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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

Part Part Number(s) Class Description Used in
12AT7 154-0039-00 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 544 546 547 556 565 570 3A2 75 3A75 1M1 A B C G H K L M N K R S Z
12AU6 154-0040-00 Vacuum Tube (Pentode) RF pentode 81 112 1L10 1L20 1L60 3L10 512 556 575 545 547 549 581 585 A B C G K H L M N O R S Z
5654 154-0206-00 154-0084-00 Vacuum Tube (Pentode) RF pentode B L
6AK5 154-0014-00 154-0206-00 154-0084-00 Vacuum Tube (Pentode) RF pentode B C CA G K L S Z 517 517A 524