Type 1A7 is a high-sensitivity differential plugin for 500-series scopes. It senses down to 10uV/div, either AC or DC-coupled. It's possible to apply up to +/-300mV offset to the reading while maintaining sensitivity. Upper and lower cutoff frequency is selectable on the front panel. Maximum bandwidth is 500kHz, and common-mode rejection is 50,000 to 1.
Type 1A7's front end is a matched quad of 7586 nuvistors, with two in parallel - to reduce noise - on each input. These tubes are in temperature-equalizing housings, and mounted on a rubber-isolated subchassis. The "coarse" balance adjustment controls the relative heater voltages on the 7586's.
Type 1A7 was introduced in 1966. In 1968 it was superseded by the FET-equipped Type 1A7A, which also doubled the bandwidth and the common-mode rejection. Type 1A7A remained available until the end of the classic plugin scope line after 1974.