Loudspeaker (Jan-Aug 1931)

Figure 2 shows the wiring of an auto-transformer. Notice that the condenser and the resistance are in the grid circuit, in the Clough transformer they are in the plate circuit. The condenser functions somewhat like it does in the Clough system. The resistance is not a coupling resistance, it is a grid leak. It keeps the grid properly biased. The electrons that gather on the grid must have some way to leak away from it, otherwise the grid will become chai’ged to such a point that the tube will “block,” it will then cease to amplify. The grid leak allows this flow of electrons away from the grid. The resistance is not too great, a value being chosen which will always allow the electrons to return to the filament faster than they can collect on the grid. Some of the signal is lost this way, but it is a very small amount and is of little consequence. The values of resistances used for this purpose are usually much higher than those used with the Clough transformer, running from Y> to 1 megohm as a rule. A few years back the values were lower, sometimes 100,000 ohms being used, and not very often was the resistance greater than >4 megohm. The purpose of the low values was to stop “motorboating,” a form of low frequency oscillation caused by feed-back. For a time the cause of the trouble was not so well understood. It was thought that the tube was blocking, that the electrons were gathering on the grid faster than they could leave via the grid leak, so a leak of smaller resistance was substituted and the trouble disappeared. This, however, caused a loss in amplification, especially at the low frequencies. Later it was learned that the real cause was feed-back from one circuit to some other circuit preceding it, so steps were taken to isolate the various circuits. This is accomplished by filtering all the grid and plate circuits, which means that the signal currents in any one circuit are kept there and are not allowed to flow through a common impedance, usually the B T ive n ty -tnxi o supply, setting up a voltage across that impedance at the signal frequency, thereby impressing the amplified signal back on the first tubes of the amplifier and thus causing the “put-put” known as motorboating. Now let us get an idea about this filtering. Referring to Figure 3, the signal currents in the last amplifier tube, after passing through the transformer, are shunted back to the filament by a condenser; an impedance, such as a choke or a resistance, is in series between this point and the B supply to hinder the signal in passing on to the supply. In this way the signal is kept from going through any associated circuits where it would interfere with their operation. The other circuits are filtered in the same way. Suppose that Figure 3 now represents the plate circuit of the first stage of the amplifier. If a tendency to feed back were present the varying voltage causing it would encounter the resistance on the way to the plate, which would tend to decrease the feed-back. Now we come to the condenser, which shunts the remaining part of the interfering voltage to the filament and so to negative B, where it can do no harm. As a result of nearly perfect filtering some amplifiers are now used with grid leaks having a resistance of 5 megohms, and without the least tendency to motorboat. Any auto-transformer wired in the conventional manner as shown in Figure 2 can be rewired so that it practically becomes a Clough transformer. A lower value of resistance is used and it is connected from the plate to positive B, the grid leak having been removed. The condenser is taken out of the grid circuit and put