About 20 years ago the unusual tube shown in Fig. 1 turned up on a list of collector tubes for sale. This tube was unlike anything previously seen by the author, and no information on George C. Clark could be found. This tube was an external control device, similar in some ways to the Moorhead Solenoid Tube. The filament was a small loop of tungsten that was positioned in the vertical plane of the large hoop that served as the plate. Just outside the plate was a spiral of wire that served as the control element. Since the control winding was not positioned between the filament and the plate, this structure avoided the DeForest triode patent. One end of the control element is tied to the normal grid pin on the base, and the other end is connected to the metal shell. No doubt the intent was to drive a signal through the winding to increase its influence on the output. The glass bulb was etched "Geo. C. Clark", and "CLARK" was stamped into the top of the plate hoop.
Numerous collectors have been shown this tube over the years, but none could provide any information as to its origins. As far as is known, no other samples have surfaced. The obvious questions remained - who was George C. Clark, and how did he come to make the tube with his name etched in the bulb? Recently, in a rare bit of luck, an event occurred that finally answered some of the questions. A Clark family member doing online research on George C. Clark found the tube on the author's website and made contact. Most of the information and photos in the balance of this article were provided courtesy of the Clark family.
George C. Clark was born in Chicago in 1874 and moved to San Francisco in 1904. A photo taken about 1935 is shown in Fig. 2. In 1904 he owned a business setting up and maintaining x-ray equipment and manufacturing static machines. Initially the company was under his name, but then became Clark & Whittemore. A photo of one of his static machines is shown in Fig. 3. After the earthquake of 1906 destroyed his business, he tried to work out of his home. However, the drastically increased rents that prevailed after the earthquake forced him and his wife Lydia to relocate across the bay to Oakland. He apparently continued in the x-ray field for a while, but by 1908 was employed by the Burlingame Telegraphing Typewriter Company. This company made an early form of Teletype machine. At the same time he was involved in his own personal research and experimentation. He designed and built the Clark Electric Typewriter, but was unable to obtain backing for it. He was associated with the Burlingame Company until perhaps 1911 or 1912. After that he worked for Marchant Calculators for several years. According to information from an October, 1922 issue of Radio magazine, he had also been associated with Thompson Houston, Western Electric, Peter Cooper Hewitt, and A. Frederick Collins in the period from 1890 to 1922.
In July of 1922 Clark stared the Clark Radio Manufacturing Corporation, located at 3045 Hopkins St. in Oakland. A picture of the workshop is shown in Fig. 4. Clark is shown seated at the bench. Their initial product was the Clark Vario-Antenna. This was a device to connect to the house wiring as an antenna input for a radio. A surviving sample is shown in Fig. 5. Although others had marketed similar products, the Vario-Antenna was unique in that it contained a tuning circuit to resonate the wiring to the desired frequency for the receiver. The Vario-Antenna was advertised in Radio magazine in Oct., Nov., and Dec. of 1922 (Fig. 6). A search of other radio magazines in this time period did not reveal any other ads. The Clark Company apparently only survived until 1923. According to family information the company was sued by RCA for patent infringement and was forced out of business at that time. The exact nature of this infringement suit is not known.
George had always been an experimenter and inventor, and in 1922 designed and built a regenerative receiver. A prototype is shown in Figs. 7 and 8. The front panel is engraved CLARK RADIO MFG CO, OAKLAND CAL. Markings inside date this receiver to June of 1922. During this time he was also working on the design of a radio tube. Fig. 9 shows one of his early experimental tubes. This tube uses the same type of spiral grid structure as the tube in Fig. 1, but has a round disc plate. The grid winding is returned to the metal shell. The grid is located between the filament and plate and would have infringed the DeForest patent. Fig. 10 shows a similar tube that has larger elements. These tubes do not have the Clark name etched into the bulb and have rather crude Clark stampings in the plates. The tube in Fig. 1 is almost certainly a later version, configured as an external grid type to avoid legal problems. About this same time external grid tubes were being sold by Moorhead (Solenoid Tube), Welsh, Delta and others. All of these tubes were useful only as detectors, they would not function as amplifiers. It is likely that the Clark tube would also only work as a detector. Unfortunately the tube in Fig. 1 has an open filament and could not be tested for function.
Work on this tube continued into 1923, and correspondence from a patent attorney in October and November of 1923 indicates that they were pursuing a patent application for this tube, called a "Magnetic Vacuum Tube". Apparently the claim was for a tube controlled by a magnetic field generated by the spiral winding. Other patents had been granted for magnetic control, but accomplished with a different structure and with the magnetic field applied in a different plane. There was no correspondence indicating that a patent was ever issued. It has not been determined if the Clark tube or receiver were ever marketed. The Clark receiver, being regenerative, could not have been sold without infringing the Armstrong patents. It is likely that the receiver was never marketed, but used for tests on the Clark tube and Vario-Antenna. The only advertising found for the Clark Company was for the Vario-Antenna. The tube in Fig. 1 may be a prototype, or a part of a small production run. The workshop of Clark Radio did not appear to include the necessary equipment for tube construction, so it's not clear who actually made the tubes. An examination of the tube in Fig. 1 shows it to be well built, and must have been assembled by someone with experience in tube fabrication.
The last reference to the Clark tube is found in a letter from a patent attorney dated November of 1924. The letter indicates that the Patent Office had found several patents related to the application for the Clark tube. The letter further states the attorney's belief that allowable claims would be obtained for the patent. It is not known if a patent was actually issued. No further information is available regarding radio activities of George C. Clark after 1924.
The author extends his thanks to the Clark family for their help with this project. Considerable communications and data exchange occurred over a period of months which made this article possible. All the items shown except the tube in Fig. 1 are part of the Clark family archives.
Photo credits: Fig. 2 and 4 are scans of Clark family photographs Fig. 3, 5, 7, 8, 9, 10 are ©copyright Roswell T. Bishop