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Robert C. Sprague's Development of the Tone Control and Midget Capacitor               (John L. Sprague, January 2018)

    Exhausted he slumped over the card table covered with papers and miscellaneous pieces of electronic parts. It was the early 1920's. The crisp, clean and pressed working khakis he had worn to the Quincy, MA Fore River Ship and Engine Building Company Shipyard early that morning were once again sodden with sweat and stained from the more than eight hours he had spent working there on the sophisticated electrical control equipment of the new USS Lexington (CV-2). He had multiple sets of such uniforms that his young wife dutifully cleaned and pressed each night until they fell apart and had to be replaced.

Born on August 3, 1900, Lt. Robert Chapman Sprague, oldest son (by second marriage) of electrical inventor Frank J. Sprague, was extremely bright, confident, and in a hurry. He left Hotchkiss School at the age of 16 to, like his father, enter the US Naval Academy. Then, because of the pressure of WW 1 and excellent grades, he graduated in only three years at the age of 19. Following a year of sea duty he received a BS in Engineering at the US Naval Post-Graduate School and then an MS from MIT. Now a well-educated but still untested electrical engineer he began work on the USS Lexington. But he wasn’t facing life alone.

While at the Academy he met a lovely young woman, Florence Antionette van Zelm, whose family summered in Twin Lakes, CT near the Frank Sprague home in Sharon. Of medium height, extremely slim with pale skin she protected from the sun, she always dressed well but modestly so never to draw attention to herself. Seemingly protected by her family, she didn’t attend college but studied classical piano, first at the New York Conservatory of Music and then the New England Conservatory. Her personality was a subtle mix of the warmth of her old New England mother and iron will of her Rotterdam-born father, who initially was strongly opposed to any relationship between the two young people. Despite this they were married in May 1921, beginning a marriage and companionship that lasted until 1987 when heart failure seized her following the tragic death of their oldest son, Robert, Jr. in a plane crash. Although over the years in business and social venues she quietly differed to her more outgoing husband, her presence was a major contributing factor to his later successes. As their family grew, children and especially grandchildren (and a couple great-grandchildren born while she was alive) adored her, in no small part because of her unconscious ability to make each feel they were the only person who was important to her. They were. And sometimes, often when her husband was absent, Mimi, as her grandchildren called her, was known to flash a largely hidden sense of humor. (One amazing image, though, from late in her life, was taken when her husband was present, and exemplifies her creative spirit. Fondly titled “Mimi and the Drum,” the photo shows her playing a conga drum at a family gathering sometime around 1980).

The USS Lexington (CV-2) was originally planned as the first of a new class of battle-cruisers, only to be redesigned as a carrier following the 1922 Washington Naval Treaty limiting world naval tonnage. Her exotic propulsion system included four-sets of General Electric turbo-electric drives and her four screws were capable of driving the ship at better than 33 knots. Commissioned late 1927, along with her sister ship, the USS Saratoga (CV-3), the Lady Lex helped the United States recover from the devastation of Pearl Harbor. Just how ill-prepared we were for the December 7, 1941 attack lies largely hidden in the records of periodic United States fleet exercises showing that in 1932, 1933 and 1938 the USS Lexington and USS Saratoga were able to repeatedly mount “successful” surprise attacks on Pearl Harbor. The USS Lexington (CV-2) was lost May 8, 1942 in the Battle of the Coral Sea.

Radio Tone Control

Getting the carrier afloat during the turmoil of the roaring ‘20s wasn’t the only thing on Lt. Sprague’s mind, however. An early ham radio operator, he had an idea which would not only take him far afield, but which he hoped might just make him as famous as his father. Not pleased with the sound of his radio, he wanted to develop a tone control device which, when placed between the output of a radio and its speakers, would allow controllable change of the audio frequency output. Until resigning from the Navy in the late 1920s, this idea was pretty much a moon-lighting operation with early devices fabricated in the kitchen of his home. Florence was his first employee (today it is hard to imagine this fragile woman assembling electronic components in her kitchen while her first child, Robert, Jr., born in December 1922, ran rowdy through the space.) And when late at night as he sat exhausted in his crowded kitchen he struggled with how to convince the US Patent Office that his idea was, in fact, unique. 

