For amplifiers with a Master Volume (MV) Control, the MV knob is used to control the overall volume level of the amplifier, when an attenuator is not being used. Often when using a MV amplifier at low volume levels (without an attenuator), the overdriven tone is generated mostly from the preamp section and not from the power tube section of the amplifier. An attenuator allows for the overdriven/distorted tone to be generated in both the preamp and power tube sections of the amplifier, allowing for a fuller tone.
When an attenuator is used, the attenuator controls the overall volume level. This frees up the MV Control, allowing the user to control the overdrive in the various sections of the amplifier.
The Channel Volume knob controls the signal level through part of the preamp section of the amplifier. The MV knob controls the signal level going into the power tube section. The MV Control circuit is always prior to the power tube section, regardless if the MV circuit is post phase inverter (PPIMV) or not.
The Channel Volume knob is used to control the desired amount of preamp overdrive/distortion, the Master Volume knob controls the desired amount of power tube overdrive/distortion. The ARACOM attenuator is used to control the overall speaker cabinet volume level.
There is a benefit to backing off the MV Control a bit, instead of having it on "10". When using your MV amplifier with or without an attenuator, with the MV Control turned down a bit, there is less current going through the power tubes and therefore, the power tubes are generating less heat. The power tubes are stressed less and therefore, they might last longer. There is also less stress to the output transformer and power supply section of the amplifier.
Fil at Solo Dalas using his ARACOM PRX150 with his vintage Marshall Master Volume Controlled Amp and his vintage Gibson guitars. Fil shows examples of using the attenuator with different Master Volume and PreAmp levels on his vintage Marshall.
