Abstract: A string instrument particularly such as a guitar is provided with an improved vibrato assembly to obtain a vibrato/tremolo effect, and further including improved tuning adjustment mechanisms for maintaining the guitar in tune and for rapid adjustment to one of several preset tuning configurations. The vibrato assembly comprises an improved tuning machine and an improved bridge unit respectively on the head and body of the guitar with the guitar strings extended therebetween. The tuning machine and bridge unit support the guitar strings independently with each string under selected tension at a primary tuning set position. At the bridge unit, the strings are supported by individual rocker arms engaged by respective cams which are movable together by a manual spring-loaded control lever to achieve the vibrato/tremolo effect with raised or lowered pitch, as desired, with the cams displacing the rocker arms and thus altering tension individually for each guitar string.