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the (416), grid (144), anode (121), and (94), #screen (73), tube (66), voltage (56), tetrode (55), was (48), valve (41), current (41), transistor (39), diode (35), control (34), with (31), for (30), radio (29), this (27), from (27), power (27), tubes (25), beam (25), space (25), charge (24), frequency (24), triode (21), than (21), which (21), that (20), cathode (19), other (19), electrons (18), two (17), low (15), circuit (15), signal (15), first (15), characteristic (15), vacuum (14), valves (14), effect (14), field (14), high (14), new (14), edit (14), are (14), output (13), oscillator (13), pentode (13), amplifier (13), amplification (13), receiver (13), cell (12), region (12), were (12), when (12), type (12), used (12), york (11), tetrodes (11), voltages (11), both (11), retrieved (10), solar (10), electron (10), john (10), also (10), secondary (10), grids (10), these (10), greater (10), between (10), inc (9), wireless (9), electric (9), higher (9), typical (9), positive (9), one 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ska українська tiếng việt 中文 edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikimedia commons wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia vacuum tube with four active electrodes this article is about the four element vacuum tube for other meanings see tetrode disambiguation a tetrode is a vacuum tube called valve in british english having four active electrodes the four electrodes in order from the centre are a thermionic cathode first and second grids and a plate called anode in british english 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 1 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 the last of these appeared in two distinct variants with different areas of application the screen grid valve proper which was used for medium frequency small signal amplification and the beam tetrode which appeared later and was used for audio or radio frequency power amplification the former was quickly superseded by the rf pentode while the latter was initially developed as an alternative to the pentode as an audio power amplifying device the beam tetrode was also developed as a high power radio transmitting tube tetrodes were widely used in many consumer electronic devices such as radios televisions and audio systems until transistors replaced valves in the 1960s and 70s beam tetrodes have remained in use until quite recently in power applications such as audio amplifiers and radio transmitters the diacrode is a specialized type of high power tetrode used in some radio transmitters particle accelerators and nuclear fusion reactors it serves as a high power rf amplifier and it is a double ended tetrode with two output cavities and two screen and anode connections it places two symmetrical tetrode structures facing each other in a single envelope so that the coaxial line created by the arrangement of the grids in a tetrode becomes a short circuit thus it is closed on both ends and not open on one end like a tetrode it is designed for output powers higher than 300 kw and or frequencies higher than 200 mhz 2 3 4 5 how it works edit 4 1000a 1 kw radial beam power tetrode in an amateur radio transmitter the tetrode functions in a similar way to the triode from which it was developed a current through the heater or filament heats the cathode which causes it to emit electrons by thermionic emission a positive voltage is applied between the plate and cathode causing a flow of electrons from the cathode to plate through the two grids a varying voltage applied to the control grid can control this current causing variations in the plate current with a resistive or other load in the plate circuit the varying current will result in a varying voltage at the plate with proper biasing this voltage will be an amplified but inverted version of the ac voltage applied to the control grid providing voltage gain in the tetrode the function of the other grid varies according to the type of tetrode this is discussed below space charge grid tube edit the space charge grid tube was the first type of tetrode to appear in the course of his research into the action of the audion triode tube invented by edwin howard armstrong and lee de forest irving langmuir found that the action of the heated thermionic cathode was to create a space charge or cloud of electrons around the cathode this cloud acted as a virtual cathode with low applied anode voltage many of the electrons in the space charge returned to the cathode and did not contribute to the anode current only those at its outer limit would be affected by the electric field due to the anode and would be accelerated towards it however if a grid bearing a low positive applied potential about 10v were inserted between the cathode and the control grid the space charge could be made to extend further away from the cathode this had two advantageous effects both related to the influence of the electric fields of the other electrodes anode and control grid on the electrons of the space charge first a significant increase in anode current could be achieved with low anode voltage the valve could be made to work well with lower applied anode voltage 6 second the transconductance anode current with respect to control grid voltage of the tube was increased the latter effect was particularly important since it increased the voltage gain available from the valve 7 8 9 space charge valves remained useful devices throughout the valve era and were used in applications such as car radios operating directly from a 12v supply where only a low anode voltage was available the same principle was applied to other types of multi grid tubes such as pentodes as an example the sylvania 12k5 is described as a tetrode designed for space charge operation it is intended for service as a power amplifier driver where the potentials are obtained directly from a 12v automobile battery the space charge grid was operated at 12v the same as the anode supply voltage 10 another important application of the space charge tetrode was as an electrometer tube for detecting and measuring extremely small currents for example the general electric fp54 was described as a space charge grid tube designed to have a very high input impedance and a very low grid current it is designed particularly for amplification of direct currents smaller than about 10 9 amperes and has been found capable of measuring currents as small as 5 x 10 18 amperes it has a current amplification factor of 250 000 and operates with an anode voltage of 12v and space charge grid voltage of 4v 11 the mechanism by which the space charge grid lowers control grid current in an electrometer tetrode is that it prevents positive ions originating in the cathode from reaching the control grid 12 note that when a space charge grid is added to a triode the first grid in the resulting tetrode is the space charge grid and the second grid is the control grid bi grid valve edit in the bi grid type of tetrode both grids are intended to carry electrical signals so both are control grids the first example to appear in britain was the marconi osram fe1 which was designed by h j round and became available in 1920 9 the tube was intended to be used in a reflex circuit for example the single valve ship receiver type 91 13 where the same valve performed the combined functions of rf amplifier af amplifier and diode detector the rf signal was applied to one control grid