1960, Radio Corporation of America launched in the world market a
new type of thermionic valve made in ceramic material, rather than
the classic glass and mica components. Such new valve was very small
in size circa 2 cm and was christened as NUVISTOR.
This kind of thermionic device was an effort of the electronic industry
to compete with the then-blossoming TRANSISTOR, whose development
started in 1955 under the surpevision of George Rose working at RCA
It was known by the valve industry from the beginning that the valves
dimensions were oversized since the manufacturing process was dependent
of the available technology.
In this way, when a valve structure is proportionally scaled down
in size, some characterisitics remain constant, others improve, and
still others degenerate.
For example, while the mutual transconductance and plate resistance
remain substantially the same, the cathode efficiency and high frequency
By other hand cathode current density, however, increases and the
grid and plate operate at much higher temperatures tend to act adversely
on the valve life and reliability.
Thus, it was discovered if the valve inner elements were spaced down
these side effects could be offset and in the same time the use of
much lower electrodes voltage could improve even more the cathode
life, as well as reducing the high-voltage insulation required in
the valve, the socket, and associated circuit elements.
This differential scaling made possible also more efficient thermal
eviroment among the valve inner elements. Thus while the heat should
be kept between the cathode and its frame assembly, it is very important
that the grid and the plate should be made of high thermal conductivity
materials to stay cool and in the same time avoiding the production
The NUVISTOR revolutionized the art of valve making by minimizing
or even eliminating the defects that had plague the industry in such
away it became "state of art".
During the researches many technologies were investigated. For example:
the planar, cylindrical and ceramic spaced stacked structures; metal-to-glass
seals, metal-to-ceramic seals.
Finally the cylindrical concetric open-ended cantilever configuration
was selected which allowed for ease and simplicity of construction
and readily lent itself to miniaturization. Fig 265
In the valve construction, the electrodes made into a small, light
cylinders comprising a conical strucuture were assembled in ceramic
base-wafer. Materials and techniques generally used in valve manufacturing
like glass and mica as well as spot-welding were eliminated. Fig 266
Such a kind of design using a sturdy metal-ceramic strucuture gave
birth to a small, light and reliable thermionic device.
A full family of NUVISTORS was launched in the market, including single
and double-ended triodes, double-ended diodes, triodes, tetrodes and
finally as 5 Watt pentode. The valve main electrical characteristcs
are indicated in the table. Fig 267
NUVISTOR was used for many industrial applications including civilian
and military purposes and it was mainly responsible for the success
of the RCA TV color sets in the early sixties.
Later in 1961 RCA production rate was greatly increased reaching 1
milion units until its plant closed in 1976. Fig 268
Certainly NUVISTOR was a step ahead in the valve manufacturing technology
and after almost 20 years in the market as a direct competitor for
the TRANSISTOR, its industrial production ceased when Sylvania closed
its plant in 1980 due to the boom of the solid-state domain.
|Fig. 265 - Cross section of a Nuvistor
5) Metal shell
6) Ceramic base wafer
7) Indexing lugs
|Fig. 266 - Nuvistor manufacturing details.
|Fig. 268- From the Galena to Nuvistor:
Left: Lead sulphide or Galena crystal used as an early
Center: The thermionic valve invented by Lee De Forest;
over more than 40 years it was used by the electronic
industry as detector, amplifier;
Right: The Nuvistor considered the
state of the art in valve manufacturing.
|Figure A- Table showing the electrical data
of the Nuvistor valve.