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Tektronix 1A4
50 MHz quad input amplifier plugin
Tektronix 1A4 front

Compatible with 500-series scopes

Produced from 1967 to 1974

Manuals – Specifications – Links – Pictures

The Tektronix Type 1A4 is a 1-series vertical plug-in for 500-series scopes. It has four input channels and 50 MHz bandwidth.

Type 1A4 was introduced in 1967 and lasted until the end of the 500-series scopes line.

The four channels are grouped into two groups of two channels. Type 1A4 has a special "trace slaving" mode which, when used in a 547 mainframe, allows channels 1 and 2 to be displayed using the A time base, and channels 3 and 4 to be displayed with the B time base. This allows high-speed and low-speed signals to be displayed together, similar to what a dual-beam scope can do, but without some of the cost and complexity.

Like the Type 1A2 and unlike the Type 1A1, the Type 1A4 has a Trigger Output selector that controls which channel provides triggering when the mainframe trigger is in Plug-In mode, for scopes having this feature (544, 546, 547, 549, 556, late 555).

Key Specifications

Plug-in Bandwidth DC to 50 MHz
Maximum Input Sensitivity 10 mV/div
Rise time 7 ns
Input Impedance 1 MΩ // 20 pF
Maximum Input Voltage 600 V combined DC and peak AC
Signal Output Impedence 50 Ω

Cascaded operation using the Signal Out jack can achieve a sensitivity of approximately 200 μV/div AC with about 20 Hz - 1 MHz bandwidth.


The signal path of each channel begins with the signal passing through the attenuators. Then it is clamped by two diodes and applied to the gate of a JFET source follower. The source of the source follower is the trigger pickoff point. From there the vertical signal path goes through an NPN emitter follower, then an NPN differential amplifier, at which point the signal becomes differential. Next, the signal passes through an analog multiplexer made with 1N4152 silicon switching diodes.

After the multiplexer network, the signal passes through a common base amplifier followed by 6DJ8 tube operating in common grid configuration.

There is an "identify" switch for each channel, which when pressed shifts the trace up by ~3 mm, to aid identification of the corresponding signal/channel. This is achieved by offsetting position control circuitry with a small resistance to shift position.

The 12.1 V and −15 V power for the plug-in is generated onboard using a step-up transformer, whose primary is connected to the 6.3 V heater supply from the mainframe and the two secondaries drive the onboard series regulators.



Some Parts Used in the 1A4

Part Part Number(s) Class Description Used in
6DJ8 154-0187-00 154-0305-00 Vacuum Tube (Dual Triode) dual triode 132 161 310A 316 317 502 502A 503 504 506 515 516 519 526 529 RM529 533 535 536 543 544 545 545A 545B 546 547 549 555 556 561A 561S 564 565 567 581 581A 585 585A 661 1A4 1S1 60 2A60 63 2A63 67 2B67 3A1 3A1S 3A2 3A3 3A6 3A7 72 3A72 75 3A75 4S2 51 3B1 3B1S 3B2 3B3 3B4 3M1 3S76 3T77 3T77A 9A1 9A2 1121 80 81 82 86 B O W Z Telequipment D56 Telequipment S32A Telequipment D52 S-311 Telequipment TD51 Telequipment S52 Telequipment S51 Telequipment Type A TU-4