For a given semiconductor compound, the maximum operational speed of a transistor is governed pretty much by its gate thickness. Capacitance and impurities along with lithography precision and accuracy are the culprits. Shrinking gate sizes and growing crystals with greater purity has driven operational speeds upward significantly over the years. An equivalent set of issues plagued vacuum tube development a century ago. The physical spacing of grid elements wrt each other as well as to the cathode and plate placed an upper limit on amplification bandwidth. As always, judicious study of the underlying causes led to the development of new designs that, along with improved manufacturing techniques, overcame existing barriers and, also as always, exposed yet a new set of limiting criteria for conquering. That's the way of science and engineering. This advertisement from a 1954 edition of Radio & Television News featured breakthrough development by Western Electric of the 436A vacuum tube for tripling the number of voice calls that could be sent over a single coaxial transmission line.|
Bell Telephone Laboratories AdSplitting Hairs to Speed Calls
This coaxial system electron tube amplifies more voices at the same time because of wider frequency band - made possible by bringing grid and cathode closer together.
Grid is shown above on left. Picture at right, enlarged 15 times, shows how wires are anchored by glass bond. They will not sag despite nearness of red-hot cathode.
To triple the voice-carrying capacity of coaxial cable, Bell Laboratories engineers had to create new amplifying tubes with the grid placed only two-thirds of a hair's breadth from the cathode. Furthermore, the grid wires had to be held rigidly in position; one-quarter of a hair's shifting would cut amplification in half.
Working with their Bell System manufacturing partners at Western Electric, the engineers developed precise optical means for measuring critical spacing insulators. On a rigid molybdenum grid frame they wound tungsten wire three ten-thousandths of an inch thick. To prevent the slightest movement they stretched the wire under more tension for its size than suspension bridge cables, then bonded it to the frame by a new process.
The resulting tube increases coaxial's capacity from 600 to 1800 simultaneous voices - another example of how Bell Telephone Laboratories research helps keep your telephone system growing at the lowest possible cost.
Bell Telephone Laboratories
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Click on the thumbnail to the right to view the original 436A datasheet that is still on the Western Electric website.