English: One of the first vacuum tube AM radio transmitters, a two way radiotelephone set sold by the Marconi Wireless Telegraph Co. designed by their chief engineer, Henry J. Round in 1914. It used a prototype triode vacuum tube designed by Round. It was designed for shipboard use. The large tube on the frame was the transmitting tube, while the small one was for the receiver circuit. The transmitter used an Armstrong type (tickler) oscillator circuit with the carbon microphone directly in the antenna lead, modulating the RF antenna current. The input plate power was 5 to 10 W at a plate voltage of 500 V. The set was guaranteed to have a 30 mi (50 km) range when used with a 100 ft (30 m) high, 200 ft long (60 m) wire antenna, but could be pushed to have an antenna current of 1 A which would give a 100 mi (160 km) range. See also "Marconi's Wireless Telephone", The Wireless Age, Vol. 2, No. 7, April 1915, p. 489