and the af signal to the other this type of tetrode was used in many imaginative ways in the period before the appearance of the screen grid valve revolutionised receiver design 14 15 tetrode of the bi grid valve type circuit using bi grid tetrode oscillator as am transmitter h is a source of high voltage one application is shown in the illustration this is recognisable as an am telephony transmitter in which the second grid and the anode form a power oscillator and the first grid acts as a modulating electrode the anode current in the valve and hence the rf output amplitude is modulated by the voltage on g1 which is derived from a carbon microphone 16 a tube of this type could also be used as a direct conversion cw radiotelegraphy receiver here the valve oscillates as a consequence of coupling between the first grid and the anode while the second grid is coupled to the antenna the af beat frequency is audible in the headphones the valve acts as a self oscillating product detector 17 another very similar application of the bi grid valve was as a self oscillating frequency mixer in early superhet receivers 18 one control grid carried the incoming rf signal while the other was connected into an oscillator circuit which generated the local oscillation within the same valve since the anode current of the bi grid valve was proportional both to the signal on the first grid and also to the oscillator voltage on the second grid the required multiplication of the two signals was achieved and the intermediate frequency signal was selected by a tuned circuit connected to the anode in each of these applications the bi grid tetrode acted as an unbalanced analogue multiplier in which the plate current in addition to passing both input signals includes the product of the two signals applied to the grids the superheterodyne receiver edit main article superheterodyne receiver the principle of the modern superheterodyne or superhet receiver originally named the super sonic heterodyne receiver because the intermediate frequency was at an ultrasonic frequency was invented in france by lucien levy in 1917 19 p 66 though credit is usually also given to edwin armstrong the original reason for the invention of the superhet was that before the appearance of the screen grid valve amplifying valves then triodes had difficulty amplifying radio frequencies i e frequencies much above 100 khz due to the miller effect in the superheterodyne design rather than amplifying the incoming radio signal it was first mixed with a constant rf oscillator the so called local oscillator to produce a heterodyne of typically 30 khz this intermediate frequency if signal had an identical envelope as the incoming signal but a much lower carrier frequency so it could be efficiently amplified using triodes when detected the original modulation of the higher frequency radio signal is obtained 20 a somewhat complicated technique it went out of favor when screen grid tetrodes made tuned radio frequency trf receivers practical citation needed however the superheterodyne principle resurfaced in the early 1930s when their other advantages such as greater selectivity became appreciated and almost all modern receivers operate on this principle but with a higher if frequency sometimes higher than the original rf with amplifiers such as the tetrode having surpassed the triode s limitation in amplifying high radio frequency signals the superheterodyne concept could be implemented using a valve as the local oscillator and a separate valve as the mixer which takes the antenna signal and the local oscillator as input signals but for economy those two functions could also be combined in a single bi grid tetrode which would both oscillate and frequency mix the rf signal from the antenna 18 in later years this was similarly accomplished by the pentagrid converter tube a similar two input amplifying oscillating valve but which like pentode tubes incorporated a suppressor grid and in this case two screen grids in order to electrostatically isolate the plate and both signal grids from each other in today s receivers based on inexpensive semiconductor technology transistors there is no cost benefit in combining the two functions in one active device screen grid valve edit view of the interior of an osram s23 screen grid valve in this valve the anode is in the form of two flat plates the wires of the screen grid can also be seen the anode connection is at the top of the envelope to minimise anode grid capacitance at anode voltages less than that of the screen grid the tetrode characteristic curves are kinked due to secondary emission from the anode in the normal range of anode voltages the anode current is substantially constant with respect to anode voltage both features are quite unlike the corresponding curves for a triode for which anode current increases continuously with increasing slope throughout the marconi osram s625 the first commercially produced screen grid tube the screen is a cylinder with a metal gauze face that completely surrounds the anode and the tube is double ended with the anode terminal at one end and the grid at the other to improve isolation between the electrodes the screen grid tube provides much smaller control grid to anode capacitance and much greater amplification factor than a triode radio frequency amplifier circuits using triodes were prone to oscillation due to the grid to anode capacitance of the triode 21 in the screen grid tube a grid referred to as the screen grid shield grid or sometimes accelerating grid is inserted between the control grid and the anode the screen grid provides an electrostatic shield between the control grid and the anode reducing the capacitance between them to a very small amount 21 22 to reduce the influence of the anode s electric field on the cathode space charge and on the control grid during 1915 1916 physicist walter h schottky developed the first tubes having a grid positioned between the anode and the control grid to provide an electrostatic shield 23 24 schottky patented these screen grid tubes in germany in 1916 and in the u s in 1919 25 26 these tubes were produced in germany and known as siemens schottky tubes 24 in japan hiroshi ando patented improvements to the construction of the screen grid in 1919 27 during the latter half of the 1920s neal h williams and albert hull at general electric h j round at mov and bernard tellegen at phillips developed improved screen grid tubes these improved screen grid tubes were first marketed in 1927 28 feedback through the anode to grid capacitance miller effect of the triode could cause oscillation especially when both anode and grid were connected to tuned resonant circuits as is usual in a radio frequency rf amplifier 29 for frequencies above about 100 khz neutralizing circuitry was necessary a typical triode used for small signal amplification had a grid to anode capacitance of 8 pf while the corresponding figure for a typical screen grid valve was 0 025 pf 30 neutralizing circuits were not required for a well designed screen grid tube rf amplifier stage 31 32 the screen grid is connected to a positive dc voltage and at ac ground as insured by a bypass capacitor to ground 21 the useful region of operation of the screen grid tube as an amplifier is limited to anode voltages greater than the screen grid voltage at anode voltages greater than the screen grid voltage some electrons from the cathode will hit the screen grid producing screen current but...