21 (plug-in)

Template:Plugin Sidebar 2 The Tektronix Model 21 and successor 21A are time base plug-ins for the 555 dual-beam scope.

Type 21 is suggested as the upper beam time base, whereas sister model Type 22(A) is for the lower beam as it contains delayed trigger circuitry, and only the lower beam horizontal section is provided with delayed trigger pickup circuitry. The Delay multiplier control pot is also provided on the lower beam section.