Editing Triode (section)

== Construction ==
[[Image:Triode-english-text.svg|right|thumb|Structure of a modern low-power triode vacuum tube. The glass and outer electrodes are shown partly cut away to reveal the construction.]]
[[File:Triode schematic labeled.svg|thumb|[[Schematic symbol]] used in [[circuit diagram]]s for a triode, showing symbols for electrodes.]]

All triodes have a hot [[cathode]] electrode heated by a [[electrical filament|filament]], which releases electrons, and a flat metal [[plate electrode]] (anode) to which the electrons are attracted, with a [[Control grid|grid]] consisting of a screen of wires between them to control the current.  These are sealed inside a glass container from which the air has been removed to a high vacuum, about 10<sup>−9</sup> atm.  Since the filament eventually burns out, the tube has a limited lifetime and is made as a replaceable unit; the electrodes are attached to terminal pins which plug into a socket.  The operating lifetime of a triode is about 2000 hours for small tubes and 10,000 hours for power tubes.

=== Low power triodes ===
Low power triodes have a concentric construction ''(see drawing right)'', with the grid and anode as circular or oval cylinders surrounding the cathode. The [[cathode]] is a narrow metal tube down the center.  Inside the cathode is a [[electrical filament|filament]] called the "heater" consisting of a narrow strip of high resistance [[tungsten]] wire, which heats the cathode red-hot (800 - 1000&nbsp;°C).  This type is called an "[[Hot cathode|indirectly heated cathode]]".  The cathode is coated with a mixture of [[alkaline earth]] oxides such as calcium and [[thorium oxide]] which reduces its [[work function]] so it produces more electrons.  The grid is constructed of a helix or screen of thin wires surrounding the cathode.  The anode is a cylinder or rectangular box of sheet metal surrounding the grid.  It is blackened to radiate heat and is often equipped with heat-radiating fins. The electrons travel in a radial direction, from cathode through the grid to the anode. The elements are held in position by [[mica]] or ceramic [[Insulator (electricity)|insulators]] and are supported by stiff wires attached to the base, where the electrodes are brought out to connecting pins. A "[[getter]]", a small amount of shiny [[barium]] metal evaporated onto the inside of the glass, helps maintain the vacuum by absorbing gas released in the tube over time.

=== High-power triodes ===
High-power triodes generally use a [[electrical filament|filament]] which serves as the cathode (a directly heated cathode) because the emission coating on [[Hot cathode|indirectly heated cathodes]] is destroyed by the higher ion bombardment in power tubes. A [[thoriated tungsten]] filament is most often used, in which [[thorium]] added to the tungsten diffuses to the surface and forms a monolayer which increases electron emission.  As the monolayer is removed by ion bombardment it is continually renewed by more thorium diffusing to the surface.  These generally run at higher temperatures than indirectly heated cathodes. The envelope of the tube is often made of more durable ceramic rather than glass, and all the materials have higher melting points to withstand higher heat levels produced. Tubes with anode power dissipation over several hundred watts are usually actively cooled; the anode, made of heavy copper, projects through the wall of the tube and is attached to a large external finned metal [[heat sink]] which is cooled by forced air or water.

=== Lighthouse tubes ===
{{More citations needed section|date=April 2022}}[[Image:Scheib3.jpg|thumb|Soviet lighthouse tube '''6С5Д''' (6S5D)]]
A type of low power triode for use at [[Ultrahigh frequency|ultrahigh frequencies]] (UHF), the "lighthouse" tube, has a planar construction to reduce interelectrode [[capacitance]] and lead [[inductance]], which gives it the appearance of a "lighthouse".  The disk-shaped cathode, grid and plate form planes up the center of the tube - a little like a sandwich with spaces between the layers.  The cathode at the bottom is attached to the tube's pins, but the grid and plate are brought out to low inductance terminals on the upper level of the tube: the grid to a metal ring halfway up, and the plate to a metal button at the top. These are one example of "disk seal" design. Smaller examples dispense with the octal pin base shown in the illustration and rely on contact rings for all connections, including heater and D.C. cathode.

As well, high-frequency performance is limited by transit time: the time required for electrons to travel from cathode to anode. Transit time effects are complicated, but one simple effect is input conductance, also known as grid loading. At extreme high frequencies, electrons arriving at the grid may become out of phase with those departing towards the anode. This imbalance of charge causes the grid to exhibit a reactance that is much less than its low-frequency "open circuit" characteristic.

Transit time effects are reduced by reduced spacings in the tube. Tubes such as the 416B (a Lighthouse design) and the 7768 (an all-ceramic miniaturised design) are specified for operation to 4&nbsp;GHz. They feature greatly reduced grid-cathode spacings of the order of 0.1&nbsp;mm.

These greatly reduced grid spacings also give a much higher amplification factor than conventional axial designs. The 7768 has an amplification factor of 225, compared with 100 for the 6AV6 used in domestic radios and about the maximum possible for an axial design.

Anode-grid capacitance is not especially low in these designs. The 6AV6 anode-grid capacitance is 2 picofarads (pF), the 7768 has a value of 1.7 pF. The close electrode spacing used in microwave tubes ''increases'' capacitances, but this increase is offset by their overall reduced dimensions compared to lower-frequency tubes.