The Tektronix Type A is a single-channel preamp introduced in 1959 for 500-series scopes. A 545 with a Type A has a system risetime of 18 nanoseconds. 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 12AU6 tubes. The second stage drives the output stage, a pair of cathode followers made of both halves of at 12AT7 tube. The plate of each output tube is connected to the cathode of the other output tube by a 0.005 microfarad capacitor. This is intended to improve high-frequency balance.