Radio Broadcast (May 1928-Apr 1929)

The 222 Tube as an R. R Amplifier By GLENN H. BROWNING AS THE readers of Radio Broadcast know. /-\ the screen-grid tube is designed primarily as a radio-frequency amplifier. The second grid screens the control grid from the plate, as its name indicates, so that the effective capacity between the two is very small. Consequently, the tendency to oscillate in a multiple stage tuned radio-frequency amplifier, due to the capacity between grid and plate in the amplifier tube, is considerably reduced. Another characteristic of screen-grid tubes is the high amplification factor. Coupled with this is a high plate resistance, so that the ordinary tuned radio-frequency transformer is not suitable if even fair efficiency is to be obtained. The writer has been able to obtain three distinct types of these tubes. The first type is the R.C.A. or Cunningham 222 tube. This is a d.c. operated tube requiring 3.3 volts across its filament. The essential dynamic characteristics are shown in Fig. 1. These curves were taken with 135 volts on the plate and 675 volts on the screen grid. The amplification varies, as will be noted, from 170 to 99, being about 152 with a grid bias of minus i§ volts. With this same grid bias, the plate resistance is 385,000 ohms and the mutual conductance 0.000395 mhos. Fig 2 shows the dynamic characteristics of the Arcturus screen-grid tube. This is an a.c. 15-volt tube of the heater type. At the same operating point, i.e., 135 volts on the plate, 67J volts on the screen grid and 1 5 volts negative bias, the amplification factor is 106, the plate resistance 220,000 ohms, and the mutual conductance 0.000482 mhos. Fig. 3 gives the same characteristics for a CeCo a.c. 22 tube. The heater operates at 2.25 volts a.c. as in the 227. With \\ volts negative grid bias, the amplification factor in this case is 362, the plate resistance 560,000 ohms, and the mutual conductance 0.000646 mhos. It will be noted that both types of a.c. screen grid tubes have a higher mutual conductance than the d.c. tube and consequently are better from the theoretical standpoint of CT'HE author's purpose in this short ■*■ article is simply to give the readers of this magazine some data and measurements which have been made using the screen-grid tube. Perhaps some of us have thought of using this tube in an impedance or resistance coupled amplifier, and to those the data given here hhII be especially interesting. — The Editor show that only a small amount of amplification may be expected at radio frequencies with either resistance or impedance coupling. The capacity, O, between the control grid and the filament of the screen-grid tube is about 10 mmfd., and the capacity, Co, between plate and filament about 40 mmfd. under usual operating conditions. The first capacity, as far as alternating current is concerned, is across R> in Fig. 4, while the second is across Ri. Thus the maximum impedance ob < 4 £ 3 2 200 180 160 140 120 100 80 60 40 20, \ \ 1 1 CX-222 Tube lp t; = Esg = 67^ 'olts M s '•^ •-* < 5^ 500.000 in the coupling device is — where Cw 300,000 £ tainable C = 50 mmfd. and w = 2tz X frequency. At 200 meters — — = 21,200 ohms., and the maxiCw mum amplification of the system using a tube with a plate resistance of 560,000 (CeCo a.c. 22) ohms, and an amplification factor of 362, would 1 200.000 be = 13.2. The amplification may also RB + — C<il 100,000 Eg VOLTS 100,000 \&CoScre< find \ 'AC22TL \iEp=135V. fc, /■ 1 ' X s \ \ \ > V A N * % Eg VOLTS ABOVE FIG. 2 -1 0 Eg VOLTS I. LEFT — FIG. 2. RIGHT — FIG. 3 350 400 450 WAVELENGTH, METERS ABOVE — FIG. 4. BELOW — FIG. 5 designing a suitable t.r.f. transformer for their use. There are two general methods, aside from regeneration and the super-heterodyne principle, of obtaining radio frequency amplification. One is by an untuned amplifier such as is shown in Fig. 4, where a resistance, choke, or a fixed transformer is used as a coupling device. The second is a method of tuning the amplifier to the incoming signal by means of a tuned radio-frequency transformer coupled either directly or through a primary winding as shown in Fig. 6. A mathematical analysis plus a few measurements on the capacities between the elements of the screen-arid tube will be simply calculated by multiplying the load impedance by the mutual conductance. 21,000 times 0.000646 gives 13.6 as the amplification, agreeing quite well with the former figure. This value of amplification can never be fully obtained, as the resistance or choke used as coupling is in parallel with these capacities and reduces the effective impedance slightly below 21,000 ohms. Fig. 5, curve A, shows the amplification obtained at s different frequencies with one stage ° of choke coupled amplification using " r.f. choke coils and a 0.006-mfd. coupling condenser. It is very apparent that tube capacities play an important factor, for at the long wavelengths or low frequencies a very fair amount of gain is observed while at the high frequencies almost no amplification is obtained. Fig. 6, curve B, shows the amplification measured with an untuned radiofrequency transformer. Thus, analysis and measurements make us discard untuned amplifying systems at radio frequencies so that the solution of obtaining the highest gain possible with a high-mu tube must lie in tuning the amplifier to the signal. Input Auto-coupled tunea r.f t 252