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Guitar Amplifier Performance Reviews
Guitar Amplifier Reviews
Vacuum tubes (valves) were by far the controlling active electronic components
in most guitar amplifiers applications until the 1970s, when semiconductors
(transistors) started taking over for effectiveness and economic reasons,
including heat and weight reduction, and improved reliability. High-end tube
guitar amplifiers have survived as one of few exceptions, because of the sound
quality. Typically, one or more dual triodes are used in the preamplifier
section in order to provide voltage gain to offset losses by tone controls and
to drive the power amplifier section.
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Tube combo guitar amplifier
The output tubes are often positioned in a class AB push-pull connection to
improve efficiency; this requires another triode or dual triode to split the
phase of the signal. The tubes of the power amplifier stage are almost always of
the pentode or beam tetrode type. Some high power models use paralleled pairs of
output tubes in push-pull. Except for the slight negative feedback from the
secondary end of the output transformer to the driver stage, most amplifying
stages work in "crude" open-loop mode. Some designs employ current feedback via
fixed cathode resistors.
Since most tubes show gain non-linearity, applying an input signal high enough
to overdrive any stage tends to produce satisfying natural distortion. Today,
most guitar vacuum tube amplifiers are based on the ECC83/12AX7 (dual triode)
tubes for the preamplifier and driver sections and the EL84/6BQ5 or
EL34/6CA7/KT77 or 6L6/KT66 or 6V6 tubes for the power output section. Some use
the KT88/6550 beam power tubes in the output stage. The differing codes for
equivalent tubes generally reflect those used by the original European or U.S.A.
based manufacturers. These tubes are now mainly manufactured in Russia, China
and Eastern European countries. Some amplifiers, such as the Marshall Silver
Jubilee, use solid state components in the preamp, most commonly diodes, to
create distortion, a design feature known as diode clipping.
Tube guitar amplifiers are often equipped with lower-grade transformers and
uncomplicated power regulation circuits than those of hi-fi amplifiers. They are
usually not only for cost-saving reasons, but also are considered as a factor in
sound generation. A simple power regulation circuit's output tends to sag when
there is a heavy load (that is, high output power) and vacuum tubes usually lose
gain factors with lower power voltages. This results in a somewhat compressed
sound which could be criticized as a "poor dynamic range" in case of hi-fi
amplifiers, but could be pleasing as long sustain of sounds on a guitar
amplifier. Some tube guitar amplifiers use a rectifier tube instead of
solid-state diodes specifically for this reason.
Dreadfully, most amplifiers offer a fixed amount of sag, and this fixed amount
can only be achieved at full volumes. A small minority of guitar amplifiers
offer sag control via either multiple rectifiers or the Sag Circuit. Amplifiers
with multiple rectifiers can offer up to two sag settings (amounts), while the
Sag Circuit provides a Sag control knob, which allows range of sag control at
all volumes (by interacting with a wattage control knob).
Some models have a "spring reverb" unit that imitate the reverberation of an
echoic ambient. A reverb unit usually consists of one or more coil springs
driven by the preamplifier section using a transducer driver similar to a
loudspeaker at one end and an electro-magnetic pickup and preamplifier stage at
the other end that picks up the long sustaining spring vibration, which is then
mixed with the original signal. Some guitar amplifiers have a tremolo control.
An internal oscillator generates a low frequency continuous signal which can
modulate the input signal's amplitude or the output tubes' bias, thereby
producing a tremolo effect.
Tube amps have the following technical disadvantages in comparison to
solid-state amps. They are bulky and heavy, primarily due to the iron in power
and output transformers. Solid-state amplifiers still require power
transformers, but are usually direct-coupled and don't need output transformers.
Glass tubes are fragile, and require more care and consideration when equipment
is moved repeatedly. Tube performance can deteriorate slightly over time before
eventual catastrophic failure.
When tube vacuum is maintained at a high level, though, excellent performance
and life is possible. They are prone to pick up mechanical noises (microphonic
noise), although such electro-mechanical feedback from the loudspeaker to the
tubes in combo amplifiers may contribute to sound creation. Tubes benefit from a
heater warm-up period before the application of high tension anode voltages;
this allows the tube cathodes to operate without damage and so prolongs tube
life. This is of particular importance for amplifiers with solid state
rectifiers.
Tube amps have the following technical advantages over solid-state amps.
Compared to semiconductors, tubes have a very low "drift" (of specs) over a wide
range of operating conditions, specifically high heat/high power. Semiconductors
are very heat-sensitive by comparison and this fact usually leads to compromises
in solid-state amplifier designs. When a tube fails, it is replaceable. While
solid state devices are also replaceable, it is usually a much more involved
process (i.e., having the amplifier tested by a professional, removing the
faulty component, and replacing it).
For working musicians this is usually a huge problem by comparison to looking in
the back of a tube amp at the tubes and simply replacing the faulty tube. In
addition, tubes can easily be removed and tested, while transistors cannot. Tube
amplifiers respond differently from transistor amplifiers when signal levels
approach and reach the point of clipping. In a tube-powered amplifier, the
transition from linear amplification to limiting is less abrupt than in a solid
state unit, resulting in a less grating form of distortion at the onset of
clipping. For this reason, some guitarists prefer the sound of an all-tube
amplifier; the aesthetic properties of tube versus solid state amps, though, are
a topic of debate in the guitarist community.
