The Tektronix Type 1A5 is a differential amplifier plug-in for 500-series scopes. It was designed by Bill DeVey after consultation with John Kobbe.

Tektronix 1A5
50 MHz Differential Amplifier
Tektronix 1A5 Differential Amplifier

Compatible with 500-series scopes

Produced from 1967 to 1975


It has an Amphenol 165 series connector on the front panel for direct connection of the Type P6046 Active Differential Probe.

Type 1A5 was introduced in 1967, replacing Types W and Z, and lasted until the end of the 500-series scope line.

Key Specifications

Bandwidth 50 MHz down to 5 mV/div, 40 MHz at 1 mV/div
Deflection 1 mV/div to 20 V/div, 1−2−5
Common-mode rejection 20,000:1 from DC to 100 kHz, 10,000:1 up to 1 MHz, 200:1 at 20 MHz

Like most Tektronix differential plug-ins, the 1A5 has three modes: VA−VC, VA−VB, and VC−VB.

VC is controlled by a ten-turn potentiometer and can vary from −5 V to +5 V (0.5 V on the 5 V/div to 20 V/div ranges).


The 1A5 is solid-state except for a pair of 7586 nuvistors used in the upper half of a cascode in the second amplifier stage.

The attenuator switching is mechanically complex. A gear mechanism drives the input attenuator switch (under the internal cover) selecting attenuators ×1000 (20 V/div to 5 V/div), ×100 (2 V/div to 500 mV/div), ×10 (200 mv/div to 50 mV/Div) or no attenuators (20 mV/Div to 1 mV/div).

Additionally, in the 500 mV/div and 50 mV/div positions, the operator can pull the V/Div knob outward when switching to one of the next two more sensitive ranges, in which case the higher attenuator (and higher input and comparison voltage range) remains in place.



Switches detail


Some Parts Used in the 1A5

Part Part Number(s) Class Description Used in
7586 154-0306-00 Vacuum Tube (Triode) Nuvistor triode M 1A1 1A2 1A5 10A2 10A2A 11B1 11B2A 321 321A 3A1 3A1S 3A3 3A5 3A6 3A7 3A8 3A74 3S76 3T77 3T77A 3B5 4S1 4S2 6R1 6R1A 9A1 9A2 82 86 S-311