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The Twilight Years of Vacuum Tube Op Amps

In the late 1950's and early 1960's, vacuum tube op amps had more or less reached their peak of technical sophistication, at least in terms of the circuitry within them. Packaging and size issues of course made a big impact on the overall appeal for the system designer, and there was work done in these areas.
An interesting design using three 9-pin miniature dual triodes is shown in Figure 6 below. This compact design was done by Bela (Bel) Losmandy of Op Amp Labs (Reference 56: Opamp Labs Inc., 1033 N Sycamore Avenue, Los Angeles, CA, 90038 (323) 934-3566 http://www.opamplabs.com/bel.htm), then working with Micro-Gee Products, Inc. in 1956 (Reference 57: Bela Losmandy, Three Stage Micro-Gee Products, Inc.Op Amp, 1956).
A fully differential op amp design by Bela Losmandy, for Micro-Gee Products, Inc.
Figure 6: A fully differential op amp design by Bela Losmandy, for Micro-Gee Products, Inc.
There are several interesting aspects of this circuit. First, it is entirely differential, right up to the 12AU7 output stage. This allows it to handle CM inputs with lower errors, and improves the drift characteristics. As can also be noted, there is only one (dual) level shift circuit, following the V1-V2 directly coupled differential stages. This minimizes gain loss, and improves the overall performance. The entire amplifier operates on supplies of ±300V @8mA, and has a gain of more than 10,000 operating into a 25kΩ load.
In the late 1950's and 1960's, two more publications appeared chronicling op amp developments. One was a long overview paper by Konigsberg, which appeared in 1959 (Reference 59: R. L. Konigsberg, "Operational Amplifiers," Advances in Electronics and Electron Physics, Vol. 11, 1959, pp. 225-285. (An overview of vacuum tube and [some] solid state op amps, with 75 references)). The other was the analog and digital oriented computing handbook by Harry Husky and Granino Korn, Computer Handbook, in 1962 (Reference 60: Harry Husky, Granino Korn, Computer Handbook, McGraw-Hill, 1962. (An encyclopedic reference work on analog computing. Includes detailed op amp design section by Edward Billinghurst, plus a comprehensive list of op amp circuit examples, with specifications)). While this book was perhaps one of the last hurrahs for the vacuum tube op amp, it does contain a wealth of detailed design information on them.
By the time the 1960's rolled around, the solid-state era was already in progress. Vacuum tube op amps were on the wane, and smaller, low power, solid-state devices would soon take over op amp applications.
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