ’80s, the Bass 400, and its more powerful successor, the Bass 400+.
The Modern Age
Until the 1990s, the evolution of the bass amp featured just a handful of key players. Applying the same scrutiny to the explosion of bass amp manufacturers in the ’90s and beyond would take up most of this magazine. Rather than illuminate the history of each of these important contributors, let’s focus instead on the technological developments that characterize today’s bass amps.
Six landmark high-powered heads (clockwise from left): Gallien-Kreuger 800RB, Eden WT-800, Ashdown ABM 1000, Carvin B1500, Traynor YBA 200, and Hartke LH500. From the ’90s onward, solid-state amps became the vastly dominant technology. While many manufacturers continued to produce all-tube models, the advantages of solid-state were significant, especially when power, durability, and cost were factors. There were also new entrants into the industry, plus legacy companies that began to make a bigger presence in bass. A bass player could choose amps from a multitude of manufacturers— companies like Aguilar Amplification, Ashdown Engineering, EBS, Genz-Benz, Epifani, Euphonic Audio, Hartke, Ibanez, MarkBass, Peavey, Orange, Tech 21, Traynor, Warwick, Yamaha, Phil Jones Bass, TC Electronic, and more.
The biggest technological advances in bass amplification address the age-old problem of weight and portability. For decades, loud amps were heavy, and so were the cabinets capable of contending with their high output. Now, it is not uncommon to find 1,000- watt heads weighing less than ten pounds. Let’s take a closer look at the technology behind the weight savings.
Light Amps: Class D & SMPS
Ultralight amps predominantly make use of two key pieces of technology. First, the power amps generally operate in a Class D topology. Peavey was a big innovator in this realm, releasing the first Class D amp for live audio applications in 1984. Class D amps achieve much greater efficiency than conventional Class AB amplifiers, so less output power is wasted as heat, and more of the input power is converted directly into sound. The resulting amps require fewer output devices, less heat-sinking, and a physically smaller power supply. The industry approached the technology tentatively in the early 2000s, but now virtually every manufacturer (except for the hardcore all-tube specialists) makes at least one bass head that utilizes a Class D power amp.
A new breed of featherweight heads (clockwise from left): Warwick LWA 1000, TC Electronic RH750, Orange Terror Bass, Fender Rumble 200, Aguilar Tone Hammer 500, EBS Reidmar. The other critical component of today’s ultra-lightweight amps is the switched-mode power supply (SMPS). All amplifiers require a section of the circuit dedicated to converting the line power (from the wall) to the appropriate voltage, current, and phase for the operation of the amp; this conversion is done by the power supply. In a traditional linear power supply, AC (alternating current) line power is regulated at the frequency it exits the wall, 60Hz in America. To contend with this low-frequency power, a large power transformer, consisting of iron laminations and copper windings, is required. Additionally, an array of other discrete components, like high-voltage capacitors, resistors, and inductors are required to filter and smooth the power for distribution through the amp. Linear power supplies are not particularly efficient, losing energy as heat and as a consequence of voltage regulation.
SMPS designs take a different route. First, an SMPS rectifies the AC line power into DC (direct current). Then, a “chopper circuit” or “switching regulator” converts the DC signal back into AC, but at a much higher frequency than its original 60Hz. This frequency is typically above the audible spectrum (20kHz) and can go as high as 100kHz in some SMPS amps. Finally, the high-frequency current hits a transformer again to step the voltage down or up for appropriate use in the amp. The trick is that since the power is at such a high frequency, a much smaller power transformer is necessary than the bulky iron anchors used in linear power supplies. The reasons get deep into physics-class territory, but suffice it to say that an SMPS can nearly eliminate the weight a transformer contributes to the overall heft of a bass head. Coupled with a lightweight and efficient Class D power amp, you get the insanely lightweight heads of today. Today's bass amps are smaller, lighter, and more powerful than ever before. So are speaker cabinets, which now make extensive use of lightweight neodymium magnets. Innovation may be a constant, but the current crop of technology feels like it's at a point of temporary stasis, with the focus going toward further refinement and cost reduction. Given how capable today's amps are, it's a wonder what may come next
Amplification 101 How Do Tubes Work?
To put the emergence of solid-state amplifiers into context, it’s important to understand the technology behind an all-tube amplifier. Prior to amplification, electrical devices were passive, only able to subtract from a signal. Vacuum tubes allowed electronic devices to add gain (amplification) to a signal, resulting in some of the 20th century’s most important inventions. These included radio, radar, recording devices, and yes, bass amps—all devices that require gain to operate, and therefore required vacuum tubes, until the transistor’s invention in 1947.
