The constant attention of electronic engineers concerned with the design and manufacture of receiving valves is towards providing improved performance, quality and reliability, and with the advent and growth of the transistor the question of size is now of paramount importance.
From recent research work. RCA engineers have presented the Nuvistor, which is now in the advanced stages of development. Many factors were considered in the design of the Nuvistor, including planar structures. cylindrical structures. metal-to-glass seals and metal-to-ceramic seals. For example, ease of assembly led to research in using planar tube elements. but any advantage to be gained by this was offset by disadvantages in electrical and thermal characteristics.
In contrast, cylindrical tube elements offered electrical and thermal stability and efficiency, and at the same time could be easy and economical to manufacture. The new design employs concentric cylinders supported in an open-ended cantilever construction, together with a combination of new materials, processes and manufacturing techniques.
For ruggedness a strong ceramic base-wafer is used as a platform on which is erected an array of valve electrodes, each solidly held in place by a tripod-like structure. The electrodes are thus strongly supported in an open-ended cantilever construction. These electrodes are small. light cylinders which, because of their form and low mass, are able to withstand a high degree of shock or vibration.
There are no micas to fray under vibration or to interfere with high-temperature exhaust processing of the tube. There is no glass to limit the processing temperature or to break under mechanical or thermal shock.
All joints in the valve are brazed together in one simple operation at high temperatures, and thus the parts are joined in a strain-free assembly. Because the valve elements are accurately secured in their original strain-free position, the possibility of shorts developing in the valve during operation is remote.
Comparison With Competitive Devices
Some of the more significant advantages of electron valves, and in particular the Nuvistor, over competitive devices can be summarised as follows:
- In a valve the electrode spacing can be 50 times larger than the electrode spacing in a transistor intended for comparable performance. Valve spacings and the associated tolerances can, therefore, be more easily controlled in manufacturing operations.
- The valve is economical in initial cost, has high impedance and gain, and generally requires less expensive associated circuit components.
- At high frequencies the noise factor and the gain of the valve are superior.
- The valve has a high uniformity of initial characteristics and does not require a costly selection process.
- The valve is capable of handling momentary overloads.
- The valve is inherently less susceptible to radiation damage.
- Valves, and particularly the Nuvistor design, maintain their characteristics over a wide range of ambient temperatures.