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Evolution of the Vacuum Tube Op Amp

Nevertheless, Julie’s op amp design was notable in some regards. It had a better input stage— due to the use of a long-tailed 6SL7 dual triode pair, with balanced loads. This feature would inherently improve drift over previous single-ended triodes or pentodes.
As will be seen shortly, a truly key feature that Julie’s circuit held over previous single-ended input designs was the basic fact that it offered two signal inputs (inverting and non-inverting) as opposed to the single inverting input (Fig. 3, above). The active use of both op amp inputs allows much greater signal interface freedom. In fact, this feature is today a hallmark of what can be called a functionally complete op amp— nearly 60 years later! The differential input stage not only improved the drift performance, but it made the op amp immeasurably easier to apply. Ironically however, there was still some time before the application of op amps caught up with the availability of that second input.
Much other work was also done on the improvement of direct-coupled amplifiers during the war years and shortly afterwards. Stewart Miller, Edward Ginzton, and Maurice Artzt wrote papers on the improvement of direct-coupled amplifiers, addressing such concerns as input stage drift stabilization against heater voltage variations, inter-stage coupling and level shifting schemes, and control of supply impedance interactions (References 43: Stewart E. Miller, "Sensitive DC Amplifier with AC Operation," Electronics, November, 1941, pp. 27-31, 106-109. (Design example of a stable, high-gain direct-coupled amplifier including 'cathode-compensation' against variations in filament voltage, use of glow tube inter-stage coupling, and a stable line-operated DC supply), References 44: Edward L. Ginzton, "DC Amplifier Design Techniques," Electronics, March 1944, pp. 98-102 (Various design means for improving direct-coupled amplifiers), References 45: Maurice Artzt, "Survey of DC Amplifiers," Electronics, August, 1945, pp. 112-118. (Survey of direct-coupled amplifier designs, both single-ended and differential, with emphasis on high stability)). Some additional examples of improved dc amplifiers can be found in the Valley-Wallman book (Reference 46: George E. Valley, Jr., Henry Wallman, Vacuum Tube Amplifiers, MIT Radiation Labs Series No. 18, McGraw-Hill, 1948. (A classic WWII Radiation Lab development team textbook. Chapter 11, by John W. Gray, deals with direct-coupled amplifiers)).
Before the 1940's expired, companies were already beginning to capitalize on op amp and analog computing technology. Seymour Frost wrote about an analog computer developed at Reeves Instrument Corporation, called REAC (Reference 47: Seymour Frost, "Compact Analog Computer," Electronics, July, 1948, pp. 116-120, 122. (A description of the Reeves Electronic Analog Computer [REAC], which used as the computing amplifier a circuit similar to the M9 op amp. Uses a Miller-compensated triode input stage)). This computer used as its nucleus an op amp circuit similar to the Swartzel M9 design. In the Reeves circuit the first stage was changed to a 6SL7 dual triode, used in a Miller-compensated low drift setup (Reference 43, again).
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