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The Tektronix Type A is a single-channel preamp introduced in 1959 for 500-series scopes. It provides two inputs and has a switch to select between them. A 545 with a Type A has a system risetime of 18 nanoseconds and a bandwidth of 20 MHz.
The plug-in uses a 12AU6 in the front end as a cathode follower. This input stage drives the second stage, a differential amplifier made of two more 12AU6 tubes. The second stage drives the output stage, a pair of cathode followers made of both halves of a 12AT7 tube. The plate of each output tube is connected to the cathode of the other output tube by a 5 nF capacitor. This "cross-coupling" extends the high-frequency response.
Although 1959 was the first year one could find a "Type A" in the catalog, the same instrument with earlier "Type 53/54A" nomenclature was in the August 1956 Short Form catalog, and its slower Type 53A ancestor was in the August 1954 catalog which introduced the first 500-series scopes, the 531 and the 535. Type 53A has only three tubes, and a bandwidth of 10 MHz in the 530 mainframes. Tek never documented its performance in the 540 mainframes.
Type A was the first letter-series plug-in to be discontinued, in 1962. Customers were told to get Type B instead, although it cost more.