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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. 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.

Diode D5972 on the first 5654/6AK5W amplifier V5942 tube's cathode is used to protect the cathode bypass capacitor C5972. C5972 is a 500 μF/6V cathode bypass Capacitor. In case someone pulls out V5942 and power on the plug-in, the cathode bias of -150V will come across C5972 and can potentially destroy it. This diode is used to prevent such scenarios and to drive the voltage to ground via R1958 (12K) if such condition occurs.

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.