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English: The Marconi "N" tube, an early "soft" triode manufactured by the British Marconi Co. and designed by its chief engineer H. J. Round, developed around 1913 contemporaneously with the De Forest Audion. It was used in Marconi's No. 16 wireless receivers, as well as in some of the first oscillator circuits and amplitude modulated (AM) radio transmitters around 1914. It was also used in Britain's first vacuum tube telephone repeaters. The scale is in inches.

It had a Wehnelt coated filament consisting of a platinum wire coated with alkaline earth oxides, which reduced its work function so it didn't have to be heated to such a high temperature to release electrons. The grid was a wire screen completely surrounding the filament, while the plate (anode was a sheet metal cylinder surrounding the grid. The filament was attached to the screw base, while the grid and plate (anode) were brought out to the wires passing through the side of the tube. The tube required 3-4 amps at 2 V for the filament and 40 to 80 V on the anode. It was used in the Marconi No. 16 reflex receiver, where it functioned as both an RF and audio amplifier. Like the early Audion this was a "soft" vacuum tube, which contained some gas and operated by ionization. The small nipple at the top contained an asbestos pellet impregnated with a material that released a small amount of gas. The ionized gas served to decrease the resistance so a lower plate voltage was needed, and also give it a more nonlinear (curved) grid voltage/plate current characteristic which improved its amplification. To increase the gas in the tube, the user could heat the pellet with a flame. Information from: Gerald F. J. Tyne, Saga of the Vacuum Tube, Part 19, Radio News magazine, September 1945, p. 54

Fleming wrote (p. 87): "...the most sensitive tubes [for radio detectors] have been low-vacuum or "soft" tubes, but the action in such tubes depends so acutely on gas pressure, which changes by occlusion, or the reverse process, and by temperature, that soft tubes required fine adjustment and expert handling. For example, the triode produced by H. J. Round...would generally give wonderful results in expert hands, but it had to be carefully nursed; and the vacuum was actually controlled by the operator from time to time by warming... the pellet of asbestos sealed in a pocket of the tube..."