Richardson's handbook of projection (1930)

1216 HANDBOOK OF PROJECTION FOR in the plate current for each volt that the grid potential changes, which is, of course, a measure of the degree of magnification obtained, is not always the same, unless the changes in the grid potential are relatively moderate. For example, suppose we had an amplifier where the normal plate current was 10 mils. Then suppose we put on the grid an alternating voltage varying between + 1 volt and — 1 volt, and that this caused the plate current to vary correspondingly between 12 mils and 8 mils, giving therefore an A. C. plate output varying between +2 mils and — 2 mils. The change in the plate current is in this case proportional to the change in the grid potential and we are getting undistorted amplification. But suppose we try to get more output by increasing our A. C. voltage on the grid to +3 volts and — 3 volts, and suppose we now find that the plate current varies correspondingly between 17 mils and 5 mils, so that the A. C. plate output varies between -\-7 mils and — 5 mils, whereas it should really be +6 and — 6 mils. The proportion is no longer maintained; the positive half of the wave gets too much amplification and the negative half gets too little. The tube is said to be overloaded. This is one form of what is called distortion, and is noticeable as causing a harsh, noisy quality in the reproduced sound. The necessity of avoiding this leads to the use of several stages of amplification, since each stage can handle only a certain part of the total increase or magnification required. But to a certain extent we can eat our amplification cake and have it as well, if we notice that, as just mentioned, in the overloaded tube there is too much amplification on the positive half of the grid voltage wave and too little on the negative half. Suppose we take two tubes and connect the grid of the first tube to one