page_title: Tetrode
section_title: Screen grid valve
hierarchical_section_title: Tetrode / Screen grid valve
caption_attribution_description: English: s23 screen grid valves in music magnet receiver
context_page_description: A tetrode is a vacuum tube having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate. There are several varieties of tetrodes, the most common being the screen-grid tube and the beam tetrode. In screen-grid tubes and beam tetrodes, the first grid is the control grid and the second grid is the screen grid. In other tetrodes one of the grids is a control grid, while the other may have a variety of functions.
  The tetrode was developed in the 1920s by adding an additional grid to the first amplifying vacuum tube, the triode, to correct limitations of the triode. During the period 1913 to 1927, three distinct types of tetrode valves appeared. All had a normal control grid whose function was to act as a primary control for current passing through the tube, but they differed according to the intended function of the other grid. In order of historical appearance these are: the space-charge grid tube, the bi-grid valve, and the screen-grid tube.
context_section_description: In the screen grid class of tetrode, the main function of the second grid is to act as an electrostatic screen between the anode and the control grid (i.e. the first grid) in order to reduce the internal capacitance between control grid and anode.  The first true screen-grid valve, with a screen grid designed for this purpose, was patented by Hiroshi Ando in 1919, and the first practical versions were built by N. H. Williams and Albert Hull at General Electric  and Bernard Tellegen at Phillips in 1926.
  This type of tetrode was developed to correct deficiencies in the triode tube which became apparent when attempts were made to use triodes as small-signal radio-frequency amplifiers.  In the triode, the control grid was next to the plate.  Capacitance between these two electrodes caused instability and oscillation when both anode and grid were connected into tuned resonant circuits, as was the case in early radios, or in any application where the anode circuit presented an inductive load to the valve. Oscillation could only be avoided by using very small stage gain at frequencies above about 100 kHz, and at frequencies above 1 MHz, triodes are virtually useless in tuned amplifiers in which both anode and grid circuits are tuned to the same frequency. A second advantage of the tetrode, which also arose from reduced anode-grid capacitance, was that, when the anode voltage was in a certain range, changing its value had much less effect on the anode current than was the case with triodes. This corresponds to an increased anode slope resistance, and allowed much higher voltage gain by permitting a larger external load resistance.  At the time of the introduction of screen-grid valves (around 1927) a typical small triode used for small-signal amplification had an anode slope resistance of 20 kilohms or less, and a grid-anode capacitance of 1 to 5 pF, while the corresponding figures for a typical screen grid valve were 1 megohms and 0.004 pF - in some cases much less capacitance.
  Screen-grid valves exhibited greater voltage gain and higher frequency capability than triodes, and permitted the development of the first true RF amplifiers in the MF and HF frequency ranges in radio equipment.  They were commonly used as the first stage of radio-frequency amplification in domestic radio receivers in the period 1927 to 1930, after which they were superseded in this application by RF pentodes. Triodes were ill-suited to this type of application on account of their relatively high anode-grid capacitance and low anode resistance.
  In normal operation the screen grid is connected to a positive DC voltage slightly less than the plate voltage, and bypassed to the cathode with a capacitor, so it was at AC ground.  To take full advantage of the very low internal grid-anode capacitance, the valve must be used in circuits in which the shielding between anode and grid is continued externally.  In the case illustrated (S625), the valve was intended to be inserted into a hole in an external, grounded, sheet-metal shield aligned to correspond with the position of the internal screen-grid.  The input, or control-grid circuit was on one side of the shield, while the anode, or output circuit was on the other.  In the case of the Osram Music Magnet, each entire stage of the 2-stage rf amplifier, as well as the tuned detector stage, was enclosed in an individual large aluminium screening box.  These boxes have been removed in the illustration, but the up-turned edges of the bases of the boxes can be seen.
caption: Two S23 screen-grid valves in a 1929 Osram Music Magnet receiver
exif: {}