The Tektronix Type 191 Constant Amplitude Signal Generator is a sine wave source introduced in 1966.
The 191 covers frequencies from 350 kHz to 100 MHz in seven bands. The output amplitude can be adjusted in calibrated steps from 5 mV to 5 V in three ranges.
The oscillator is a Colpitts oscillator using a 7119 tube.
A peak-to-peak detector made with two GaAs diodes measures the swing of the oscillator and produces a DC feedback signal that controls a series voltage regulator that supplies power to the oscillator.
The calibration procedure in the manual makes use of a 661 to accurately view the output waveform.