Power attenuator (guitar)

In electric rock guitar, attenuators are used to dissipate some or all of the amplifier's power in the attenuator's built-in, mostly resistive dummy load instead of letting that power drive the speaker, in order to silence or reduce the output volume.

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 power
* Adding line-level effects between a guitar amplifier and a guitar speaker.

With guitar amplifiers, power-tube distortion is often the desired sound but it can only be achieved with the amp running at or near maximum power, and attenuators are frequently used to reduce the actual sound from the speakers to more practical levels while allowing the amp circuitry to still run at full volume.

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-soaker approach and the power-supply reduction approach. In the power-soaker approach, the power tubes are pushed into their maximum possible power output, and then the unwanted resulting power 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 power to the guitar speaker. In the power-supply reduction approach, as with a Variac, Power Scaling, 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 power 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.

Power Dampening, while sounding like a power attenuator method, is not actually power attenuation. This method of output level control involves replacing the phase inverter tail resistor with a potentiometer and resistor in series; varying the potentiometer causes a limitation of the phase inverter signal peak-to-peak voltage, causing reduced output, similar to a Post Phase Inverter Master Volume (PPIMV) control.

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.

If the amplifier is being driven to heavy overdrive, use of an attenuator will neither increase nor reduce the potential of damage to the amplifier. Some production attenuators are the Rockman Power Soaker, the Marshall Power Brake, THD Hot Plate, and Weber MASS [http://www.tedweber.com/atten.htm] .

See also

*Guitar amplifier
*Attenuator (electronics)
*Distortion (guitar)
*Isolation cabinet (guitar)
*Soundproofing

External links

* [http://www.amptone.com/powerattenuatorfaq.htm Amptone's Guitar amp power attenuator FAQ]
* [http://amps.zugster.net/articles/attenuation Adam's amps - Basic attenuator circuits]
* [http://londonpower.com London Power] - Power Scaling circuit design book and kits
* [http://www.rockman.fr/Story/SRD.htm Rockman.fr] - Tom Scholz's Power Soak