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. 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. For this duty, not much testing was needed.
The earliest of these was the model AC-4600 which required external power but its five meters instrumented nearly everything a serviceman could want. Note that only three sockets were required to support the tubes of its time.
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. 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).
This was carried forward as exemplified by the Diamond Point Junior. Note that this small tester from about 1933 has eight tube sockets while the otherwise identical appearing 1934 model (J-34-P) had an additional seven sockets in a row along the top. The small meter on the earlier model is centered for each tube type, an early form of the GOOD-?-BAD english settings used on later Hickoks. By 1938 Hickok had managed to consolodate the fifteen sockets of the 1934 model into ten sockets and had dispensed with the centering meter.
The AC-49 a followon of the early design and the proliferation of tube types as triodes were enhanced by supressor and screen grids was quickly crowding out space on the panel. In 1937 the AC-49 required 14 sockets and there was no longer room for a standard round meter.
As with the E-38-P this was corrected in the AC-51 by a major panel resdesign and the introduction of the six-knob setting mechanism that allowed a return to only one of each socket type. Note the lack of minature tube sockets which dates this tester to pre-1939. This also was the first Hickok to use a roll chart for tube settings.
This same combination of tube tester and VOM was also available as two instruments in a single case for simultaneous measurements. This combination was known as the AC-4800