Guitar Amplifier - Types - Vacuum Tube Amplifiers

Vacuum Tube Amplifiers

Vacuum tubes (valves) were by far the dominant active electronic components in most instrument amplifier applications until the 1970s, when semiconductors (transistors) started taking over for performance and economic reasons, including heat and weight reduction, and improved reliability. High-end tube instrument 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 sufficient voltage gain to offset losses by tone controls and to drive the power amplifier section.

The output tubes are often arranged 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 (also known as "kinkless tetrodes", hence the KTxx nomenclature). Some high power models use paralleled pairs of output tubes (four or more in total) in push-pull. Except for the light negative feedback from the secondary end of the output transformer to the driver stage, most amplifying stages work in "raw" open-loop mode. Some designs employ current feedback via unbypassed cathode resistors.

Since most tubes show "soft clipping" gain non-linearity, applying an input signal high enough to overdrive any stage tends to produce favorably natural distortion. Today, most vacuum tube amplifiers are based on the ECC83/12AX7/7025 (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 use solid-state components in the preamp, most commonly diodes, to create distortion, a design feature known as diode clipping.

Tube instrument amplifiers are often equipped with lower-grade transformers and simpler 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 creation. For example, 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 desirable 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.

Unfortunately, most amplifiers offer a fixed amount of sag, and this fixed amount can only be attained at full volumes. A small minority of amplifiers offer sag control via either multiple rectifiers or the Sag Circuit (a non-traditional power supply design patented by Maven Peal® Instruments). 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 simulates 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 some 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 also have some 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.