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Tektronix 2A61
Differential amplifier plug-in
Tektronix 2A61 Front

Compatible with 560-series scopes

Produced from 1963 to (?)

(All manuals in PDF format unless noted otherwise)
Manuals – Specifications – Links – Pictures

The Tektronix 2A61 is a high gain differential AC amplifier plug-in for the 560 series oscilloscopes, introduced in 1963. It has independently switchable low frequency and high frequency limits. The low frequency limit can go well below 1 Hz, but it can't go to DC.

Key Specifications

Input impedance 1 MΩ // ~100 pF
Deflection 10 μV/div to 20 mV/div in 1−2−5 sequence
Bandwidth Switch selectable:
Low Limit: ~0.06 to ~600 Hz
High Limit: ~60 Hz to ~0.3 MHz
CMRR 50,000:1 at Frequencies below 10 kHz
Common Mode Range -5 V to 5 V in A-B mode
Input DC Bias Max: ±0.1 V differential
Line Filter Switchable: at Least 50:1 attenuation


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 a PC06 8-4P / MS3106 8-4P. The standard cable for connecting the input signal to the 2A61 is the 012-072.