Control grid
Encyclopedia
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.
.
The addition of just a single control grid to a diode
valve produces a triode
.
alloy
with a gold
plating is frequently used. 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).
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.
changes the mutual conductance 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.
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.
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
used in thermionic valves
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
(vacuum tubes) used to modulate the flow of electrons in the cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
to anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
or plate
Plate electrode
A plate is a type of electrode that formed part of a vacuum tube. The plate is impressed with a positive charge so that it may capture and flow electrons within a circuit....
circuit.
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 amplifierAmplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
.
The addition of just a single control grid to a diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
valve produces a triode
Triode
A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
.
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. MolybdenumMolybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
with a gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
plating is frequently used. It is wound on soft copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
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).
Effects of grid position
By placing the control grid closer to the filament/cathode relative to the anode, a greater amplificationAmplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
results. This degree of amplification is referred to in valve data sheets as the amplification factor
Amplification factor
In structural engineering the amplification factor is the ratio of second order to first order deflections.In calculations this is represented by the symbol AF...
. It also results in higher transconductance
Transconductance
Transconductance, also known as mutual conductance, is a property of certain electronic components. Conductance is the reciprocal of resistance; transconductance, meanwhile, is the ratio of the current change at the output port to the voltage change at the input port. It is written as gm...
, which is a measure of the anode current change versus grid voltage change. The noise figure
Noise figure
Noise figure is a measure of degradation of the signal-to-noise ratio , caused by components in a radio frequency signal chain. The noise figure is defined as the ratio of the output noise power of a device to the portion thereof attributable to thermal noise in the input termination at standard...
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.
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.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 biasGrid bias
Grid bias is a DC voltage applied to electron tubes with three electrodes or more, such as triodes. The control grid of these devices is used to control the electron flow from the heated cathode to the positively charged anode...
changes the mutual conductance and hence the gain of the device. This variation usually appears in the pentode
Pentode
A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid vacuum tube , which was invented by the Dutchman Bernhard D.H. Tellegen in 1926...
form of the valve, where it is then called a variable-mu pentode or remote-cutoff pentode.
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
Miller effect
In electronics, the Miller effect accounts for the increase in the equivalent input capacitance of an inverting voltage amplifier due to amplification of the effect of capacitance between the input and output terminals...
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
Mullard
Mullard Limited was a British manufacturer of electronic components. The Mullard Radio Valve Co. Ltd. of Southfields, London, was founded in 1920 by Captain Stanley R. Mullard, who had previously designed valves for the Admiralty before becoming managing director of the Z Electric Lamp Co. The...
EC91 operated at up to 250 MHz.