Radio broadcast .. (1922-30)

300 350 300 will deliver about 2000 milliwatts with a plate cur- rent of about 40 milliamperes at 250 volts, and re- quires a grid vol- tage (a.c.) of only 12 volts peak. This may be com- pared to a 245- type tube which delivers 1600 mil- liwatts on ap- proximately the same amount of d.c. power and with a grid volt- age of 50 volts. Fig. 4 TABLE II Fig. 5 Name Ef If Rp Cossar 230 2 0.3 20 X 10 3 415 4 0.15 20 Marconi PT 240 2 0.40 55 PT 625 6 0.25 Mullard PM 24 4 0.15 28.6 PM 24A 4 0.275 PM 22 2 0.3 62.5 Six Sixty ss 230 pp 2 0.3 64 415 4 0.15 27 4pen 4 0.275 Mazda 425 4 0.25 Philips C443 4 0.25 40 PE 7 2.5 1.75 40 Ef—filament voltage If—filament current Rp—plate resistance M u —amplification factor Is—current to high-voltage grid Mu 40 40 90 65 80 80 60 60 80 2000 2000 1650 1850 2300 1550 1300 180 180 150 250 150 300 150 Es 120 120 150 200 150 200 150 E c 9 9 9 15 12 21 10 1250 150 150 2200 150 150 1550 300 200 2000 150 150 1500 300 200 2000 250 250 Gm—mutual conductance E p —plate voltage Es—high-grid voltage EC —control-grid bias IP —plate current Po—power output (milliwatts) It 14 14 16 26.5 12 18 13 13 12 18 18 28 40 500 2000 500 2000 350 350 500 2000 750 Is 1.6 1.6 6.0 7.0 3.0 5.0 3.5 3.5 3.0 5.0 5.0 2200 10.0 Characteristic Curves _ Plate current-plate voltage characteris- tics of the Arcturus PE-? tube are shown in Fig. 4. There is still some secondary emission at low plate voltages. The effect of varying the potential of the cathode grid is shown in Fig. 2. At high negative voltages on this grid, all the electrons are speeded back to the plate and there is no secondary emission. While it is probably not correct to use the usual method of laying out load lines on the Ep-Ip curves to determine the proper load resistance and the second harmonic distortion, some idea of the respective values can be discovered by so doing. These data are presented here with the knowledge that they may mean very little indeed. Thus it may be cal- culated that the Arcturus experimental tube with an internal resistance of about 40,000 ohms will work best into a load resistance of from 4000 to 8000 ohms. Within these two values the second har- monics (according to usual methods of calculation) will be less than 5 per cent., the usual criterion for distortionless ampli- fication. At the same time the power out- put does not increase appreciably for values of load resistance in excess of 8000 ohms. (Fig. 3). It is probable that the third harmonics are the worst offenders in the pentode and at the present time there seems to be no generally accepted and easily worked method by which the percentage of third harmonics can be calculated from the characteristic curves. Use of the Pentode In Europe the pentode has been worked with magnetic loud speakers whose well- known impedance characteristics are any- thing but straight flat lines. As a rule the impedance of these loud speakers in- creases rapidly with frequency. Since greater and greater distortion is the result -40 (/Vote change in grid current scale below abscissa.). Fig. 6 TABLE III Filament volts Filament amperes Control grid bias Space-charge grid volts Screen-grid volts Plate volts Mutual conductance Plate resistance AmpliGcation factor Plate current Screen current Space charge current Maximum amplHcation (into 100,000 ohms) of using a high- resistance load with the pentode, the fidelity of re- sponse is not very good. The high frequencies devel- op very high volt- ages across the load and within the tube, and are reproduced all out of proportion to the low tones. In this country the magnetic loud speaker is practi- cally on the shelf in favor of the electrodynamic type which has a flatter impedance characteristic. Since the pen- tode should be worked into a load lower in resistance than itself, it can be coupled to an electrodynamic loud speaker through the same transformer which usually couples it to a 4000-ohm tube. In the Laboratory the fidelity from such a tube and a Peerless loud speaker compared very favorably with that obtainable from a 245-type tube, and with a given grid volt- age input the output was some 15 DB higher. In other words, feeding 12 volts into a 245-type tube produced about 100 milliwatts but the same voltage fed into an Arcturus pentode produced about 2000 milliwatts. This is a very appreciable difference in volume. Practical Applications One of the important applications of this tube may be in the detector socket as a true power detector. Thus it may be used with the loud speaker in its plate circuit. However, whether it will make a good power detector tube has not been deter- mined by experiment. A glance at its grid voltage-grid current curve (Fig. 6) in- dicates that it will make a good grid-circuit detector. The problem then becomes one of obtaining sufficient power from it. Development of more efficient loud speak- ers will make it possible to eliminate the audio-frequency amplifier entirely and to use only the pentode as a power detector working directly into the loud speaker. It has been suggested that the tube in this capacity might serve in automobile radio receivers and in other places where the space limitations are severe. The fidelity obtainable from a small loud speaker under the best conditions cannot be extremely good, and so the use of a small magnetic or electrodynamic loud speaker in connection with a pentode detector supplying perhaps 500 milliwatts may be an important application. In this particular tube, which may not (Continued on page 293) RADIO BROADCAST FOR MARCH • 255