Radio technology began in 1906 with Lee de Forest’s invention of the audion amplifier (the crude forerunner of the three terminal vacuum tube amplifier). By the 1920s it was common practice to place a low value condenser (today called a capacitor) across the output of a telephone amplifier (#18 in Fig. 1 of Figure 2) to its speakers. Citing prior work on telephone circuit design by inventors Armstrong and McCaa, the patent examiner argued that, while not specifically claimed, making such a condenser variable should also allow tone control. Not so, said Sprague and his first patent (US #1,675,831: Tone-Control Means, filed 5/29/1925 and issued 7/3/1928) became an early treatise on radio design. In it he argued that the output of an amplifier contains both radio frequency (RF) and audio frequency (AF) components and the purpose of the shunt condenser in a telephone circuit is to bypass the RF component and thus improve telephone reception. Therefore even if #18 is variable, since the AF component is absent in the shunt path no means is available for varying the tone of the audio component. Also Sprague noted that the rating of the condenser used in the patents of both Armstrong and Mcaa is too small to accomplish such a task.

                             Figure 2: Tone Control Circuits

On the other hand in a simple radio receiver (Fig. 2, Figure 2) addition of an audio frequency transformer (#17) and second stage of amplification (#21) means that only the audio frequency component reaches the radio’s output leads. Therefore placing a variable condenser (#29) of sufficient value across these leads can provide frequency or tone control of this output. The examiner finally agreed with the argument and Sprague’s second patent, US # 1,616,832; Condenser, filed 9/18/25 was issued 2/8/1927 (see Figure 3) claiming the variable condenser. It was both a sophisticated and very complex construction and it is hard to imagine that it could be manufactured in volume without use of very complicated equipment. However the condenser in the tone control did employ inexpensive materials, such as tin foil, paper and film, a key to Sprague’s next innovation.

                     Figure 3. R.C. Sprague's Variable Condenser

With working samples and urged by friends and family, in June 1926 the Sprague Specialties Company was incorporated with an initial capitalization of $25,000, $3,200 of which were Robert and Florence’s total savings. While people liked the sound, few bought the part, and by mid-fall half of the original investment was gone. As things continued to deteriorate, Robert’s younger brother, Julian, who had joined the firm, recommended scrapping the multiple device and developing a simple single condenser using the same inexpensive materials and some of the unique construction techniques learned from the tone control device.

               Fig. 4:  Advertisement for the R.C. Sprague's Tone Control

                                            Midget Condenser

In a much later November 1958 promotional presentation to the Newcomen Society, Robert Sprague (by this time almost universally referred to as RC) described in glowing terms how the Midget saved the company and detailed its superior characteristics: “It was ½ the size, 1/8 the weight, much less expensive, and had nearly equivalent electrical performance to any competitive parts” (for more detail on the Newcomen presentation and capacitors per se see pages 11 through 13 of Sprague Electric). However unlike Newcomen, which describes replacing expensive cleaved mica in competitive devices, the Midget C patent (US # 1,767,412 Electrical Condenser and Method of Making the Same, filed 9/14/1926 and issued 6/24/1930) makes no such claim. In fact the competitive device described in the patent and the Midget both use the same basic materials, paper, film, and metal foil. Rather the patent claims a device that is smaller, lighter, and cheaper, because it uses much less material than competitive devices for the same rating, and is manufactured by a greatly simplified process. While competitive devices are manufactured using a stacked configuration of alternating metal foil and film/paper dielectric pressed in a mold, the Midget’s interleaved dielectric film and foil are rolled around an insulated substrate and use of a mold is completely eliminated (see Fig. 2 and Fig. 3 in Figure 6 below).

         Fig.6: Images from the Patent for the Midget Condenser (Capacitor)

Not only was the Midget a much simpler construction than the Variable C, in customer qualification testing the Midget C performed just as well as advertized. Accordingly, despite some concern about the financial viability of the infant company, Sprague Specialties was soon swamped with orders, not just from small customers, but from large corporations such as Burroughs, Columbia Radio Corporation, GE, Magnavox, Raytheon, Westinghouse, and Philco. As demand grew production of the Midget had moved from the kitchen of the Sprague home to the basement and then to a series of ever larger facilities in Quincy, MA. By 1929 550 employees were manufacturing more than a half million dollars of product while squeezed into too crowded Quincy space that was also in violation of local sanitation laws. Possible relief increasingly beckoned some 150 miles to the west – North Adams – a paradise where there seemed to be an almost infinite supply of both adequate manufacturing space and trainable labor.

     Fig. 7: Florence Sprague playing the conga drum. Her fierce spirit played a larger role in her husband's success than many people knew