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The 551 shown here is one of the largest Teks. It comes with a separate power supply that holds a full 5 6080's and supplies the 551 mainframe with regulated DC power. The 551 is a true dual beam instrument, that can hold two plugins, and has a bandwidth of about 24 MHz. When equipped with two 4-channel plugins, 8-channel displays are a real possibility, though probably of limited use, with less than 1 cm of vertical deflection per channel.
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The Tektronix Type 551 is a DC-to-27 MHz dual-beam oscilloscope. It features two vertical-deflection systems, common horizontal deflection, 4 x 10cm display per beam, and it accepts multi-trace, differential, sampling and spectrum analyzer plug-in units.
It uses a Tektronix two-gun cathode-ray tube with two pairs of vertical-deflection plates. A single pair of horizontal-deflection plates is common to both electron beams. The two wide-band main amplifiers in the Type 551 are designed for Tektronix Letter-Series and 1-Series Plug-In Units, providing a high degree of signal handling versatility in both channels. Both electron beams are simultaneously deflected horizontally at any one of many sweep rates provided by an accurately-calibrated time base generator.
Tektronix Catalogue, Oscilloscopes and Associated Instruments, 1968