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George Philbrick and GAP/R

After WWII, George Philbrick also continued with op amp development work. Shortly thereafter he formed a company bearing his name, George A. Philbrick Researches, Inc., in 1946 (GAP/R). Philbrick's work was instrumental in the development of op amp technology in many regards. His company was to see growth over the span of the vacuum tube technology days and well into the solid-state era.
Not too long after forming GAP/R, Philbrick introduced the world’s first commercially available op amp, known as the K2-W. This modular 8-pin octal plug-in op amp was developed in 1952, and appeared in January 1953 (Reference 52: Data Sheet For Model K2-W Operational Amplifier, George A. Philbrick Researches, Inc., Boston, MA, January 1953. See also "40 Years Ago," Electronic Design, December 16, 1995, pp. 8. (The George A. Philbrick Research dual triode K2-W, the first commercial vacuum tube op amp)). A photo and schematic of this $20 op amp are shown in Figure 5 (opposite).
The K2-W used two 12AX7 dual triodes, with one of the two tubes operated as a longtailed pair input stage, which offered fully differential operation at the input. With the K2-W operating on ±300V supplies, the input stage's 220kΩ tail resistor was returned to the –300V supply, fulfilling the long-tailed pair biasing requirement.
Half of the remaining 12AX7 dual triode was operated as a second gain stage, which in turn drove the remaining section as a cathode follower output, through a level shifter part 8355037 (typically thyrite devices). Overall gain of the K2-W was enhanced by positive feedback through the 150kΩ resistor, connected back to the cathode of the second stage.

Operating from the ±300V power supplies @ 4.5mA, the K2-W was able to achieve a ±50V rated signal range at both input and output. DC gain was typically 15,000, and the entire circuit was packaged in a convenient, plug-in octal tube-based package.
Some vacuum tube op amp manufacturers provided chopper-stabilizer "add-on" units. In the case of GAP/R, this was the GAP/R K2-P. This unit, when used with the K2-W, formed a combination low drift, high gain op amp (Reference 53: Henry Paynter, Ed., Applications Manual for PHILBRICK OCTAL PLUG-IN Computing Amplifiers, George A. Philbrick Researches, Inc., Boston, MA, 1956. (The first op amp application manual)).
Early on GAP/R set an excellent standard for application information, publishing a popular 1956 manual for the K2-W and related amplifiers which went through at least 10 printings by 1963 (Reference 53, again). GAP/R not only made available applications literature for their devices; they published a periodical devoted to analog computation, the Lightning Empiricist. It contained technical articles and new product information.
GAP/R also published what is now a classic set of reprints, the "Palimpsest" (Reference 54: Jerry Roedel, "An Introduction to Analog Computors," within A Palimpsest on the Electronic Analog Art, George A. Philbrick Researches, Inc., Boston, MA, 1955. (Roedel's paper is one of many analog computing papers, [including Ragazzini et al, above])). Some researchers see George A. Philbrick as a veritable op amp founding father. For example, Roedel, in his "An Introduction to Analog Computors" (Reference 54, again), gives Philbrick and Lovell credit for being the first op amp users.
The GAP/R K2-W op amp, photo and schematic diagram (courtesy of GAP/R alumnus Dan Sheingold - schematic values in megohms and pF.)
Figure 5: The GAP/R K2-W op amp, photo and schematic diagram (courtesy of GAP/R alumnus Dan Sheingold - schematic values in meg-ohms and pF.)
It is also undoubtedly true that the GAP/R organization produced some of the best documentation and application support for op amps, both vacuum tube and solid state.

Because of the longevity of so many op amp principles, much of the wisdom imparted in the GAP/R app notes is still as valid today as it was in the 1950's. Although it did not appear until several years later, the best example of this is GAP/R's classic 1965 op amp book edited by Dan Sheingold, Applications Manual for Computing Amplifiers… (reference 55: Dan Sheingold, Ed., Applications Manual for Operational Amplifiers for Modeling, Measuring, Manipulating, and Much Else, George A. Philbrick Researches, Inc., Boston, MA, 1965. (The classic GAP/R op amp application manual, now a collector’s item)).
next The Twilight Years of Vacuum Tube Op Amps
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