A vacuum tube consists of a sealed envelope, usually glass, which contains a number of elements. The air is removed from the tube to allow the free flow of electricity between the components. The simplest tubes contain three elements: an anode (also called the “plate”), cathode, and heater. Naming conventions ignore the heater as an element, since nearly all tubes contain one. Thus, the simple two-element tube (plate and cathode) is called a “diode” (short for “dielectrode”). The cathode is made of a material that sheds electrons when heated. Electrons have a negative charge, so when the cathode is heated and a positive charge is applied to the anode, electrons flow from the cathode to the anode. Critically, electrons do not flow from the anode to the cathode. Thus, a diode allows current to flow in only one direction.
What’s the point, you may ask? A diode is a critical component of an amplifier’s power supply, the part of an amp that manages the power coming from the wall. Wall power emerges from a socket as an alternating current (AC), the polarity of which changes 60 times per second. If plotted on a graph, the power would look like a sine wave, with peaks and valleys equidistant from a centerline. One way to imagine AC as compared to direct current (DC) is that AC flows forward and backward, while DC flows in only one direction. Since diodes only allow current to flow in one direction, a diode can convert the AC from the wall into DC. When used this way, a diode tube is called a “rectifier” (the conversion of AC to DC is called rectification). Tube amps need DC to function, so a rectifier creates this type of current for use throughout the amp. It performs this function in concert with other components in the power supply, including a transformer for modulating the voltage of the wall power, and filter capacitors that help smooth out the DC emerging from the rectifier. In contemporary tube amps, tube diodes are often replaced by solid-state components that perform the same function, but some players still prefer the sound of a tube rectifier.
The other role of a tube in an amplifier is amplification itself. Whereas rectifier tubes simply convert AC to DC, the rest of the tubes in an amp add gain to the signal. They accomplish this by adding an additional element to the tube, a control grid. The control grid is placed between the anode and the cathode, regulating the flow of electrons from the cathode to the anode. When an appropriate negative voltage is applied to the grid, it can effectively “turn off” the tube, preventing electrons from making their way to the anode. (This voltage is a tube’s bias.) When an AC signal is applied to the grid, however, the grid allows electrons to flow. Since the output of a bass pickup is AC (in fact, all audio signals are AC), applying a small audio signal to the grid results in a much larger fluctuation in the anode current. A small signal voltage is controlling the creation of a large current; this is amplification. The simplest amplifier tube is a triode, so called because of its three active components (anode, cathode, and grid). Some tubes, called tetrodes and pentodes, add additional grids to further filter and regulate the flow of current.
Hartley Peavey with an early Class D amplifier.Amplification 101 Amplifier Class
There are a number of methods for designing an amplifier’s output section, whether it’s tube or solid-state. Each type or “topology” is categorized according to its output class. In bass amps, the primary output classes in use are Class AB and Class D (the latter used only in solid-state amps). To understand Class AB power amps, let’s take a quick look at the simplest power amp topology, Class A.
In a Class A amp, the active amplifying component—whether tube or transistor—remains conducting at all times. The component is idling or “biased” such that its base level of operation is roughly half of the operating current. That means that even at low signal levels, a significant amount of the power-supply voltage must be applied to the tube or transistor. This makes Class A amps inefficient and their accompanying power supplies large and hot. They have a cult following among some guitar players and hi-fi purists for their perceived higher-fidelity sound, but at the output levels required for bass, they’re of little practical use.
Until the dawn of Class D, Class AB was by far the most prominent amp topology in bass amps. In a Class AB amp, the audio signal is divided into two halves. An audio signal being a form of alternating current, it has positive and negative phases. A section of the amp called the phase inverter divides the audio signal and sends the positive side to one or more output devices (solid-state or tube) and the negative half to another set of output devices. Since each array of output devices is only responsible for amplifying half the audio signal, it can spend half of its time in a low-power state. The resulting DC signals are then recombined to constitute an amplified audio signal proportional to the input signal. This process introduces some distortion, but the distortion is mitigated in a well-designed Class AB output stage. Class AB topologies are also referred to as “push–pull” amps, because of the phase inversion integral to their operation.
Class D amps exclusively use solid-state output devices, typically MOSFETs, a specially designed form of transistor. Through a function called pulse-width modulation (PWM), Class D amps produce a high-frequency series of pulses, the duration of each being proportional to the input signal’s level at that moment. In this way, a high-frequency signal can act as a “carrier” for the low-frequency bass signal. Because this signal consists of on-and-off pulses only, and not the complex undulations of a typical analog audio signal, the output devices can operate like switches, either fully on or fully off. This results in extremely high efficiency. After the amplification, a lowpass filter removes the high-frequency content to reveal the low-frequency audio.