until the terminal voltage of the cells when supplying a load of 4 ampere falls below 0.9 volt. A dry cell should not be required to deliver more than 4 ampere for any length of time. If the current drain exceeds this value cells may be connected in parallel to supply the load.

Dry cell B batteries marketed by various manufacturers have proved very satisfactory when checked frequently for voltage and noise, and when chosen with regard for the service required. 221⁄2 volt units may be used unless noise develops until the voltage falls below 17. A high resistance voltmeter must be used in checking B battery voltages. For sets employing more than three tubes, it is more economical to use heavy duty B batteries. If B batteries deteriorate rapidly when in service, the trouble is probably caused by excessive current drain, which in turn may be caused by insufficient negative grid bias or C voltage.

The most satisfactory bias or C battery in the long run is the large 4 in. x 18 in. x 3 in., 41⁄2 volt type. Smaller sizes deteriorate rapidly and require frequent replacement.

5. VACUUM TUBES

Tubes in the audio amplifier may be divided into two general classes: (1) Voltage amplifiers, and (2) Power amplifiers. In a

transformer coupled amplifier all the tubes are, properly speaking, power tubes; and their rating should increase as we progress through the amplifier toward the final tube which works into the speaker. On the other hand the tubes used in a resistance or impedance coupled amplifier, with the exception of the tube which works into the speaker, may be properly termed voltage amplifiers. The following table gives the characteristics of the more common types of both voltage amplifiers and power amplifiers.

Whereas condensers and inductances discriminate against certain audio frequencies, vacuum tubes are free from this defect. They must, however, be used with proper voltages and coupling units or volume distortion will result. The significance of the data in the foregoing table, and their bearing on the problem of distortionless amplification may perhaps be best shown by a consideration of the curves of Fig. 9, which gives the relation between

grid volts and plate current for a typical vacuum tube. Curve A was taken with a plate battery of 94 volts and no external load in the plate circuit. It is called the static characteristic. Curve B is a similar static characteristic with a plate battery of 130 volts. We can now obtain a rough estimate of the plate impedance and voltage 94 + 130 amplification factor at a mean plate voltage of 2

=112.

The plate impedance is defined as the ratio of the change in plate voltage to the resulting change in plate current. At zero grid volts for example we see that the plate current has changed 8 mils for the change of 36 volts plate potential. The plate impedance is therefore roughly 36/0.008 or 4500 ohms at zero grid potential. At -8 volts grid potential it increases to 36/0.005 or 7200 ohms. In an audio-frequency amplifier this impedance is practically pure resistance, and we shall adopt for it the symbol Rp.

The plate current in a tube may be changed by changing either the plate potential or the grid potential. The voltage amplification factor is defined as the ratio of the change in plate potential to the change in grid potential required to maintain the same value of plate current; and we may also make a rough determination of this factor from the curves of Fig. 9. Following horizontally along the 10 milliampere line, we find the grid voltage for curve A is zero, and for curve B is —6, a change of 6 volts on the grid to counteract 36 volts change on the plate. The amplification factor is therefore 36/6 or 6. The conventional symbol for this factor

is μ.

The effect of this factor is explained by considering that an alternating voltage of value e impressed on the grid of a tube has the same effect as an alternating voltage of μe in the plate circuit. An explanation more near the physical facts, however, is that the alternating grid voltage, produces corresponding alternations in the plate impedance, thus allowing the plate battery to supply a pulsating direct current which contains a pure direct current component and an alternating current component.

Whereas condensers and inductances discriminate against certain audio frequencies, vacuum tubes are free from this defect. They must, however, be used with proper voltages and coupling units or volume distortion will result. One simple precaution to be observed is that sufficient negative bias or C potential be supplied so that the impressed alternating signal voltage can never exceed this value and drive the grid positive. If the grid is driven positive it will draw current, absorb power, and distort the output.

The curves A and B of Fig. 9 show that the relation between grid voltage input and plate current output is not linear. We should therefore expect distortion. But we should remember that these curves were taken with no external load in the plate circuit; and such a system has no practical value. To pass on the output

of this tube to another we must have an external impedance in the plate circuit. Curve C was taken with a 6000 ohm resistance in series with the 130 volt B battery, and we notice that this is nearly a straight line. Practically, a nine volt bias is specified for this circuit condition; and from this operating point we see that the grid may swing from -18 to zero without departing seriously from the straight line relation with plate current. There will be practically no distortion if the alternating voltage impressed on the grid never exceeds 9 volts maximum value or 6.4 volts effective value.* Thus we see that the effect of an external load in the plate circuit is to straighten out the static characteristic, and give a dynamic characteristic which permits a relatively wide operating range of grid voltages. (Note that as the load impedance approaches zero for this tube we should be obliged to use only 4 volts bias and restrict our input signal voltage to this value for distortionless amplification). With curve C we are limited to an alternating input of 9 volts; but we can increase this operating range by increasing the plate battery voltage to give curve D. This shows a 12 volt bias permissible with a similar alternating input voltage range. The life of the tube is the factor to be considered when such voltage increases are made. The values given in the table are the maximum values consistent with reasonable life of the tube. With any marked change in plate voltage there should be a corresponding change made in grid bias. An approximate rule is to make the grid bias equal to the plate potential divided by twice the amplification factor.

When the tube works into an inductance instead of a pure resistance, the dynamic characteristic is of more complex form; but a characteristic determined for a pure resistance equal in value to the impedance of the load provides a satisfactory basis for approximation. Thus an impedance coil for a 6000 ohms load would require at 40 cycles an inductance of

*Effective values are those read by almost all of our alternating current meters. The effective value of voltage or current is equal to the maximum value divided by 1.41. In our 110 volt lighting circuits the maximum voltage is 1.41 x 110 or 155 volts.

6. COUPLING SYSTEMS-THE STAGE OF AMPLIFICATION

We are now ready to show how the various electrical units, tubes with their coupling elements, may be combined in a stage of amplification. We shall deal first with the resistance coupled amplifier.

Resistance coupling. The resistance coupled amplifier is capable of giving the most perfect performance of any of the types to be discussed, provided it is properly designed. A stage of resistance coupled amplification is shown in Fig. 10, and its equivalent circuit is given in Fig. 11. It may be shown that the

efficiency of this coupling system (i.e. the ratio of voltage input to tube No. 2 to the voltage developed in the plate circuit of tube No. 1) is

Rp, P, and G being expressed in ohms and C in microfarads. Of the alternating signal voltage developed in the plate circuit, a part is used up within the tube in overcoming the resistance Rp, and the remainder is impressed across the coupling system P-C-G. This is the useful voltage and should be made as large a part of the