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Tektronix Type L
High sensitivity amplifier plugin
Tektronix Type L

Compatible with 500-series scopes

Produced from 1957 to 1973

(All manuals in PDF format unless noted otherwise)
Manuals – Specifications – Links – Pictures

The Tektronix Type L is an amplifier plug-in for 500-series scopes. It was designed by Ron Olson.

The Type L is a Type K plus a 10x-gain AC-coupled preamp that extends the maximum sensitivity to 5 mV/div.

Type 53/54L was introduced in 1957 along with the 536 and its companion 53/54T sweep plug-in. It was renamed the Type L in 1959. It was the fastest 5 mV plug-in until the 1A1 in 1964. It was dropped after 1972.

K, L, and T are tied for longest production life, at 16 years. B, D, and G are next at 15 years.

Late versions have a BNC connector instead of a UHF connector.

Lavoie Laboratories produced an unauthorized clone, the LA-265-L.

Key Specifications

Bandwidth 30 MHz at 50 mV/div and above, 3 Hz – 24 MHz at 20 mV/div and below (in a 545)
Rise time ≤6 ns at 50 mV/div and above, ≤10 ns at 20 mV/div and below
Deflection 5 mV/Div to 20 V/Div, 1–2–5


The input signal traverses a frequency-compensated attenuator network controlled by the Volts/cm switch, and is then coupled to the first cathode follower tube (V5832), a 5654/6AK5W RF pentode. The screen supply of this cathode follower is switched from +100 V to a decoupling network fed from the +225 V rail when the ×10 Mode is selected.

The output of input amplifier is then switched via a ×10 amplifier (if ×10 mode is selected), or directly to the phase inverter stage made up of two 12AU6 tubes. This stage converts the input signal to a differential push-pull output. This output is passed to the final output drivers, which are two cathode follower sections with a pair of 12AT7 tubes. The differential output is then passed to pin 1 and 3 of the connector to drive the vertical amplifier inside the chassis.

The ×10 amplifier stage is built with three 5654/6AK5W tubes, the first two of which are configured as amplifiers, and the last one as an output cathode follower.

The diode D5972 on the first 5654/6AK5W amplifier tube's cathode (V5942) is used to protect the cathode bypass capacitor C5972, a 500 μF/6 V electrolytic, in case someone would pull out V5942 with power applied, in which case the cathode bias of −150 V would come across C5972 and destroy it.

Heater supplies for the tubes are derived from the +75 VDC supply available at pin 15 of the mainframe interface, to avoid any mains interference/hum from the regular AC heater supply.


Some Parts Used in the L

Page Class Description Used in
12AT7 Vacuum Tube (Dual Triode) dual high-gain triode 161 180 310 310A 315 316 360 502 502A 511A 512 513 513D 514 514AD 514D 516 524 529 RM529 544 546 547 556 3A2 3A75 1M1 A B C G H K L M N K R S Z
12AU6 Vacuum Tube (Pentode) RF pentode 81 112 512 556 575 545 549 581 585 A B C G K H L M N O R S Z
5654 Vacuum Tube (Pentode) RF pentode B L
6AK5 Vacuum Tube (Pentode) RF pentode B C CA G K L S Z 517 517A 524