So many great guitar tones are the result of valve power amplifier distortion. Sometimes the power amplifier is the only source of distortion and other times it is in addition to distortion created in the pre amplifier or by a pedal. Sometimes the distortion can be from an amplifier that is heavily overloaded and on other occasions just a slight enhancement to an otherwise clean tone. There are so many great tones that are wildly varied but all the result in some degree to the overloading of a valve power stage.
It's a complex process when power amp clipping occurs - the interaction of the phase splitter with the power valves, the instantaneous changes in impedance of the power valves when they go non-linear, and the filter effects of the output transformer and speaker which are impedance-sensitive. The Variable Headroom feature on 633 amplifiers is designed to exploit these phenomena to the max allowing the player to drive the power valves well into saturation at a selection of power levels, without affecting the crucial interaction between the power amplifier and the speaker, and consequently maintaining the 'big' tone.
Many players who like to work this way use a selection of amplifiers of various sizes to cope with different types of venues or bands. For example a player may use a Fender Princeton for a small gig but for a bigger venue where he or she needs more volume he may take a Deluxe Reverb or even a Super Reverb. Or they may use a Marshall 18w for a small gig and a JCM800 for a bigger one. 633 Engineering's Variable Headroom feature means the player can effectively have a selection of amplifiers in one box. The five position switch offers five output power levels and three different power amplifier topologies. This feature is not to be confused with simple power scaling and there is no tone-robbing attenuator between the amplifier and the speaker.
Whether it be just an extra bit of sparkle, hair or edginess to the tone, some mild compression, classic rock crunch or sweet sustaining overdrive that's desired from pushing the amp hard, Variable Headroom let's you get it at a volume to suit any situation.