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.
|1 MΩ // ~100 pF
|10 μV/div to 20 mV/div in 1−2−5 sequence
Low Limit: ~0.06 to ~600 Hz
High Limit: ~60 Hz to ~0.3 MHz
|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
|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.
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.
Schematic (early version)
Schematic (late version)