Radio Broadcast (May 1928-Apr 1929)

WHAT IS A 4lr GOOD TUBE m "^HREE years ago a series of articles was published in -«tC2> I Radio Broadcast as a result of a serious investigation I Y| of the characteristics of the radio vacuum tubes then ■ LA fl on the market. Nearly one hundred tube manufacturers ^**~»H sent samples of their products to the Laboratory to be tested. Many of these manufacturers have since demonstrated the fact that they had no business in the tube market — their names are forgotten. At that time, radio listeners and dealers did not have the means of finding out the characteristics of the tubes, and probably knew little about what the characteristics meant if they had been able to obtain them. No one knew how long a tube was supposed to last, least of all the ultimate user. Then the market for radio receivers was rising rapidly, but was still small. And so the data collected in the Laboratory and published in this magazine not only gave the possible users an idea of how to judge good tubes, but how to use them intelligently, and — if they desired — simple methods of measuring their characteristics as well. To-day the picture is different. Th ere are perhaps a dozen reputable manufacturers of tubes for receivers and power apparatus — and yet the market for tubes has increased to a degree never dreamed of by the forefathers of radio. The tubes made by these manufacturers are more uniform in their characteristics, have longer lives, and look and act more like their brothers from other factories. The tube business is "shaking down;" it is becoming more a matter of engineering and less of cut-and-try. The names of tube manufacturers we still see on tube cartons are the names of people who have learned by experience how to make a good tube. Now the question is, " What is a good tube? " The data presented in the following pages is an attempt to answer that question. It is presented for the benefit of radio dealers and servicemen whose responsibility to the users of tubes is great. These men are in intimate contact with radio listeners and must daily answer the question, "What tube do you recommend? " The data should be interesting to the ultimate user of the tube too, for he ought to know what to expect and when his expectations have or have not been realized. The more the serviceman-dealer knows about the tubes he sells, the better he can satisfy the demands of the user who wants long life more than he wants a cheap price. The data are in a form so that they can be clipped from the magazine and filed in a note book. W e suggest each sheet be mounted on a heavier sheet in the notebook. This information represents the latest obtainable data on the best-known tube manufacturers and until something startling occurs in the tube business, something entirely unforeseen, the data presented here will be representative of what these better known manufacturers are building into their tubes. Answering the question " What is a good tube " involves two factors, the electrical characteristics of the tube and the factor of economics. How long will the tube last? The characteristics of general and special-purpose tubes have undergone no radical changes in a year or more. Whether or not this is fortunate, we are not prepared to state. It is true that every reputable manufacturer who is interested in his future is doing his best to build good tubes, tubes whose characteristics are good when the user purchases them, and good for a long time afterward. The uniformity in the characteristics, as shown by comparing the data in the following pages, was not true three years ago. Then there was a wide disparity between the characteristics of one tube as compared with a similar type made in another plant. Now a 201A-type tube is a 201A-type tube no matter who makes it. Standard Tube Types are Similar f DIFFERENCES among the products of the tube manufacturers are differences of construction, differences of material, differences of packing for shipment, etc. The electrical characteristics are much the same no matter what the name of the tube. But the differences in mechanical construction, in pumping, and in the choice of materials used are the differences that determine life, the second criterion by which one may judge a good tube. The first factor, the electrical characteristics of tubes, involves the manufacturer alone. The second involves both the manufacturer and the user. The best of tubes will blow up if the proper wires of the receiver are crossed, or if a screw driver in the experimenter's or engineer's laboratory or in the serviceman's shop falls into the power supply by accident. The best tube will have short life if its filament or plate is overloaded with voltage. The electrons which make the radio wheels go round are tireless workers, but their supply is limited. The best tube will suffer with fatigue if it is forced to work under conditions for which it was not designed. Generally speaking, tubes from one of the manufacturers represented in the following pages should last as long as from any other,, and, although there may have been a time when manufacturers deplored the fact that their tubes lasted too long, it is true now that every manufacturer must make his tubes as good as is possible. Competition takes care of that. And yet all the responsibility for long tube life does not rest with the manufacturer. The serviceman who recommends the tube, the dealer who sells it, and the user who employs it must share this responsibility. The dealer-serviceman's share is particularly great for it is he who knows whether or not the set-owner is operating the tubes of his set at voltages recommended by the manufacturer. It is he who must tell the user not to raise the filament and plate voltages until the last "ounce" of signal strength or "DX" is extracted from his radio. Importance of Correct Voltage A TUBE manufacturer is much more familiar with his product than anyone else can be, and when the tube manufacturer sets the voltage limits which his tubes can stand, the user has no right to assume a new set ratings for himself and to expect the manufacturer to replace tubes which seem to have premature failure of emission. In other words, if the correct filament or heater voltage of a certain tube is 2.5 volts, the experimenter should not use 3.0 volts nor should he use 2.0 volts and expect long healthy tube life. Let us consider the case of a power tube, a full-wave filament rectifier, for example. It seems natural to suppose that it will last much longer if we don't overload the filament and such is true. But let us reduce the voltage across the filament, burn it at a lower temperature, and see what happens. All tubes lose emission as they get old. Rectifier tubes have a certain lower emission limit beyond which they cannot hope to keep up with the drain of electrons imposed not only by the receiver, but by the filter and voltage divider as well. Once the tube emission falls below this figure, the voltages supplied to a receiver begin to fall. a.c. hum begins to come up — because the wave-form of the rectified current is no longer halves of sine waves, but has a flat-topped form, hard to filter — and the regulation goes bad because of the increase in effective resistance of the rectifier and hence of the plate-voltage supply system. When such a tube is operated below its rated temperature the supply of electrons is not as great as when the temperature is raised. This means that the useful life of the tube is decreased, because the lower limit in emission is reached sooner. The tube is still good, and probably would continue to supply plenty of voltage if its temperature were increased — but the only safe and economical method is to supply the heating voltage that the tube needs throughout its life. Low can the user tell a good tube from a bad one? He must first of all buy a tube whose name is known to him. It is safe to state that a purchaser of tubes gets exactly what he pays for, and if he wants freedom from replacement worries, he should purchase a tube whose name is one he is sure of, one that is nationally advertised and sold, and backed by a manufacturer who has a reputation for making good products out of glass, nickel, and electrons. A purchaser cannot tell by looking at a tube whether it is good or not. He must rely either on the tube maker's reputation or upon the advice of his dealer or serviceman. It is certain that the latter must not trick the former into buying "just as good" products on the supposition that they can get away with it. A dealer who has a monopoly on the radio sales in his community might sell the tubes that cost the least and had the shortest life, but he could be sure that someone would soon furnish his customers with better tubes — even at a higher price, and that his short-sighted policy would get its merited residt. There is a practice in "gyp town" of showing a prospective customer how good a tube is by stepping on a foot switch which boosts the plate voltage while the tube is being tested. Of course, the plate current will be high, and the customer thinks is he receiving a highemission product. The chances are he is getting somebody's shrinkage. The answer is to buy tubes from well-known dealers, who safeguard their customers as well as themselves. If tubes get weak, as indicated on a reputable dealer's or service organization's tester, within a short time, look at the name. In the majority of cases it is one you never heard of. From the dealer's standpoint, the rules for successful tube sales are but three — 1. Sell only well-known tubes, 2. Insist that your serviceman install these tubes so that the voltage will be correct, 3. Make sure that the purchaser operates the tubes according to the conditions the manufacturer recommends. From the user's standpoint, there are three rules, 1. Buy a wellknown tube, 2. Buy it from a well-known dealer, 3. Operate the tube at correct voltages. + march page 335 •