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Actually, there never was a Type C, but that name is sometimes used as shorthand

for the official "Type 53C" and "Type 53/54C" nomenclatures.

The Tektronix Type 53C is a dual-trace plug-in for 500-series scopes. It was introduced along with the 531 and 535 mainframes in 1954. It is very possibly the first dual-trace scope or plugin ever.


In the 53x scopes (Tek never documented its performance in the 540's), its bandwidth is 8.5 MHz, and like most letter-series plug-ins, its maximum vertical sensitivity is 50 mV/div. Unlike the Type A and Type B, the Type C had the ability to invert the signal before display.

Type 53C was replaced by Type 53/54C in 1956 when the 541 and 545 were introduced. The Type 53/54C's bandwidth is 24 MHz in those scopes. In 1959 it was replaced by Type CA.


At 15 tubes, the Type 53/54C and Type CA have the highest tube count of any letter-series plug-in. Each channel consists of a 6AK5 cathode-follower driving a 12AU6 differential pair which drives a 6AU6 differential pair. The two channels converge on a 12AT7 cross-coupled cathode-follower. Four more tubes, a 6AL5 and three 12AT7's, comprise a logic circuit which determines which channel drives the output at any given moment. The options are A, B, CHOPPED, and ALT.