The Nuvistor vacuum tube (valve) was developed and introduced by RCA in 1959 (see this historical photograph). It was one of the last major advances in basic vacuum tube technology, following on the heels of the cold cathode tube. The Nuvistor was heralded as a threat to the transistor; they were initially attractive because they handled momentary overloads better and they were higher impedance devices than the transistors of 1959 (those advantages disappeared with improvements in semiconductors). RCA indicated the Nuvistor technology was particularly well suited to mechanized production. Interestingly, the Nuvistor was announced at nearly the same time as the first integrated circuit; the Nuvistor's fate was sealed from the very start.|
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There are historical hints that the Nuvistor was originally intended primarily for the military and industrial markets. But by April of 1959, RCA triode and tetrode versions had already been demonstrated in TV tuners, reduced to one third the volume of the conventional tuners of the day. Thousands of Nuvistors were used in TV tuners in the 1960's, and many were in fact used in military and industrial equipment (prior to the commercial availability of FETs, some were used as high impedance input stages in otherwise solid state equipment) (see our Vintage Oscilloscopes page). Nuvistor style tubes were eventually offered by RCA, Sylvania, Raytheon, GE, and others. Though the only current manufacturer seems to be a Russian concern, there has been renewed interest in Nuvistors by the nutty tube audiophile community (see this Stereophile Magazine article from December, 1999.)
The Nuvistor is smaller than a thimble (not much larger than the transistors of 1959), and is more rugged and efficient than glass envelope designs. A ceramic base is sealed to the metal case with polarized extensions of the metal case protecting the delicate pins, and ensuring the device can only be inserted into the special socket in the correct orientation. The 13CW4 and the 8393 Nuvistors commonly used in logging tools have only five pins extending down from the 12 pin grid that supports the internal elements. The Nuvistor is built with very small coaxial electrodes, resulting in low inter-electrode capacitances. Nuvistors were characterized for satisfactory operation to 350° C, explaining why some logging tool manufacturers continued to use them up into the 1980's.
The 13CW4 was used in countless logging tools for at least two decades. It is a high mu triode with 13.5 volt, .06 amp filaments, and a maximum plate rating of 1.5 watts and 135 volts. A few radiation tools used the 8393, a medium mu triode with 13.5 volt, .06 amp filaments, and a maximum plate rating of 1.0 watts and 110 volts. In most cases the 13CW4 and the 8393 are interchangeable, but there are a few specific applications where they are not (actually we have seen devices with 8393 and 13CW4 dual markings).
A common Nuvistor tool failure mode is a burned out heater filament. In most logging tools, the heaters in the Nuvistors are connected in series in the same manner as old Christmas tree lights. If any single heater filament goes open, the tool voltage and current will be grossly abnormal, and the tool will of course not function. Check filament continuity for each Nuvistor (an ohmmeter between pins 10 and 12 on the socket should read around 35 ohms) until locating the defective tube, then replace it.
AnaLog Services, Inc. stocks both the 13CW4 and the 8393 for the repair of our customer's tools. Believe it or not, we still have a tube tester capable of checking Nuvistors! We have also replaced nuvistor circuitry with solid state circuitry in customer cement bond tools, radiation tools, and elsewhere.