The Triode Valve

The Triode Valve

This cut-away view of a triode electrode assembly shows the three main electrodes, plus the heater. (b) Circuit symbol for an

indirectly heated triode.

The addition of one further electrode into the space between the cathode and the anode makes three altogether, and accounts

for the name, triode.

Suppose a metal mesh is placed between the cathode and the anode. Electrons will flow through the gaps in this mesh, or grid

and reach the positive anode as before. But if an electric potential is placed between this grid and the cathode, the flow

of electrons will be greatly affected. If there is a negative voltage on the grid, the electrons will be repelled and slowed down, and fewer will reach the anode. In fact, if the grid is made sufficiently

negative, all the electrons will be repelled back, and none will pass to the anode, despite its positive potential. We say the

valve is cut of or in electronics, switched off at the grid, when this condition occurs.

The importance of this third electrode can be appreciated now: small changes of the grid voltage can produce much greater

changes in the current passing through the valve. Not without justification is this third electrode frequently referred to as the control grid. You can see that a triode can be looked on as a

voltage-change to current-change converter, and is in fact, an amplifier of signals. Amplifications of 50 to 100 times are

possible with modern valves.

The graph shows how the current flowing through a triode is influenced by the voltage applied to the control grid. As the grid

is made more negative, a point is finally reached where the anode current ceases altogether. This is the 'cut-off' value of

grid bias.

In practice, the grid is held at a small negative voltage with respect to the cathode. This is called the grid bias and the small signal voltage variations increase and decrease the value of the

grid voltage about this point. Only in exceptional cases is the grid allowed to rise positive to the cathode, and therefore attract

electrons to itself.

Valve Construction

Most substances must be heated to a very high temperature before in appreciable number of electrons are given off. Early

valves used tungsten filaments, which had to be heated to 2,200 Centigrade before electrons were emitted. They glowed like an

electric lamp, and much power was consumed to heat them.

An alloy of tungsten and thorium came into use next, and operated at about 1,600C. The great break-through came when

oxide coated filaments (and soon after, separate cathodes) became possible. The filaments of directly heated valves are

usually made of tungsten. The separate cathodes are cylinders of nickel or molybdenum. The carbonate of the metal barium or

strontium is painted onto the cathode, and this is heated to quite high temperatures during the pumping of the vacuum,

which forms the oxide. This delicate oxide coating emits electrons readily at temperatures of about 750C, so that only a

dull red heat is required.

The anode plate is usually of nickel or molybdenum, blackened to radiate heat. It is held by welding thick wires to it, these passing through mica end supports wedged into the glass

envelope. In use, the bombardment of the anode by the speeding electrons produces heat. The anode must remain cool,

or electrons might be emitted from it also, although other damage is usually done long before the temperature rises to the

electron emission point.

Now to the grid. There are very few materials suitable for this electrode, and again tungsten or molybdenum are the usual

metals employed. The grid is formed by winding a spiral of fine wire round two stout support wires (held in the mica pieces).

DESCRIPTION :Current cannot flow if the Grid is held at a negative potential to the cathode because the electrons are repelled from the grid back towards the cathode. As the Grid is raised in potential slightly higher than the Cathode, current begins to flow through the tube and through the load. A very small change in voltage on the Grid causes a large change incurrent flow from anode to Cathode. The Grid has a small potential applied and carries a very small current (a few micro amps) but it is able to control much larger power passing through the tube. The effect when a very small change in applied energy can control a large amount of energy is called ‘Amplification’

Applications

Although triodes are now largely obsolete in consumer electronics, having been replaced by the transistor, triodes continue to be used in certain high-end and

professional audio applications, as well as in microphone preamplifiers and electric guitar amplifiers.

Some guitarists routinely drive their amplifiers to the point of saturation, in order to produce a desired distortion tone. Many people prefer the sound of triodes in such an application, since the distortion of a tube amplifier, which has a "soft" saturation characteristic, can be more pleasing to the ear than that of a typical solid-state amplifier, which is linear up to the limits of its supply voltage and then clips abruptly. However, this typically only applied to the power stage of a tube amplifier.

Characteristics

In triode datasheets, characteristics linking the anode current Ia to anode voltage Va and grid voltage Vg were usually given. From here, designer would choose the operating point of the particular triode.

In the example characteristic shown on the image, we can select our anode voltage Va=200 V. Next we can select grid DC voltage bias, let's use -1 V. This corresponds to the yellow curve on our graph. We can see that anode current of about 2.25 mA corresponds to this operating point.

Principle of voltage amplification

In the class A triode amplifier, anode resistor would be connected between anode and the positive voltage source. If we select value of Ra=10000 Ohms, voltage drop on it will be

VRa=Ia*Ra=22.5 V,

if anode current of Ia=2.25 mA is chosen.

Now, if input voltage amplitude (at grid) changes from -1.5 V to -0.5 V (difference of 1 V), anode current will change from 1.2 to 3.3 mA (see image). This will result in anode resistor voltage drop changes from 12 to 33 V (difference of 21 V).

Since grid voltage changes from -1.5 V to -0.5 V, and anode resistor voltage drop from 12 to 33 V, amplification of signal resulted. Amplification factor is 21 - output voltage amplitude divided by input voltage amplitude

The Mutual Conductance

Very often we are interested in the change of the plate current for different changes in the grid voltage. This is expressed by means of a quantity called mutual conductance, gm, or, as it is sometimes called, the transconductance sm. It is defined as the change of plate current for a change of 1 volt on the grid. Thus

Since the plate resistance changes with the voltages on the tube, so also does the mutual conductance. For different triodes on the market, the mutual conductance ranges from 200 to 5,000 micro-mhos (units of conductance). A value of 5,000 micro-mhos means that the plate current will change by 5 milliamperes when the voltage change of the grid is 1 volt.

There is a relationship between transconductance, plate resistance, and amplification factor:

µ = S x Ra

The static amplification factor represents the best-case voltage gain that could be obtained if the ratio of load resistance to the tube's plate resistance can be increased to any large number. This means virtually a load resistance line running horizontally on the plate-characteristic family of curves, corresponding to an ideal constant-current source. Using a practical load resistance, a voltage gain only less than µ is available that can be calculated as V.