1A4
The 1A4 is a 1-series vertical plug-in with four input channels and 50MHz bandwidth. The four channels are grouped into two groups of two channels. The 1A4 has a special "trace slaving" mode which, when used in a 547 mainframe, allows channels 1 and 2 to be displayed using the A time base, and channels 3 and 4 to be displayed with the B time base. This allows high-speed and low-speed signals to be displayed together, similar to what a dual-beam scope can do, but without some of the cost and complexity.
The signal path of each channel of the 1A4 is as follows. First the signal passes through the attenuators. Then it is clamped by two diodes and applied the the gate of a JFET source follower. The source of the source follower is the trigger pickoff point. From there the vertical signal path goes through an NPN emitter follower followed by an NPN differential amplifier, at which point the signal becomes differential. Next, the signal passes through an analog multiplexer made with 1N4152 silicon switching diodes. After the multiplexer network, the signal passes through a common base amplifier followed by 6DJ8 tube operating in common grid configuration.