The control grid is an electrode used in thermionic valves (vacuum tubes) used to modulate the flow of electrons in the cathode to anode or plate circuit. In electronics, a vacuum tube (U.S. and Canadian English) or (thermionic) valve (outside North America) is a device generally used to amplify, or otherwise modify, a signal by controlling the movement of electrons in an evacuated space. ... Diagram of a copper cathode in a Daniells cell. ... Diagram of a zinc anode in a Daniells cell. ... Diagram of Vacuum-Tube Diode A plate is a type of electrode that formed part of a vacuum tube. ...

Operation

A relatively small variation in voltage on the control grid causes a significantly large variation in anode current. The presence of a resistor in the anode circuit causes a large variation in voltage to appear at the anode. Thus the device will function as an amplifier. For the British rock band of the same name, see Amplifier (band) An amplifier can be considered to be any device that uses a small amount of energy to control a source of a larger amount of energy, although the term today usually refers to an electronic amplifier. ...

The addition of just a single control grid to a diode valve produces a triode. Types of diodes In electronics, a diode is a component that restricts the direction of movement of charge carriers. ... Simplified diagram of a triode. ...

Construction

The grid in the first triode valve consisted of a zig-zag piece of wire placed between the filament and the anode. This quickly evolved into a helical grid placed between a single strand filament (or later, a cylindrical cathode) and a cylindrical anode. The grid is usually made of a very thin wire that can resist high temperatures and is not prone to emitting electrons itself. Molybdenum alloy with a gold plating is frequently used.333333333ghhgh by dog General Name, Symbol, Number molybdenum, Mo, 42 Chemical series transition metals Group, Period, Block 6, 5, d Appearance gray metallic Atomic mass 95. ... An alloy is a combination, either in solution or compound, of two or more elements, at least one of which is a metal, and where the resulting material has metallic properties. ... General Name, Symbol, Number gold, Au, 79 Chemical series transition metals Group, Period, Block 11, 6, d Appearance metallic yellow Atomic mass 196. ...

 It is wound on soft copper sideposts, which are swaged over the grid windings to hold them in place. A 1950s variation is the frame grid, which winds very fine wire onto a rigid stamped metal frame. This allows the holding of very close tolerances, so the grid can be placed closer to the filament (or cathode). 

sweaty man General Name, Symbol, Number copper, Cu, 29 Chemical series transition metals Group, Period, Block 11, 4, d Appearance metallic brown Atomic mass 63. ...

Effects of grid position

By placing the control grid closer to the filament/cathode relative to the anode, a greater amplification results. This degree of amplification is referred to in valve data sheets as the amplification factor. It also results in higher transconductance, which is a measure of the anode current change versus grid voltage change. The noise figure of a valve is inversely proportional to its transconductance; higher transconductance generally means lower noise figure. Lower noise can be very important when designing a radio or television receiver. For the British rock band of the same name, see Amplifier (band) An amplifier can be considered to be any device that uses a small amount of energy to control a source of a larger amount of energy, although the term today usually refers to an electronic amplifier. ... Transconductance, also known as mutual conductance, is a property of certain electronic components. ... In telecommunication, noise figure (NF) is the ratio of the output noise power of a device to the portion thereof attributable to thermal noise in the input termination at standard noise temperature (usually 290 K). ...

Multiple control grids

A valve can contain more than one control grid. The hexode contains two such grids, one for a received signal and one for the signal from a local oscillator. The valve's inherent non-linearity causes not only both original signals to appear in the anode circuit, but also the sum and difference of those signals. This can be exploited as a frequency-changer in superheterodyne receivers. The Super Heterodyne receiver (or to give it its full name, The Supersonic Heterodyne Receiver) was invented by Edwin Armstrong in 1918. ...

Grid variations

A variation of the control grid is to produce the helix with a variable pitch. This gives the resultant valve a distinct non-linear characteristic. This is often exploited in R.F. amplifiers where an alteration of the grid bias changes the mu and hence the gain of the device. This variation usually appears in the pentode form of the valve, where it is then called a variable-mu pentode or remote-cutoff pentode. Electron tubes (or valves in British English) are electronic devices with two or more electrodes; grid bias is a term used on devices with three electrodes or more, such as triodes. ... In electronics, a vacuum tube (American English) or (thermionic) valve (British English) is a device generally used to amplify a signal. ...

One of the principal limitations of the triode valve is that there is a capacitance between the grid and the anode. A phenomenon known as the Miller Effect causes this capacitance to be magnified in proportion to the amplification factor of the valve. This can severely limit the upper operating frequency. This can be overcome by the addition of a screen grid, however in the later years of the tube era, constructional techniques were developed that rendered this 'parasitic capacitance' so low that triodes operating in the upper VHF bands became possible. The mullard EC91 operated at up to 250 MHz. In electronics, the Miller effect describes the fact that a capacitance between input and output of an amplifier is multiplied by (with is the voltage gain) in a electrical circuit. ... A grid introduced into a thermionic valve or tube to greatly reduce the capacitance between two other parts of the electrode structure. ...

References

• [1] (regarding variable mu)