An explanation of the multi-grid HF valve, and its application to modern receivers.
The Mullard SP4 HF pentode of 1933.
It is an open secret that the coming season will almost certainly see the general use of high-frequency pentodes as radio-frequency amplifiers. Although, at first, they may only make their appearance in sets of the most advanced design, there is no doubt that in due course valves of this type will, to a very large extent, replace the familiar four electrode or screened-grid valve in the high-frequency stages.
For this reason, it is not too early for listeners to learn something of the new method of amplification, and the reasons for its adoption. In the first place, it must be stated right away, that actually high-frequency pentodes have been available for some little time, although so far they have not been used very widely. Now, however, their appearance in commercial form, for the use of the average listener, is imminent.
Where is the Difference?
In external appearance, and in general design, high-frequency pentodes do not differ greatly. from screened-grid valves, the only addition being the third grid which, like the low-frequency, pentode third grid, is connected to the cathode of the valve. In application, also, the high-frequency pentode and the screened-grid valve are identical, so that the user will have no difficulty whatever in accustoming himself to the new valves.
What, then, will. be the difference between the two types of amplifier? Briefly, the main difference will lie in the much greater sensitivity of the pentode, giving a much larger overall gain per valve stage, but in addition, the pentode allows a considerably wider degree of latitude as regards operating conditions, as will be explained. First of all, however, because the high-frequency pentode must be considered as a development of the screened-grid valve, it is necessary to consider briefly the facts which led up to the introduction of the screened-grid valve itself.
Up till about five years ago, the only type of valve available for use as a high-frequency amplifier was the ordinary three electrode valve. Theoretically, such a valve should be quite a good amplifier of radio-frequency signals, but in practice its possibilities could not be fully realized. You see, owing to the physical design of the valve (the position and closeness of the electrodes and the leads coming from them) the metal parts of the valve acted as the plates of a very small capacitor. For low-frequency work, this capacitor effect, or inter-electrode capacity as it is called, was not very serious, but on radio frequencies this small capacitor allowed a considerable part of the amplified high-frequency energy in the anode circuit to pass back to the grid circuit within the valve.
Feed Back Difficulties
You will scarcely need to be reminded that the return of energy from the anode circuit to the grid circuit in any valve is equivalent to the process known as reaction, the energy so fed back being re-amplified in the valve. At first sight, this may seem a very efficient and useful arrangement, but the difficulty was that owing to this re-amplification, more high-frequency energy was built up than could be utilized, or, to put the matter in technical language, the valve generated free oscillations. This excess energy produced an unstable condition of the circuit, and was manifest by howling and other distressing symptoms.
The screened-grid valve was the final and best solution to the difficulty. In addition to the usual control grid, this valve has a second grid which is kept at a positive potential of approximately half the high tension voltage applied to the anode. Two effects follow. In the first place, the second grid, being arranged between the true grid and the anode, and being connected to earth via the high-tension battery, acts as an electrostatic screen between the grid and anode, and thus prevents, high-frequency energy from passing back from the anode to the grid. The circuit thus remains stable, especially if care is taken to screen effectively all other components in the high-frequency circuits. The second point is that the amplification factor of the screened grid valve is very much greater than that of the triodes it replaced, and because of the stable condition of the circuit, this high amplification factor can be utilized to good advantage.
SG Valve not Perfect
Why, then, is it now found necessary to introduce a further type of high-frequency valve? Well, in spite of the high efficiency of the screened-grid valve, it is not perfect in certain respects. I do not want to go too deeply into technicalities, so it must suffice to say that in order to obtain stable, working with a screened-grid valve, a certain rough proportion between the screen voltage and anode voltage must be maintained. The respective values are not very critical, as anyone who has used a screened-grid valve will know, but the fact remains that the value of the anode current is not absolutely and at all times governed by the voltage applied to the grid of the valve. In certain circumstances, an interaction takes place between the screen and the anode, and the anode current variations do not follow the variations of grid voltage. Not only does this mean distortion, but also an unstable condition of the circuit.
In the high-frequency pentode, a third grid is situated between the screen grid and the anode, and is connected to the cathode or filament, as the case may be. It thus acts as an electrostatic screen and shields the anode from the screen grid, preventing the flow of electrons from the anode to the screen grid - the cause of the unstable condition referred to above.
Thus, We have the screen grid used to prevent high frequency energy passing from the anode to the control grid, and the third or earth grid employed to prevent secondary emission from the anode to the screen grid. As a result, the high-frequency pentode can be relied upon to give efficient and stable amplification because the anode current is governed almost entirely by the signal applied to the control grid, and is not effected by the relation between the screen voltage and anode voltage to the same extent as in the screened grid valve.
High Impedance Necessary
In addition to this, the characteristics of the high-frequency pentode are much higher than those of a screened-grid valve. That is to say, we may expect higher values of amplification factor and of mutual conductance. Other things being equal, therefore, a high-frequency pentode should give a greater degree of amplification than a screened-grid valve. There is, however, one little snag. On account of its special construction, the high-frequency pentode has a considerably higher impedance than a screened-grid valve. Now the overall amplification obtained from a valve, while it depends to a large extent upon the amplification factor of the valve, is also governed by the impedance of the valve and of the apparatus connected in its anode circuit. Actually, the overall amplification, or stage gain as it is termed, is given by the formula:-
Stage gain = (Amplification factor x load impedance)/(Valve impedance + load impedance)
It will be seen, therefore, that the stage gain can never be as great as the amplification factor of the valve, but that, subject to certain practical limitations, the higher the impedance of the load in the anode circuit, the larger the proportion of the amplification factor which can be utilized. In order to make full use of the high amplification factor of the high-frequency pentode therefore, it will be necessary to use tuned high-frequency couplings of the highest possible efficiency as a high efficiency tuned circuit forms a load of high impedance. The introduction of the high frequency pentode, therefore, will stimulate manufacturers to produce coils of higher and higher efficiency, for without these the advantages of the new valves cannot be exploited to the fullest possible extent. At the same time, even when using what are now termed high-efficiency coils, such as are employed in the best home constructed and commercial sets of to-day, the high-frequency pentode will give better amplification than the screened grid valve.
Why are radio engineers so anxious to increase the sensitivity of receivers by introducing these high-amplification valves? One reason is undoubtedly the rapidly increasing condition of ether congestion, due to more and more high-powered broadcasting stations coming on the air. In order to cope with this situation it is necessary to adopt various devices to improve selectivity, all of which have the effect of reducing signal strength so that increased amplifying power in the set is necessary. Again, more and more listeners are demanding sets which Will work quite satisfactorily and give a full choice of programmes without the use of an outdoor aerial, and it is only by using the most efficient types of radio-frequency amplifier that mains aerials and plate aerials within the cabinet can be suitably employed.
It may be asked what are the signal handling capabilities of the screened-grid pentode. The answer is approximately the same as an ordinary screened-grid valve. In other words, apart from such minor alterations as the adjustment of voltage dropping resistances, screen potentiometers and grid bias, a high-frequency pentode can be substituted simply for an existing screened-grid valve, and will work well, although better results will be obtained if more efficient coils are also fitted. When the set is tuned-in to a very strong signal, however, such as that from the local station, there will be the same risk of distortion due to overloading as with a screened-grid valve, unless some form of input control is fitted.
For this reason it is expected that valve makers will offer a choice of high-frequency pentodes, some of which will have characteristics similar to the variable-mu screened-grid valve, namely that by increasing the grids bias the valve is able to handle greater signals without distortion, but, of course, giving a smaller degree of amplification.