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The Tektronix 2A61 is a differential amplifier plug-in for [[560-series scopes]].
{{Plugin Sidebar
It was [[introduced in 1963]].
| manufacturer=Tektronix
The deflection factor ranges from 10uV/div to 20mV/div.
| series=560-series scopes
There are switch-selectable high-frequency and low-frequency cutoffs.
| type= 2A61
At 20mV/div, the minimum low-frequency cutoff is 0.06Hz and  
| summary = Differential amplifier plug-in
the maximum high-frequency cutoff is 0.3MHz. The common mode rejection ratio
| image=Tek 2a61 front.jpg
is 50,000:1 at frequencies below 10kHz.  The input impedance at each input is
| caption=Tektronix 2A61 Front
10 megohms in parallel with 100 pF.  The common mode range is +/- 5V. 
| designers=Larry Mayhew
The 2A61 has a line frequency filter that can be switched in our out of the signal path.
| introduced=1963
It is specified to attenuate line-frequency signal components by at least 50:1.
| discontinued=(?)
| manuals=
* [[Media:070-0328-01.pdf|Tektronix 2A61 Manual]] (OCR)
* [[Media:2a61 1000-1 cal input adapter.pdf|Tektronix 2A61 1000:1 Cal Div/Input Adapter (OCR)]]
}}
The '''Tektronix 2A61''' is a high gain differential AC amplifier plug-in for the [[560-series scopes | 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.


{{BeginSpecs}}
{{Spec | Input impedance | 1 MΩ // ~100 pF }}
{{Spec | Deflection | 10 μV/div to 20 mV/div in 1−2−5 sequence }}
{{Spec | Bandwidth | Switch selectable:<br>Low Limit: ~0.06 to ~600 Hz<br>High Limit: ~60 Hz to ~0.3 MHz }}
{{Spec | CMRR | 50,000:1 at Frequencies below 10 kHz }}
{{Spec | Common Mode Range | -5 V to 5 V in A-B mode}}
{{Spec | Input DC Bias | Max: ±0.1 V differential }}
{{Spec | Line Filter | Switchable: at Least 50:1 attenuation }}
{{EndSpecs}}
==Internals==
The front end consists of a differential amplifier.
The front end consists of a differential amplifier.
Up to serial number 986, the front-end 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.
used a pair of [[5842]] tubes biased with a tail current of 10mA and a  
plate-to-cathode voltage of about 100V.


[[File:2a61 frontend.png|thumb|left|upright|Nuvistor-based front-end amplifier]]
[[File:2a61 frontend.png|thumb|left|upright|[[Nuvistor]]-based front-end amplifier]]
Starting with serial number 987, the front-end differential amplifier
Starting with serial number 987, the front-end differential amplifier was made with four [[7586]] [[nuvistor]] triodes: V424, V425, V524, and V525.
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
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.
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.
However, for purposes of thermal stability, V424 is thermally connected to V524
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.
and V425 is thermally connected to V525. The tail current in the 987-and-up version is about 16mA
and the nuvistors have a plate-to-cathode voltage of about 70V.


The second gain stage is a differential amplifier made of a pair of a pair of germanium PNP transistors.
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,  
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),  
the 2A61 has a large electrolytic capactitor,  
in the emitter frequency compensation network of the second differential amplifier stage.   
C437 (13,000uF or 20,000uF 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 1V without damage.


The 2A61 manual's calibration instructions recommend using the Heathkit IG-72
Although C437 is polarized, it is operated with no DC bias and therefore sees reverse voltage as part of normal operation.
Audio Generator.
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.
* [http://bama.edebris.com/download/tek/2a61/2a61.djvu Tektronix 2A61 Manual (DjVu)]
* [http://w140.com/2a61.pdf Tektronix 2A61 Manual (PDF)]


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]].


==Pictures==
<gallery>
<gallery>
File:Tek 2a61 front.jpg|Front  
Tek 2a61 front.jpg | Front  
File:Tek 2a61 left.jpg|Left
Tek 2a61 left.jpg | Left
File:Tek 2a61 right.jpg|Right
Tek 2a61 right.jpg | Right
File:Tek 2a61 top.jpg|Top
Tek 2a61 top.jpg | Top
File:Tek 2a61 bottom.jpg|Bottom
Tek 2a61 bottom.jpg | Bottom
Image:Tek 2a61 1.jpg
Tek 2a61 1.jpg
Image:Tek 2a61 2.jpg
Tek 2a61 2.jpg
Image:Tek 2a61 3.jpg
Tek 2a61 3.jpg
Image:Tek 2a61 4.jpg
Tek 2a61 4.jpg
Tek-2a61 early.png | Schematic (early version)
Tek-2a61 late.png | Schematic (late version)
</gallery>
</gallery>
[[Category:560 series plugins]]

Latest revision as of 07:03, 10 March 2024

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

Compatible with 560-series scopes

Produced from 1963 to (?)

Manuals
(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

Internals

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.

Pictures

  • Front

    Front

  • Left

    Left

  • Right

    Right

  • Top

    Top

  • Bottom

    Bottom

  • Schematic (early version)

    Schematic (early version)

  • Schematic (late version)

    Schematic (late version)

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