Radio Broadcast (May 1929-Apr 1930)

A Study of Performance and Possibilities i liimiiriMiiilniKMiilliilllllll II 1 1 1 1 m 1 1 n 1 1, AN ENGLISH OUTPUT TUBE, THE PENTODE By W. T. COCKING Radio Engineer, Receptite Company, London T. Coding A few months ago Radio Broadcast published a brief description of the pentode, a new power output tube which is now common in England. Although, as far as the writer is aware, there is none available in America at present, it is certain that if it offers a real advantage over other tubes some firm will market it before long; but before this happens it is desirable that its advantages and disadvantages should be well known. In this article the writer gives a description of the tube's special characteristics, and explains what it will and will not do; together with data which he has collected in the course of a year's experimenting with it in receivers intended for commercial production. At first sight it may seem that there are no special conditions to be observed when using the pentode, but this is not the case, although many people in England still seem to believe so. Several Types Available The tube which the writer has chosen for the purpose of illustration in this article is the Mullard P. M. 22; he has chosen this tube because it was the first pentode on the market, and because he has therefore had more experience with it than with any other. As a matter of interest, pentodes are now available with filaments for a two-, four-, or six-volt A battery, and of several different makes. As the name pentode implies, the P. M. 22 has five electrodes, a filament, three grids, and a plate. It is fitted to the standard English four-pin base, to which the connections for filament, control-grid and plate are taken in the usual way; on the side of the base, however, a binding-post is fitted for the connection to the auxiliary grid. The electrodes are all mounted inside each other in the following order: filament, control grid, auxiliary grid, outer grid, and plate. The filament is of the coated type, and in the two-volt pattern takes a current of 0.3 ampere, but in the six-volt pattern g 2 b/ kl 1 1— only 0.1 ampere, like the majority of English tubes. The electrodes are ah assembled in a horizontal position, but they cannot be seen clearly as the tube is heavily gettered. In use, the control grid and plate are treated in exactly the same way as those of a triode, the auxiliary grid is connected to the same source of high potential The writer of this article is a radio engineer by profession and can vouch for the accuracy of the facts given; he has for the last five years been radio engineer of the Receptite Co., London, England, in whose laboratory the data given have been obtained in the course of experimental work in connection with the design of receivers intended for commercial production in England. „, — The Editor. as the plate; the outer grid, however, is connected internally to the centre of the filament, and it is impossible to make any connection to it. In Table I are given the maker's published figures for the characteristics of the P. M. 22. It is important to note that when the tube is used at its maximum plate FREQUENCY Fig. 2 500 1000 2000 5000 10,000 20,000 50,000 100,000 10AD IMPEDANCE Fig. 1 and auxiliary grid potentials, the amplification factor and plate resistance are very different from the rated figures, at the working point, A (Fig. 4) ; they are 47 and 27,000 ohms, respectively. This large difference is principally due to the higher voltage used on the auxiliary grid, the maker's curves being taken at 100 volts only, although individual specimens vary far more than is the case with triodes. The plate-current plate-voltage curves of the P. M. 22 with a voltage of 150 on the auxiliary grid are given in Fig. 4; it will be seen that they are totally different from those of any triode. If a perfect pentode could be made, its curves would be horizontal straight lines, just as the curves of a perfect triode would be straight lines inclined at an angle to the base. Although the perfect pentode cannot be obtained, Fig. 4 shows that for plate voltages above about 60 volts the curves are almost completely straight. When voltages lower than 150 are applied to the auxiliary grid the curves are not nearly so straight, they commence curving at about 120 volts plate potential; but when the auxiliary grid potential is higher than 150 volts the curves approach more nearly to the ideal, for they are straighter at low plate voltages and they are nearer to the horizontal. More than 150 volts, however, is in excess of the maker's rating, and if it were used it would probably result in a short life for the tube. To those accustomed to high-resistance triodes the high plate current of the pentode comes as a surprise. A triode of the same internal resistance will take perhaps 2 mA., but the pentode takes 22 mA., and requires a negative C bias of 10| volts with 150 volts on the plate; while the total current requirements are still greater, for the auxiliary grid takes a current equal to about 25 per cent, of the plate current, so at 150 volts the total current will be in the neighborhood of 27 mA. Question of Power Output In investigating the conditions under which a power tube must operate, the first thing to do is to find out what the maximum power output is with various load impedances in the plate circuit, and at the maximum rated plate voltage and current for the tube. The easiest way of doing this is by drawing straight lines through the point A, the working point, Fig. 4, which represent the effect of a pure resistance in the plate circuit, the slope of the fines being inversely proportional to the value of the resistance. The maximum output is calculated by taking the values for plate voltage and plate current at the points where the resistance load line crosses the grid voltage curves for the maximum signal voltage which can be applied without distortion. In this particular case, if the load is such that it is possible to apply to the control grid a signal voltage the peak value of which is equal to the negative C bias, then the plate voltage and plate current should be taken at the points where the load line crosses the curves for 0 and negative 21 1.0 0.8 0.6 i- 0.4 0.2 D~~ B ~ C i i — "50 100 200 500 1000 2000 5000 FRE0UENCY Fie. 3 360 • • OCTOBER 1929 •