The RCA Selectron -- US Patent 2,494,670 Cylindrical Selectron

Jan A. Rajchman, Radio Corporation of America
Applied: 26 April, 1946
Granted: 17 January, 1950

Side view of structure, envelope, 
base and pins


The '670 patent covers the original 4096-bit memory tube which was intended for the Institute for Advanced Study digital computer. '670 describes the memory effect of a bombarded continuous surface having the appropriate secondary electron emissive characteristics. It first describes George W. Brown's "all-of-four" gating logic which allows partial internal address decoding and the resultant need for fewer electrical connections.

The cylindrical topology can be thought of as a digitally addressed Williams-Kilburn storage tube. Unlike the Williams tube, which was a conventional CRT with analog voltages on orthoganally aligned deflection plates steering a single electron beam, the Selectron digitally selected from 4096 simultaneous "beams."

The simplicity of the digital addressing structure was also the downfall of this Selectron topology. Where the Williams tube could sweep the beam over the stored locations at different speeds, paths, and focus for both writing and reading, the Selectron could deliver only a single beam profile and control was limited to presence or absence. This is important in dealing with the tendency of the charge patterns on the storage target to bloom or grow over time. The Williams tube based memories could use various techniques to reduce charge leakage (spreading) over the continuous storage surface. RCA chose to solve this problem in subsequent designs by developing a discrete storage element target to eliminate charge leakage. The time lost in this additional research and development gave the Williams tube an advantage -- availability -- and therefore, many design wins in early computer storage systems.

US Patent 2,494,670 images (OCR'd) 577 kB
US Patent 2,494,670 images (OCR'd) 577 kB

US Patent 2,494,670 text (OCR'd) 119 kB
US Patent 2,494,670 text (OCR'd) 119 kB



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