The three-electrode lamp constituted the first amplifying device historically of an electronic signal. This lamp was invented by the engineer Lee De Forest in 1906.

See also: Historical of the electron tubes

Operation

The triode is composed of a transmitting Cathode of electron S, of a receiving Anode, and of a grid placed between the two.

• cathode is heated by a filament placed behind. (Sometimes the filament and cathode form one element). The heating of cathode confers sufficient energy on the electrons of cathode to enable them to release themselves (one speaks about cathodic emission) and to join a “electronic cloud” in the Vide surrounding cathode. Cathode has a Potentiel (often the 0V) smaller than that of the anode causing the migration of the electron S towards the anode, because the electrons of negative charge are attracted by the positive loads of the anode.
• While exploiting the potential (tension) of the grid compared to cathode, a number less less large of electrons emitted by cathode will arrive until the anode, creating a flexible current between anode and cathode: amplification is carried out.

Diagram

Characteristics of the triode

The factor of amplification μ of a triode expresses the relationship between the tension amplified on the anode and the tension of entry applied to the grid: $\backslash \; driven\; =\; \{Ua\; \backslash \; over\; Ug\}$

It is also equal to the relationship between the stray capacity roasts - cathode Cgc and stray capacity anode - Cac cathode: $\backslash \; driven\; =\; \{Cgc\; \backslash \; over\; Cac\}$

This second equation shows that a reduction in the distance between the grid and cathode allows augment factor of amplification of the tube, because that will cause to increase the capacity between the grid and cathode.

The law of Child makes it possible to describe the current of anode of a triode: $Ia\; =\; has\; \backslash \; left\; (\backslash \; driven\; Ug\; +\; Ua\; \backslash \; right)\; ^\; \backslash \; frac\; \{3\}\; \{2\}$ where has is a constant which depends on the geometry of the tube.

Expansion of Taylor of this equation in the vicinity of the point of work of the tube, one finds:

The first derivative partial of this equation is the slope of the characteristic of transfer to the point of operation and it is called the mutual conductance $g\_m$ or transconductance , which is expressed EM mho , or more commonly in mA/V (1mmho = 1 mA/V). The second partial derivative is the slope S characteristic of anode. However in the trade, one kept name " S" slope; (in mA/V) to indicate, in Europe, the equivalent of the transconductance, in the United States. The reciprocal one of this slope is called the resistance interns anode $r\_a$ which is expressed in ohm .

For $Ia=0$ and by introducing these definitions, this equation can be written: or.

The term of right-hand side is equivalent to the definition of the factor of amplification. The sign $-$ means that the tension of anode dimunue when the grid voltage increases. This last equation can be written: $\backslash \; driven\; =\; r\_a\; g\_m$, very useful formula to find one of the parameters of the tube when the two others are known.

Application

This invention allowed the appearance of the speaking Cinéma, TSF (radio), and later of the Informatique, as well as progress in the telephone Communication S by allowing a important amplification signal without effects of distortion.

• the triode was quickly improved by addition of one (for the tetrode) then, two additional grids, avoiding undesirable effects, in particular the effect “dynatron”. This tube Pentode will quickly be adopted the majority of the tube amplifiers, for its best output. Let us note that those are always used today, and very appreciated guitarists (amplifying) and amateurs of HI-FI (in spite of the need for replacing the tubes regularly, whose cathode becomes exhausted) because of their excellent qualities for the reproduction of the sound, among which one can note:
• the enviable harmonic spectrum in linear mode,
• the good answer with the transients producing a more natural considered sound,
• and incidentally the entry in soft saturation during the chopping (although it is not use in Hi-fi to enter the zones of saturation and chopping, if one excludes the rather particular case of an amplifier in class B!)

Apart from the marginal field of the collectors and music lovers, the triode is largely used like amplifier of strong power (500W and beyond) until 100MHz approximately, in assembly " roast commune" , with the advantage of absence of neutrodynage, contrary to the Pentode. These triodes for linear amplifiers HF can provided 500W to 15KW. The principal suppliers are EIMAC and AMPEREX.

Standard models

Principal triodes used in audio

For the preamplifiers

For the amplifiers (power)

Principal industrial triodes

• 3CW30000H3 maximum dissipation 30kW with 100MHz
• 3CW20000A7 maximum dissipation 20kW with 140MHz
• 3CX15000H3 maximum dissipation 15kW with 90MHz
• 3CX10000A7 maximum dissipation 10kW with 160MHz
• 8161R maximum dissipation 4kW with 110MHz
• 833A maximum dissipation 0.4kW with 30MHz
• TH5-4 maximum dissipation 4kW with 110MHz
• TH6-3 maximum dissipation 10kW with 140MHz
• 3-500Z maximum dissipation 1000W with 50MHz

(indicative values being able to vary according to the manufacturer)

See too

Random links:

Sarracénie crimson | Beginning of Evening | Moving sand | Federal minister with special attributions | Hermann Talmadge