Valve Theory Continued.
The device can be given a different function, by the addition of another metal plate inside the envelope of the valve. This third metal conductor is known as the grid. The grid is placed between the cathode and the anode, and has gaps or perforations to allow electrons through. If a voltage is applied to this grid, it can be used to influence the number of electrons that the grid allows to pass through itself. The actual rate of electron flow through the grid is determined by how positive or negative the voltage on the grid is when compared to the voltage on the anode. When a high positive voltage is applied to the grid, many electrons will be attracted to it and thence onwards to the anode. As the positive voltage to the grid is reduced, fewer electrons are attracted to the grid, and therefore fewer continue to the anode. As the grid voltage turns negative, it repels the negative electrons and the flow of electrons to the anode is diminished, instead the electrons return back to the cathode. Eventually a point will be reached where the electron flow to the anode ceases completely. Therefore is can be seen that the grid is controlling the number of electrons able to reach the anode. A valve with three electrodes as described is commonly referred to as a triode. If the valve has more than one grid (see next sections) this first grid is known as the control grid. The amount by which the current is increased at the anode is known as the amplification factor (which is not the same as the absolute amplification). The amplification factor is the ratio of the change in anode voltage compared to the change in grid voltage to produce a given change in anode current.
Screen Grid Valve.
The triode is highly suitable for low frequency (LF) amplification, detection, and also as an audio frequency output valve. However for radio frequency (RF) amplification it has a number of shortcomings. Screen grid valves are designed to reduce capacity inside the valve, which might otherwise cause instability or unwanted oscillation from internal feedback from the anode the grid. This achieved by the provision of the screen grid that is physically situated between the control grid and the anode, thereby providing a shield or "screen". When the screen grid valve is used as an RF amplifier, the screen grid is connected to the HT that is already appearing on the anode via a resistor. This means that it is at positive DC voltage to the cathode. The screen grid is also connected to the negative connections (both HT and LT) through a capacitor to provide the shield effect. The screen grid therefore behaves similarly to the anode as it attracts electrons through the control grid.
Screen grid valves are unable to handle large signal voltages. This is because it was found that under certain conditions further electrons were spuriously released which were attracted back to the screen grid, causing the anode current to be reduced and hence a decrease in amplification. This effect is known as secondary emission, and may be overcome to some degree by the provision of a further grid. This grid is positioned between the screen grid and anode, and in normal operation is connected to the power supply negative connections or cathode. This increases the signal voltage that the valve is able to handle, and the third grid is known as the suppressor grid. Pentodes are commonly used in the power output stage of a radio receiver, and various manufacturers also promoted the virtues of this valve type using the names "Beam Tetrode" or "Kinkless Tetrode".
It is possible to incorporate even more electrodes into the valve envelope, and the valve nomenclature indicates this following the expected sequence after pentode, namely hexode (six electrodes), heptode (seven electrodes), octode (eight electrodes), nonode (nine electrodes). More than one device may be provided inside the envelope, for example it is possible to provide two additional diodes inside a triode envelope.
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indirectly heated valves on the next page
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