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.
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
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.
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
|Mesa/Boogie Bass 400+
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.
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.
|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.
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.
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.
|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.
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
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.
Amplification 101 Amplifier Class
|Hartley Peavey with an early Class D amplifier.
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,
In a Class A amp, the active
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