The Tektronix 123 is a miniature preamplifier introduced in 1955. The gain is 100, input impedance is 10 MΩ, and output impedance is 31 kΩ. Frequency response is from 3 Hz to 25 kHz.
The 123 uses two CK512AX pentodes for amplification and is powered by batteries.
Although the 123 is an AC amplifier, the output is taken directly from the plate of one of the tubes, without an output coupling (DC block) capacitor. So the output signal rides on about 15 volts of DC.
The intended use of the 123 is as a preamplifier for other instrumentation such as an oscilloscope, and the oscilloscope's input coupling can simply be set to "AC".