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The Tektronix 112 is a DC-coupled differential amplifier introduced in 1950. The voltage gain can be set from 0.5 to 5000 V/V. The frequency response is DC to 2 MHz when the gain is set below 166, and DC to 1 MHz for gain settings greater than that. The 121 is intended for use with the 511, 512, 514, and 524 oscilloscopes. The input impedance is 1 MΩ in parallel with 47 pF. The output impedance is 8000 Ω. The amplifier's schematic and response are identical to that of a Tek 512.

The first stage is a differential amplifier using a pair of 5879 pentodes. Next is a differential amplifier using a pair of 12AU6 pentodes.

The 112 has an unusual output coupling stage. It provides output short circuit protection and shifts the common-mode DC level of the differential output to 0 V. The plate voltage of the final gain stage (just before the output coupling stage) is about +150 V. When enabled, the output coupling stage is a a pair of constant current sinks. It uses 6CB6 pentodes for this. This shunt current causes a constant voltage drop across output resistors, resulting in 0 V at the output. The cathodes of the output coupling stage are supplied by the -150 V rail, allowing them to operate as proper current sources when their plates are at 0V.

Internal photos needed.

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