The VT-Class I (is a Type B tube) is a "soft" tube for detector application with 18 - 40 volts at the anode and similar to the VT-Class II (is a Type A tube), which is a "hard" tube for 60-90 volts. There is a series of similar tubes: Type A, Type B, Type C, Type D, Type E and Type F (see Tyne page 172 for more information) and spherical Type G with British standard four-pin base.
The tubes advertised and sold by the American Marconi Company in 1919 and early 1920 as the Marconi VT were Type A and Type B tubes. A life of 1500 hours was claimed for these tubes. The first consignment of tubes (about 25,000) delivered to Marconi bore no Marconi or de Forest markings. They were stamped on the glass bulb with the legend "Moorhead Audion - San Francisco". The class is rubber-stamped on the base. About 800 of these tubes have been sold. The remainder got the words "Patented Nov. 7, 1905. Sold for amateur and experimental use". According to Tyne page 173 this was stamped on the bulb before being sold. Subsequently both the Marconi and de Forest patent numbers and the restrictive legend were steel-stamped at the brass base.