Solid State Amplifier Review
Most inexpensive guitar amplifiers currently produced are based on semiconductor
(solid state) circuits, and some designs incorporate tubes in the preamp stage
for their subjectively warmer overdrive sound. Tubes create warm overdrive
sounds because instead of cutting the peaked signal off, they more or less pull
the peaked audio information back (like natural compression) which creates a
fuzzy overdrive sound. While this is a desirable attribute in many cases, the
tube's characteristic will "color" all the sounds at any volume, unlike solid
state. However, solid state in general have the quickest response time, perhaps
even more so than modeling amps.
High-end solid state amplifiers are less common, since many professional
guitarists tend to favor vacuum tubes. Some jazz guitarists, however, tend to
favor the "colder" sound of solid-state amplifiers, preferring not to color the
sound of their guitar with the tube distortion and compression so popular with
rock, blues, and metal musicians. Solid-state amplifiers vary in output power,
functionality, size, price, and sound quality in a wide range, from practice
amplifiers to professional models. Some inexpensive amplifiers have only a
single volume control and a one or two tone controls.
Hybrid Solid State Tube Amplifier Review
A tube power amp may be fed by a solid-state pre-amp circuit, as in the Fender
Super Champ XD and the Roland Bolt amplifier, which is thereby classed as a
'hybrid' amp. Randall Amplifier's current flagship models, the V2 and T2, use
hybrid amp technology. Alternatively, a tube pre-amp can feed a solid state
output stage, as in models from Kustom and Vox. This approach dispenses with the
need for an output transformer and allow modern power levels to be easily
achieved.
Modeling Amplifiers Review
Modeling amplifiers use amplifier modeling to simulate the sound of well-known
guitar amps, cabinets, and effects, as well as simulating the way traditional
speaker cabinets sound when mixed with different types of microphones. They may
also be an original creation not meant to simulate any particular real world
guitar amp at all, instead allowing the user to create their own unique sound.
This is usually achieved through digital processing. Modeling technology offers
several advantages over traditional amplification. A modeling amp typically is
capable of a wide range of tones and effects, and offers cabinet simulation, so
it can be recorded without a microphone. Most modeling amps digitize the input
signal and use a DSP, a dedicated microprocessor, to process the signal with
digital computation. Some modeling amps incorporate vacuum tubes, digital
processing, and some form of power attenuation.
Acoustic Guitar Amplifiers Review
These amplifiers are designed to be used with acoustic guitars, especially for
the way these instruments are used in relatively quiet genres such as folk and
bluegrass. They are similar in many ways to keyboard amplifiers, in that they
have a relatively flat frequency response, and they are usually designed so that
neither the power amplifier nor the speakers will introduce additional
coloration.
To produce this relatively "clean" sound, these amplifiers often have very
powerful amplifiers (providing up to 800 watts RMS), to provide additional
"headroom" and prevent unwanted distortion. Since an 800 watt amplifier built
with standard Class AB technology would be very heavy, some acoustic amplifier
manufacturers use lightweight Class D amplifiers, which are also called
"switching amplifiers."
Acoustic amplifiers are designed to produce a "clean", transparent, "acoustic"
sound when used with acoustic instruments with built-in transducer pickups
and/or microphones. The amplifiers often come with a simple mixer, so that the
signals from a pickup and microphone can be blended. Since the early 2000s, it
has become increasingly common for acoustic amplifiers to be provided with a
range of digital effects, such as reverb and compression. As well, these
amplifiers often contain feedback-suppressing devices, such as notch filters or
parametric equalizers.
Tube Distortion Review
Power-tube distortion is required for amp sounds in some genres. In a standard
master-volume guitar amp, as the amp's final or master volume is increased
beyond the full power of the amplifier, power tube distortion is produced. The
"power soak" approach places the attenuation between the power tubes and the
guitar speaker. In the re-amped or "dummy load" approach, the tube power amp
drives a mostly resistive dummy load while an additional low power amp drives
the guitar speaker. In the isolation box approach, the guitar amplifier is used
with a guitar speaker in a separate cabinet. A soundproofed isolation cabinet,
isolation box, isolation booth, or isolation room can be used.
Volume and Distortion Review
Distortion is a feature available on many guitar amplifiers that is not
typically found on keyboard or bass guitar amplifiers. Tube guitar amplifiers
can produce distortion through pre-distortion equalization, preamp tube
distortion, post-distortion EQ, power-tube distortion, tube rectifier
compression, output transformer distortion, guitar speaker distortion, and
guitar speaker and cabinet frequency response. Distortion sound or "texture"
from guitar amplifiers is further shaped or processed through the frequency
response and distortion factors in the microphones (their response, placement,
and multi-microphone comb filtering effects), microphone preamps, mixer channel
equalization, and compression. Additionally, the basic sound produced by the
guitar amplifier can be changed and shaped by adding distortion and/or
equalization effect pedals before the amp's input jack, in the effects loop just
before the tube power amp, or after the power tubes.
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