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Tek 112 front.jpg

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 112 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 a differential output intended for directly driving the vertical deflection plates of a CRT. The output terminals of the 112 are on the right side in the rear, so the 112 can sit to the left of an oscilloscope and short wires can connect the 112 to the oscilloscope's CRT. A two-position switch on the rear panel of the Type 112 controls the common-mode output voltage. In one position, the common-mode output voltage is +150 V, which is the plate voltage of the 6AG7 tubes of the output amplifier. In the other position, the common-mode output voltage is 0 V. The shifting of the common-mode output voltage 150 V downward is accomplished by enabling a pair of 680 μA DC current sinks that cause a voltage drop on 220 kΩ series output resistors. 6CB6 pentodes form the current sinks, with their cathodes tied to -150 V. The 220 kΩ output resistors also provide short circuit protection. The Type 112 manual recommends 0 V for driving the CRT of the 511, and +150 V for driving the CRT of the 512, 513, or 514.

Internal photos needed.

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