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discontinued=(?) | | discontinued=(?) | | ||
manuals= | manuals= | ||
* [http://bama.edebris.com/download/tek/2a61/2a61.djvu Tektronix 2A61 Manual (DjVu) | * [[Media:070-0328-01.pdf|Tektronix 2A61 Manual]] (PDF) | ||
* [http://bama.edebris.com/download/tek/2a61/2a61.djvu Tektronix 2A61 Manual @ BAMA] (DjVu) | |||
* [[Media:2a61 1000-1 cal input adapter.pdf|Tektronix 2A61 1000:1 Cal Div/Input Adapter (PDF, needs OCR)]] | * [[Media:2a61 1000-1 cal input adapter.pdf|Tektronix 2A61 1000:1 Cal Div/Input Adapter (PDF, needs OCR)]] | ||
}} | }} | ||
The '''Tektronix 2A61''' is a differential amplifier plug-in for [[560-series scopes]] | The '''Tektronix 2A61''' is a differential amplifier plug-in for [[560-series scopes]], [[introduced in 1963]]. | ||
==Specifications== | ==Specifications== | ||
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There are switch-selectable high-frequency and low-frequency cutoffs. | There are switch-selectable high-frequency and low-frequency cutoffs. | ||
At 20 mV/div, the minimum low-frequency cutoff is 0.06 Hz and | At 20 mV/div, the minimum low-frequency cutoff is 0.06 Hz and the maximum high-frequency cutoff is 300 kHz. | ||
the maximum high-frequency cutoff is 300 kHz. | |||
The common mode rejection ratio is 50,000:1 at frequencies below 10 kHz. | The common mode rejection ratio is 50,000:1 at frequencies below 10 kHz. | ||
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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 10 mA and a | |||
plate-to-cathode voltage of about 100 V. | |||
[[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 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. | 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 capacitor, | in the emitter frequency compensation network of the second differential amplifier stage. | ||
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 | Although C437 is polarized, it is operated with no DC bias and therefore sees reverse voltage as part of normal operation. | ||
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. | ||
D437 and D438 shunt C437, preventing it from ever seeing a voltage | Most electrolytic capacitors are specified to handle reverse voltages of at least 1 V without damage. | ||
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 | The 2A61 manual's calibration instructions recommend using the Heathkit IG-72 Audio Generator. | ||
Audio Generator. | |||
A suitable input connector for the 2A61 is PC06 8-4p / MS3106 8-4p. | A suitable input connector for the 2A61 is PC06 8-4p / MS3106 8-4p. |