RADIO AND ITS RECEIVERS

Sun (Auckland), Volume III, Issue 657, 8 May 1929, Page 14

RADIO AND ITS RECEIVERS

R. F. HAYCOCK.

Conducted fcr THE SUN by

Latest in Receivers screen-grid Development New Era in Reception

THE curtain is rising on a new and improved type of radio receiving set. The research laboratories have succeeded in their efforts to perfect the screen-grid or shielded valve so that it can be used in a broadcast receiver of the alternating current type. The new valve is responsible for the improved receivers now being announced by several manufacturers. The all-electric, alternating current screen-grid valve set is expected to be the main attraction at the Radio Trade Show to be held in June at Chicago.

Therefore it is expected to stimulate the sale of portable sets heretofore too heavy and cumbersome to really live up to their name. Already one manufacturer has introduced a screenj grid portable outfit “of shock-proof J construction—a single drum control—with volume and clarity—an idea] portable for school, hospitals, hotels, office buildings and clubs.” What about the new valve? One manufacturer who has already announced the new models, report “new selectivity, new sensitivity and new distance range. Sensational performance, never before approached in a popular-priced radio —the latest innovation in radio.” The valve is known as the AC-22, while some refer to it as the 222 or 322. It is believed that later valves of this type may be numbered 223 or | 224. It has been used in several sets introduced last autumn in a direct j current capacity. j There are several factors that must be carefully considered if this type of i valve is to be employed successfully,

THE valve Is heralded in radio circles as one of outstanding merit, unequalled by any other valve because of its high amplification properties and sensitivity. The bulb was introduced to experimenters; more than a year ago. but, because of its extreme power, engineers were not able to use it in iadio receiving sets built for the public. It was so sensitive that its selectivity could not be controlled. But now the laboratories have succeeded in harnessing its power and it will be used in many of the 1929-30 radio receivers. It may be a rather difficult task to put the new valve in standard broadcast receivers now in use. However, this does not mean that the present sets shall become obsolete overnight. Rut when the present owners buy a new set it will no doubt be one equipped with the screen valve. One Does Work of Four Ultimately it is likely to make sets ; more compact because one screen-grid ! valve will do the work of from two to I four present general purpose valves, i

according to engineers of E. T. Cunningham, Inc. They call attention to the fact that the first requirement is that radio frequency amplifier stages of sets using the CX-322 be given at least the same amount of attention that any good four-stage radio fre- ! queney amplifier should have to make | it operate efficiently, which is often ! nothing less than complete shielding | of every radio stage and the use of j by-pass condensers and choke coils in each plate “B” lead. Special Condensers J The second requirement is that the coils and condensers used to link the 1 valves with each other must be properly designed and constructed if the valve is to amplify the incoming signals anywhere near its rating, of at least from 25 to 40 times. There is nothing about the four-ele-ment valve that will eliminate undesirable influences in the set, which cause the receiver to howl and be unstable in operation. In electrical terms this unsteadying influence may be either “electro-magnetic” or “electrostatic,” meaning respectively the presence of any magnetising force or charge of electricity which is allotved to produce an unbalanced condition. With adequate shielding, according to the engineers, it is practicable to employ even three stages of radio frequency amplification, using these valves without trouble from oscillation. Such an amplifier will provide a voltage amplification of from 10,000 to 15,000, as compared with around 125 for a similar number of stages in the average receiver and about 1,000 for an exceptionally efficient radio frequency amplifier using 301 A type tubes. To be at all effective, shielding must include not only the metallic shielding usually used to isolate each stage, but also means for preventing coupling through the battery or eliminator circuits. In a three-stage amplifier, it is advisable to also include chokes in the screen-grid leads of each stage, al- | though in a two-stage unit, choke coils | in the plate leads alone will usually ! prove adequate. Data on Transformers j Standard types of radio frequency transformers used to couple stages using 301 A. valves cannot be used in j circuits employing the 22 type be- ! cause they do not provide a sufficiently i high primary “impedance” to obtain ! the degree of amplification possible | with these valves. The old standard j type of transformer with a small primj ary winding may be used as an anj tenna coupler, but not as a coupler I between the stages, j Transformers for the valves must | have primaries having approximately j the same inductance as the second- j i aries. Impedence coupling using a j ! single coil common to both the plate circuit of one vave and the grid circuit of one valve and the grid circuit of the succeeding valve may be used, but this method does not permit of the selectivity obtained by the transformer method. Close coupling obtained by having a l transformer with equal primary and secondary winding does not broaden tuning to any appreciable extent if proper care is “taken in the design of the transformer and circuits. Voltage Amplification The voltage amplification obtained with the average radio frequency type of receiver is usually about five or less a stage, w'hile in a receiver properly designed for use with the screengrid valve a voltage amplification of 25 a stage is easily obtained. In welldesigned circuits using suitable coupling transformers and shielding, even higher amplifications can be effected. The enormous amplification possible with the new valve is sufficient to recommend it to the radio public, according to the Cunningham engineers, but when to this accomplishment is added the fact that neutralisation and delicate balancing are not required with this type of valve, its advantages can be fully realised. In using the battery-operated screen | grid-valve, the voltages recommended for most satisfactory service are 3.3 volts for the filament—l.s volts for the control grid, 45 volts for the screen-grid and 90 to 135 volts for the plate.