Below is an extract from an old RCA datasheet for their 6021 medium mµ double triode tube that gives an idea of the supreme engineering effort that went into ensuring the reliability and consistency of subminiature tubes:
RCA-6021 is a subminature medium mu twin triode of the heated cathode type having flexible leads. It is intended for is use in oscillator and amplifier at frequencies up to 400 Mc (that’s 400MHz in today’s money and overkill for audio applications). Constructed to give dependable performance under conditions of shock and vibration, this “premium” tubes is especially suited for use in mobile and aircraft equipment and is rated for service at altitudes up to 60000 feet without the use of pressurised chambers.
The design of the 6021 incorporates a compact structure in which special attention has been given to the following features:
- “U” frame construction to keep the mount rigid and prevent distortion of plates.
- Precisely made and accurately fitted tube parts, including new mica design, to lock the parts firmly in place.
- Grid side rods having high heat conductivity to provide cool operation of the grids.
- Pure tungsten heater having high mechanical strength.
- Getter shield to prevent deposit of getter flash on tube elements.
- Pure nickel plate to minimise evolution of gas.
As a result of its structural design this tube is characterised by:
- Small spread in electrical characteristics.
- Reduced microphonic effects.
- Reduced grid emission.
- Long life under frequent on-off switching.
- Low leakage currents and high leakage resistance between the elements (plate, grid and cathode).
In addition, this tube utilises separate terminals for each cathode to permit flexibility of circuit arrangement.
Manufactured under rigid controls, the 6021 undergoes rigorous tests during manufacture to insure its “premium” quality as follows: test reading at the end of 1 hour, 100 hours and 500 hours to ensure that tubes fall within the established tight characteristics limits and that early failures are held to a low percentage.
The boffins weren’t messing about—these tubes were intended to be used in extreme environments and mission critical situations—and the result of their wartime driven engineering efforts were some of the finest tubes ever made. These little devices represent the absolute pinnacle of tube technology and offer more consistent and reliable performance than the early N.O.S. germanium transistors. The Raytheon datasheet boldly states, “Tubes developed for this purpose proved so rugged that in-operative failures became very rare.”—impressive stuff. And it’s interesting to speculate a little here—on a future that could have been—that if the development of the crystal transistor had been delayed by just a few years, the vacuum tube might have attained such dizzying levels of technological refinement that it could still be the standard audio amplification device today.