Radio age (Jan-Dec 1925)

RADIO AGE for June, 1925 The Magazine of the Hour 35 Economy of Parts and Space in A 3-Tube Portable Reflex By JOHN B. RATHBUN TO BE truly portable, according to my idea, means that a receiving set should be easily carried about from place to place without seriously straining its owner's physiology, and at the same time it should be so compact that it will not take up any more room than necessary in a trunk. There are portables and portables, but the absolute zero in portability is the set made up in a traveler's sample case which weighs about 100 pounds and occupies about fifty per cent of the trunk space. On the other extreme is the freak midget set which has been variously fitted into pill boxes, fountain pens and pickle bottles, and which has absolutely no purpose in life except to exhibit the make's ingenuity. The real portable should have a good range and sufficient volume to operate a loud speaker, and yet at the same time should not take up a great deal more space than a camera, even when fully equipped with batteries. No really practicable portable has been turned out with less than three tubes, for it is impossible to operate a loud speaker satisfactorily with less tubes on anything but local stations. For this reason I will assume a three tube set from the beginning and will build up all the other data about this premise. Whether this is to be a regenerative, radio frequency or reflex still remains to be seen, but as the maximum volume is to be obtained from a minimum number of tubes and batteries, I have strong leanings for the refle-x type. The reflex circuit is not always the greatest distance getter, but what it does get, it gets good and loud. Batteries Consume Space T)ROBABLY the most important item •* in a portable is that of the batteries, for the batteries weigh more and take up more space than the rest of the equip Copyright: 1925 Reversed Capacity Feedback Cuts Out Free Oscillations ment. Our current supply system must be reduced to the lowest possible limit even at some sacrifice in the life of the batteries. If the batteries stand up for a month while traveling about, it is generally considered satisfactory for vacation tours. At home, we can substitute larger batteries installed outside the set, "EVERYTHING I NEED IN RADIO" "I bought a copy of the RADIO AGE ANNUAL for 1925 and I found that everything I wanted to know about radio, from crystal sets to complicated multi-tubers, was contained between its two covers," wrote an enthusiastic beginner. "I never knew so much could be contained in one book without crowding or omitting necessary details. But you haven't left a thing out of the ANNUAL for 1925." Letters such as the above are sent to us every day, voicing sincere appreciation of the ANNUAL for 1925, the most complete radio hookup book ever printed. And the price for the 120 pages of technical "nuggets" is but ONE DOLLAR, postpaid. Send your order now while our supply of the limited first edition lasts. Blueprints of the 3-Tube Portable Reflex on Two but in touring, the space and weight are the principal items. With this idea in mind, we will study the battery situation and the proper tubes to go with these batteries. Storage batteries are out of the question, of course, hence only dry cells are available for the filament and plate current. This means that the tubes must either be of the WD-12 or the UV199 type, which are specially designed for dry cell service. They do not give the volume of the 201A power tubes used with storage batteries, but they give excellent results if properly handled. The WD11 and WD-12 are the same tube with the exception of the base. The base of the WD-11 is a special small size, while the WD-12 fits in a standard socket. Both tubes operate on the 1.5 volts produced by a single dry cell and take 0.25 ampere per tube. Each tube therefore takes 1.5x0.25 =0.375 watt, or 3-8 watt. One No. 6 dry cell is provided for each tube, which can be connected independently to each tube of a multi-tube set or to a multiple connected battery with as many cells as tubes. As 0.25 ampere is the rated discharge rate for a No. 6 cell, it is not possible to use a smaller batterv. Next come the UV-199 or the C-299 tubes, which require 3.0 volts at the filament, and which take only 0.06 ampere of current. As the voltage of a battery falls off with use, we must use three dry cells in series, which gives us a total of 4.5 with a fresh battery. This excess is taken care of by a 30 to 40-ohm rheostat, which permits the use of a battery between the limits of 4.5 volts /and 3.0 volts, the battery being discarded iwhen the voltage drops to the latter point. The power taken is therefore: 4.5x0.06 = 0.27 watt, very much less power than is required with the WD-12. (Turn to page 38) Pages Following