Power attenuator (guitar)

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In electric Rock guitar, attenuators are used as a dummy load by sending all of the power to the power attenuator's built-in, mostly resistive dummy load and none to the speaker, in order to silence or reduce the output volume of an audio amplifier (for example, a guitar amplifier).

1 Explanation
2 Description
3 See also
4 External links

[edit] Explanation

Silencing an amplifier is useful for:

Biasing the positive and negative signal crossover
Running bench tests such as measuring the amplifier's maximum output wattage
Adding line-level effects between a guitar amplifier and a guitar speaker.

With guitar amplifiers, power-tube distortion is one of the main purposes of guitar amplifiers, and power attenuators are frequently used essentially for signal processing, as a means of obtaining power-tube distortion at a controlled volume level. A power attenuator is similar to a guitar effects pedal: it provides a useful way of getting a guitar amp to produce distortion for use in musical performance.

Whether set to full dummy load or only partial attenuation, a guitar-amp power attenuator typically offers a line-level output jack to enable recording the distortion-processed signal directly or reamplifying the distortion-processed signal through a larger or smaller amplifier for more or less volume, independently of how much power is being output by the first guitar amplifier's power amp section.

There are two approaches to power attenuation to obtain any desired amount of power-tube distortion at a quiet or independently controllable speaker volume: the power-soak approach and the power-supply reduction approach. In the power-soak approach, the power tubes are pushed into their maximum possible power output, and then the unwanted resulting wattage is directed into the dummy load portion of the power attenuator, which is placed between the output transformer and the guitar speaker, routing only a portion of the output wattage to the guitar speaker.

In the power-supply reduction approach, as with a Variac, Power Scaling, Power Dampening, or Sag circuit, the B+ plate voltage available to the power tubes is reduced, producing power-tube distortion at arbitrarily low output power levels -- the entire resulting output wattage is sent directly to the guitar speaker. This removes power-tube wear, prevents blowing an output transformer, prevents overheating and shutdown, and obviates purchasing and transporting a separate, bulky power attenuator in addition to the guitar amp.

[edit] Description

A power attenuator or dummy load is either purely resistive, or mostly resistive and partly reactive. The original guitar-amp power attenuator, the Altair Attenuator, was purely resistive, using a toaster coil with low inductance windings. Other models, such as the Marshall Power Brake, add some electrical inductance or capacitance to the electrical load (including fans, light bulbs and coils). There is debate about whether reactive attenuators do a better job of preserving a guitar amplifier's tone.

Use of an attenuator can potentially damage an amplifier just as using a speaker as a load can, if you are driving the amplifier into heavy distortion. Some production attenuators are the Rockman Power Soak, the Marshall Power Brake, and the THD Hot Plate.