English: Marconi-Osram S625 vacuum tube, the first commercially produced screen grid tube tetrode). It was designed by Marconi's chief engineer H. J. Round in 1926 and came out in 1927. The complete tube is shown at top, and the internal electrodes at bottom. The screen grid was added to the triode to reduce capacitance between the plate and the grid electrodes to prevent parasitic oscillations; in high frequency amplifiers the capacitance acted as a feedback path to couple energy from the output plate circuit back into the grid circuit, causing the tube to become an oscillator. The overall design of the S625 was directed toward isolating the grid circuit from the plate. Unlike most tubes the S625 had a linear axial construction, with the filament, grid, screen grid, and plate in line down the axis of the tube. The screen grid is in the form of a cylinder with metal gauze on the front which completely encloses the plate. The tube is double-ended, with the secreen and plate connections at one end and the filament and grid connections at the other. It was designed to be mounted horizontally, projecting through a metal partition between two shielded compartments; the grid circuit in one compartment, and the plate circuit in the other, to prevent feedback. Although it was successful in the specialist applications for which it was designed (intercontinental shortwave communication) it was less successful in the commercial radio receiver market. Due to production requirements it used a thoriated tungsten filament (called a "bright emitter") which didn't have the high electron emission of the oxide coated filaments used in other tubes.