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
Telephone Laboratories Ad
Splitting Hairs to Speed Calls
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.
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.
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
Improving telephone service for America
provides careers for creative men in scientific and technical fields
Click on the thumbnail to the right to view the original
436A datasheet that is still on the Western Electric website.