This was carried forward as exemplified by the Diamond Point Junior. Mutual conductance is really a ratio test in which an ac voltage is compared to the resulting plate current and the ratio of the two was expressed in MicroMhos (a Mho being the reciprical of resistance and a measure of conductance). The first really successful unit, the AC-47 still needed only three tube sockets (the upper socket is for an 80 tube required to develop B+) however Hickok discovered that only two meters were really needed, one for plate current (emissions) and a second for the new mutual conductance, a test that was to become the mark of a quality tube tester. Note that only three sockets were required to support the tubes of its time. The earliest of these was the model AC-4600 which required external power but its five meters instrumented nearly everything a serviceman could want. For this duty, not much testing was needed. Of course in 1928 there were not many tubes available and most of the time the unit under test would be an 01 or 01A. The early Hickok tube testers, while advanced for their time, were also a far cry from their children - characterized by simple emissions circuits, the first units had only two knobs supplanted by a number of jumpers. Early Hickok Tube Testers, 1928-1940 Early Hickok Tube Testers, 1928-1935
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