The first Fleming valve from which all our present-day multi-electrode valves have been developed, was the original detector, and to-day the diode detector has returned to favour. The diode action is briefly as follows :-
Electrons leave the heated filament or cathode, and are attracted to the anode provided this is at a positive potential with respect to the cathode. The space current flows when the anode is positive only; when negative, no current will flow. Consequently, if A.C. is applied to the anode a space current flows only on the positive half cycle, and hence a current flowing always in the same direction is obtainable.
The more usual form of this valve is as a double diode and current is then obtainable on both halves of the cycle, and we have full wave rectification. Or, as is often the case in practice, one anode is used for half wave rectification while the other is employed for automatic volume control by applying the rectified voltage dependent on the R.F. carrier to the grids of the preceding vari-mu H.F. valves.
The normal rectifier valve is merely the diode with larger electrodes and a more copious electron stream, made possible by a more generous cathode or filament area for emission. There are both directly and indirectly heated types, single phase and bi-phase, and they are designed to rectify alternating current mains supplies to give high tension energy to receivers.
An important point in rectifier design is the necessity for low voltage drop across the valve. This may be effected by making the anode closely surround the cathode, having regard to mechanical and electrical limitations. When extremely low voltage drop is necessary mercury vapour rectifiers are used, the advantage being that the mercury vapour ionises and tends to neutralise the space charge effect.
Since the introduction of the third electrode into the diode, and the consequent possibility of amplification, various types of triode for specific purposes have been developed. Broadly speaking there are three classifications : the H.F. and detector type with high magnification factor and impedance values of 10-30,000 ohms, the intermediate L.F. amplification triode with impedances of the order of 10,000 ohms, and finally the output triode with low amplification factors and impedances of from 5,000 ohms downwards.
Each of these types has been highly specialised, not only electrically to give the best results when associated with its particular circuit, but also mechanically, the finest points of structure being varied according to type. Microphony in H.F. and detector valves has been prevented by the seven point suspension, involving the threading of the filament through tiny hooks projecting inside the grid turns ; the effects of overheating on large output valves has called for much variety and skill in the methods of attacking grid and anode cooling.
The screen grid tetrode valve consists of the standard three electrodes together with a close mesh screen interposed between the signal grid and the anode. This fourth electrode is held at a high positive potential with respect to the cathode, but is usually lower than the anode potential.
The outstanding feature of the tetrode valve is that the screen grid
acts as an electrostatic shield between the control grid and anode, and
thus prevents uncontrollable feed back from the output to the input
circuit. Normal detector triodes for instance, have a grid to anode
capacity of 5-10 micro-micro F, whereas tetrodes may have this capacity
reduced to 0.001 micro-micro F. In consequence of this, a much greater
stable amplification can be obtained from tetrodes.
The anode current-anode voltage characteristic curve of the tetrode valve is of somewhat peculiar shape. Firstly, with increasing anode voltage the anode current rises and then falls, giving the characteristic negative resistance dip of the tetrode, and finally rises again, and thereafter remains practically parallel to the voltage axis. The cause of the dip in the characteristic is due to the phenomenon of secondary emission from the anode when the latter is at a lower potential than the screen. As the anode potential is progressively raised from zero, there is first an anode current rise owing to the electrons drawn through the screen being all collected by the anode. A further increase in anode voltage causes the primary electrons to strike the anode with sufficient velocity to give rise to secondary electrons which reach the screen, and if more secondaries are leaving the anode than primaries striking it, the net effect will be a fall in plate current accompanied by a rise in screen current. With still increasing anode potential all the available electrons are drawn to the anode and only a very small increase in current will result. Hence an extremely high impedance is obtained when the valve is operated at an anode potential well above the screen, and with a suitable associated anode circuit a very high stage gain may be realised.
The five-electrode, or pentode, valve is really the tetrode valve with a coarse mesh grid inserted between anode and screen electrodes. This additional grid is usually internally connected to a low potential electrode, and is termed the earth or suppressor grid. Its main function is to remove the secondary emission dip from the tetrode characteristic. This is accomplished by placing it near the anode, and while being sufficiently open mesh not to impede high velocity primary electrons, it is sufficient to repel low voltage secondary electrons from the anode back to the anode, rather than let them pass through it to the screen.
Two types of pentode have now been developed - the output and more recently the H.F. Pentode. In the case of the output valve, using the correct anode circuit load, it is possible with modern valves to get an exceptionally high anode circuit efficiency, which is a most important consideration for output circuits where dry batteries are to be relied upon for H.T. supply. With H.F. Pentodes a very high voltage amplification is possible, and the only serious limitation is the attainable associated anode circuit impedance.
It has already been pointed out that the triode valve has been developed along specialised lines according to its function in the receiver - that tetrodes and pentodes have been introduced which were really the first multi-electrode valves which fulfilled the duty of more than one triode valve. This process of developing valves, which by virtue of their characteristic, or by dual operation, are the equivalent of more than one simple valve, has been still further extended and embraces double diode triodes, double diode pentodes, Class B amplifiers, and Pentagrids.
The double diode triode consists of the usual three-electrode valve together with two diode anodes mounted round the same cathode sleeve. The diodes may be used for full or half wave rectification or for A.V.C. systems, and the triode is a straightforward L.F. amplifier feeding the output valve.
The double diode pentode also has the diodes mounted on the cathode in common with the pentode section, and the valve acts as a special vari-mu L.F. Pentode. The valve is intended for corrected A.V.C., gain being varied both in the preceding H.F. stages and on the Pentode itself.
In the case of Class B amplifiers, we have in reality two separate triode valves mounted in the same envelope, and their virtue lies in the fact that they are capable of giving extremely large output for a very low average anode current. In other words, it is an extremely high efficiency output valve which is capable of giving, with ordinary H.T. battery supply, a power output which is usually associated with mains-driven receivers.
The Pentagrid is the most recent example of multi-electrode devices designed to simplify Superheterodyne receivers. It fulfils the two functions of providing the local oscillation and frequency conversion. This is accomplished by a triode oscillator section surrounding the cathode, followed by a tetrode assembly. Since all these electrodes affect the same cathode electron stream, frequency conversion is possible by internal mixing of the local oscillator frequency with the radio frequency input to the modulator grid.