September, 1939 THE WIRELE SS ENGINEER 449 emission then has to be suppressed by the see that the points where the lines for cos space charge alone. This has not as yet I intersect Z° = I, i.e., the points repre- been achieved to a sufficient extent, so that Ropt most output tetrodes on the market are senting the normal distortion curve, are pentodes in a disguised form. In other comparatively little lower than for the words, the 3rd grid is no longer designed as Za a but is in the form of a window or is pentode. For values of greater than 2, grid opt replaced by screening plates which enhance however, output diminishes far more rapidly the effect of space charge upon the secondary electrons. 0 In these output tetrodes or beam -power 04 valves, anode current increases also very cos =1 s% cos 0.07___- rapidly with anode voltage. In this respect 0 W,.11W these valves are much better than old type Rop1. s.LCJOn are only slightly output pentodes and ° inferior to a modern output pentode such as 02 the Pen A4. These two qualities are clearly s% seen in the la - Va diagram of the beam - 01 ,. power valve (Fig. 21). It will be noticed F", zs% 0 that at low anode voltages and comparatively 06 1 2 4 to small anode currents there is still a fairly Z./Ropt large number of secondary electrons passing Fig. 22-Output at 5 per cent. and at 2.5 per from the anode to the screen -grid. This cent. distortion as a function of the load im- does not generally cause any trouble at the pedances when cos 4, = 1 and when cos 4, = 0.7 for a combination of a beam -power valve or optimum value of the load impedance, tetrode output valve with the TDD4. because, with a pure resistance load equal to Ropt, operation does not take place within with the tetrode than with the pentode, so that of the la - Va diagram in which that when Za = 3 Rom, the output of the secondary emission phenomena are con- tetrode at a given distortion is only Ì to } spicuously visible in the characteristics. that of the pentode. With the tetrode, in contrast to the pentode, distortion at higher values of the load resistance is not pre- dominantly determined by the A.C. anode voltage. This is also clearly shown by the dotted curves in Fig. 14, in which the A.C. anode voltage for a beam -power valve is plotted at 2.5 per cent. and at 5 per cent. distortion as a function of Za/Ropt. In view of this peculiarity it is sometimes recommended that the load impedance should be given a lower value than Ìa ; a this, however, results in a worse distortion seen from Fig. 22. Fig. 21.-Oscillogram of the I. - V curves for curve as can be a standard type beam -power valve. When the load is not a pure resistance, the beam -power tube even at Za = Ro,t Matters become different, however, when the is much less favourable than the pentode. load impedance acquires higher values or This is due to the fact that the Ia - Va when it is not a pure resistance but has a diagram is then traversed by a curve having certain phase angle. more or less the form of an ellipse (Fig. 23). Fig. 22 gives the output as a function of Although these curves do not actually pass the load impedance for a standard type through the region where secondary emission beam -power valve in combination with a is evident in the la - Va characteristic, this TDD4. Comparing this with the corre- phenomenon does already cause additional sponding curves of the output pentode, we curvature of the la - Va characteristics,

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