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The Tektronix Type L is an amplifier plug-in for 500-series scopes.

The Type L is a Type K plus a 10x-gain AC-coupled preamp that extends the maximum sensitivity to 5 mV/div. Bandwidth in a 545 is 30 MHz at 50 mV/div and above, 3 Hz-24 MHz at 20 mV/div and below.

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.


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The input is switched via a Frequency compensated attenuator network which is controlled by the Vols/CM Switch. Input is then coupled to first cathode follower V5832, a 5654/6AK5W RF Pentode. This is configured as a cathode follower. The screen supply of this cathode follower is switched from +100 to a decoupled network from +225V rail when the X10 Mode is selected.

The output of input amplifier is then switched via a 10X Amplifier - if the X10 mode is selected in the front panel - or directly to the paraphase or phase inverter 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 10X amplifier stage is built with 3 5654/6AK5W tubes. First two are configured as amplifier and the last one as output cathode follower.

Heater for the tubes are derived from +75VDC heater supply available at pin 15 of the connector. This is to avoid any interference/hum from the regular AC heater supply at 60Hz.