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 adequate voltage gain to offset losses by tone controls and to drive the power amplifier section.
Rear view of a 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.
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.
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 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. Such as the original creations of companies like AcmeBarGig or Peavey. 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.
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.
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.
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.
www.handstander.com 2007
marshall guitar amplifiers
bass guitar amplifiers
acoustic guitar amplifiers
electric guitar amplifiers
used guitar amplifiers
crate guitar amplifiers
fender guitar amplifiers
vintage guitar amplifiers
tube guitar amplifiers
cheap guitar amplifiers