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Tektronix 2A61
Differential amplifier
Tek 2A61 Front

Compatible with 560-series scopes

Produced from 1963 to (?)

Manuals – Specifications – Links – Pictures

The Tektronix 2A61 is a differential amplifier plug-in for 560-series scopes, introduced in 1963.


The deflection factor ranges from 10 μV/div to 20 mV/div.

There are switch-selectable high-frequency and low-frequency cutoffs.

At 20 mV/div, the minimum low-frequency cutoff is 0.06 Hz and the maximum high-frequency cutoff is 300 kHz.

The common mode rejection ratio is 50,000:1 at frequencies below 10 kHz. The input impedance at each input is 10 MΩ in parallel with 100 pF. The common mode range is +/- 5 V.

The 2A61 has a line frequency filter that can be switched in our out of the signal path. It is specified to attenuate line-frequency signal components by at least 50:1.


The front end consists of a differential amplifier. Up to serial number 986, the front-end differential amplifier used a pair of 5842 tubes biased with a tail current of 10 mA and a plate-to-cathode voltage of about 100 V.

Nuvistor-based front-end amplifier

Starting with serial number 987, the front-end differential amplifier was made with four 7586 nuvistor triodes: V424, V425, V524, and V525.

The parallel combination of V424 and V425 forms one side of the differential amplifier and the parallel combination of V524 and V525 forms the other side. However, for purposes of thermal stability, V424 is thermally connected to V524 and V425 is thermally connected to V525. The tail current in the 987-and-up version is about 16 mA and the nuvistors have a plate-to-cathode voltage of about 70 V.

The second gain stage is a differential amplifier made of a pair of a pair of germanium PNP transistors. To implement the low-frequency cutoff, the 2A61 has a large electrolytic capacitor, C437 (13,000 μF or 20,000 μF depending on serial number), in the emitter frequency compensation network of the second differential amplifier stage.

Although C437 is polarized, it is operated with no DC bias and therefore sees reverse voltage as part of normal operation. D437 and D438 shunt C437, preventing it from ever seeing a voltage more than one diode drop. Most electrolytic capacitors are specified to handle reverse voltages of at least 1 V without damage.

The 2A61 manual's calibration instructions recommend using the Heathkit IG-72 Audio Generator.

A suitable input connector for the 2A61 is PC06 8-4p / MS3106 8-4p.