The early Marshall amps, "Plexis" and "Metal Panels" had a volume knob, but no master volume knob. These amps had to be turned up really loud to produce a Rock and Roll overdrive sound. Around the mid-70s Marshall introduced the JMP Master Volume followed by the JCM 800 in the early 80's. These amps did featured a master volume knob so they could produce the rock sounds of their predecessors but at more considerate volume levels. During the 80s, rock guitarists wanted an even heavier and more saturated sound. This was achieved by "Modding" or "Hot rodding" the amp by making changes and/or additions to the amp's circuit. Hence the term "Hot Rodded Plexi" was born.