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Dynatrons are special types of tetrodes that utilize a screen grid voltage higher than the plate voltage. This causes the plate current to decrease as plate voltage increases. This characteristic, known as negative resistance, is useful in oscillator circuits. See triode and diode.
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| The Tetrode Uses Two Grids |
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| In a tetrode, the feedback and oscillation are controlled by the addition of a screen grid between the control grid and plate. |
tetrode [′te‚trōd]
Tetrode
a four-electrode electron tube containing a hot cathode that is directly or indirectly heated, two grids, and an anode, or plate. Tetrodes are used as receiving tubes or as low-power, medium-power, or high-power oscillator tubes for frequencies of up to several tens of megahertz.
Receiving tetrodes usually have an oxide-coated cathode and a nickel anode; oscillator tetrodes usually have a tungsten cathode and a tantalum or molybdenum anode. Tetrode grids are made of nickel, molybdenum, or tungsten wire wound on support rods. As a result, the grid closer to the cathode serves as the control grid; the other grid has a smaller wire spacing than the first and serves as the screen grid.
The tetrode was developed in 1919 by the German scientist W. Schottky and, in essence, is an improved version of the triode. The introduction of a screen grid made it possible to reduce the direct capacitance between the control grid and the plate by a factor of about 100. This reduction is significant in operation at high frequencies. The use of a screen grid also improves the electrical parameters of tetrodes. In particular, the amplification factor of the tube is increased by about an order of magnitude without reducing the plate current and without shifting the operating range of the grid-plate transfer characteristic toward higher grid voltages—that is, to the right along the axis of abscissas—at a given plate voltage. Furthermore, in a tetrode the plate and control-grid leads are carefully screened within the tube and are placed in opposite parts of the glass envelope, for example, in the upper and lower sections. This arrangement provides an additional reduction in the direct capacitance.
A serious disadvantage of tetrodes is the dynatron effect, whereby the main electron stream in the tube causes one electrode to emit secondary electrons, which flow to another electrode at a higher potential. Because of this effect, receiving tetrodes have been almost completely replaced by beam tetrodes and by pentodes, neither of which has this advantage. To suppress the dynatron effect in beam tetrodes, special electrodes are used that focus the main electron stream into narrow beams. As a result, a space charge, which prevents the secondary electrons emitted by the plate from reaching the screen grid, is produced in the vicinity of the plate. Oscillator tetrodes usually operate at plate voltages that are much higher than the screen-grid voltages; the dynatron effect is thus insignificant in such tubes.
REFERENCE
Vlasov, V. F. Elektronnye i ionnye pribory. Moscow, 1960.S. M. MOSHKOVICH
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