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This question has been asked by many people over the past few years. Those who have been in electronics for many years look back to the time when there were relatively few types performing all the functions necessary for successful equipment operation. They argue that most applications haven't changed enough in a few years to warrant all these new tubes. After all, there are only a limited number of fundamental tube configurations--diodes, triodes, and pentodes--and they work at various power levels. You can vary their basic characteristics just so much before they start to overlap one another, so how can you possibly need almost a thousand different tube types? The newcomer to the field of electronics is perplexed as he discovers tube after tube, many of which are apparently intended for the same general service. He naturally wonders, "Are all these overlapping and minor variations on the same theme really essential?" The answer to both of these questions is the same. No, all of these many types are not really necessary. The electronics industry as we know it today could get along with out many of the active types now on the market. Well, if this is true, then why don't they do it and make life easier for everyone? The answer is simple and direct. They can't.
Tube manufacturers cannot mutually agree to eliminate any type which has been produced if there is still a market for it. To do so would be in violation of our laws regarding monopolistic practices and restraint of trade. It follows then, that although there are many competing companies, and any one of them may choose what he wishes to manufacture, no one of them can afford to be too independent unless he is willing to give up his share of the market to his competition. For this reason, and others that will be discussed in the succeeding paragraphs, the number of tube types has been increasing and is bound to keep on increasing at an ever-expanding rate in future years.
Actually, this has been the case ever since the earliest days of this industry. It has merely accelerated in recent years because of the tendency toward specialization. To see how this has come about, we will trace the evolution of some of today's types and the reasons for their existence.
FILAMENT VOLTAGE SOURCES
One of the most prolific forces which has contributed to the development of many new tube types is the power source used to supply the heater voltage. The earliest tubes were designed around standard 1.5-volt dry cells more commonly known in those days as "door-bell batteries." Although these cells were called 1.5-volt batteries and, when new, did have a no-load reading which approached that value, they rather quickly dropped off to a value much nearer to 1.2 or 1.3 volts. Sets using tubes designed for this source of power usually included a filament rheostat and a voltmeter, right on the panel, and the user adjusted the filament voltage to 1.1 volts as the battery voltage gradually decreased.
The second phase, the DC heater era, came when wet batteries were adopted as the more popular form of filament supply. This gave rise to a number of types designed to operate with 5 volts on their filaments. Rheostats were still used, and the 5-volt rating allowed for some battery voltage fall-off before recharging was necessary. It was customary to haul the battery down to a local garage once a week for recharging. Most users hadn't discovered the trickle charges as yet, so batteries were more often partially discharged, and the 5-volt ratings were nearly optimum under these circumstances.
The first AC heater tubes were designed to work at 2.5 volts. No one seems to remember just why this was so.
A few of the heavier filamentary types could be operated directly from an AC source, provided the filaments were center tapped. This seems to have set in motion a sort of standard, some remnants of which still exist in the form
of the 5-volt filamentary rectifiers of today. In that particular era, filament ratings came in 2.5-volt steps. You had 2.5-volt, 5-volt, 7.5-volt, and some 10-volt tubes.
Things were off to a nice, easy-to-understand system.
But it didn't stay that way for long!
2-VOLT AND 6.3-VOLT TUBES
Rural electrification had not spread into too many areas of the country, but radio was reaching the remotest ham let in our land. The residents there wanted the same kind of reliable radio reception their city cousins were enjoying. To answer this need, as well as the need for better lighting because the rural folks began to stay up later to listen to their radios, wind-driven, farm-electric systems came on the market. These were fairly efficient in some areas and provided a reasonably reliable source of avail able DC power. Since they maintained a fairly high charging rate, even in a light breeze, the battery terminal volt age stayed fairly close to the theoretical 2 volts per cell of the normal lead-type storage battery. So, a group of tubes were developed to work with 2 volts on their filaments, and they became known as "farm-radio tubes." About the same time, a few radical souls began putting radios in their automobiles by connecting the 2.0-volt tubes in series across the 6-volt battery. This set in motion another major section in tube industry. It became apparent, even to those early planners of a new industry, that tube types were becoming too numerable, so they decided to bring out some more types, making a first try at standardization. The value of 6.3 volts was chosen as being representative of what batteries and electrical systems in cars of those days averaged. The tubes were made with indirectly-heated cathodes, thereby making them suitable for AC operation also. By that simple act, the 2.5-volt types became obsolete overnight and practically went out of existence in a couple of years.
About this time, another trend started in the AC powered sets which eventually broke the temporary period of standardization and re-opened the trend toward specialization. This was the elimination of power transformers and the connection of heaters in series across the 117-volt line, limited by a series-dropping resistor in the form of a ballast tube, or a line-cord resistor. These sets used the then standard 6.3-volt tubes which had been standardized at a 300-milliampere heater current. In order to fill out the complement, special higher voltage rectifiers and power tubes having 300-milliampere heaters were introduced. The AC-DC era was under way. But it wasn't long before it became apparent that specially designed tubes, just for this service, would make the elimination of ballast tubes and line-cord resistors possible.
The trick was simply to reduce the heater current to 150 milliamperes and maintain the same wattage by doubling the heater voltage. And so, the 12.6-volt types were born.
The arrival of television brought with it many new tube types, although most people have forgotten that some of the first, large-scale production models of the early TV days used the same tubes that had been in use for years in radio. They even went back for a short time and started using the old 6.3-volt, 300-milliampere tubes in series strings. But, although they appear to have been remark ably satisfactory at the time, the trend toward larger screens and wider deflection angles soon left these original TV types by the wayside. Many new types were born, using the old automotive standard heater voltage of 6.3 volts.
This continued for several years, until interest turned toward reducing the price of TV sets once more, and the series-string circuit was again explored. This time a new crop of engineers were on the drawing boards, and they decided that in order to operate in series, tubes must have identical heater warm-up characteristics. No one had thought of this before and, as a consequence, all types of equipment had been made using tubes with uncontrolled heaters, and they seem to have been none the worse for this fact. But in the new age of specialization, this fact was ignored.
A large new crop of tube types was born at first by just adding the letter A to their previous type number, or the letter B if they were already sporting an A for some other obscure reason. As new types were introduced, they automatically got the controlled-heater treatment right from their inception, so they had no letter following their type number at all. This lead to some confusion, but only for a short while, because a new class of tubes began to appear and confusion over these took the attention away from the former ones.
The miniature or portable TV had made its appearance, and with it a whole new group of tubes having 450-milli ampere heaters in a series-string circuit. Why 450 milli amperes? Well, the 600-milliampere tubes being used in the larger TV sets proved to be too hot in the smaller compact sets. Heater wattage had to be reduced in order to decrease the amount of heat liberated in the smaller enclosures. Why they didn't go back to the 300-milliampere versions of the same tubes they were accustomed to using, no one has ever fully explained. Instead, the 450 milliampere value was chosen. This meant that almost the entire TV complement had to be redesigned and brought out as additional new tube types. Even new picture tubes had to be designed to fulfill the requirements of series string filament circuits.
The introduction of transistors into the entertainment electronics field, and especially into the auto radio field was spearheaded by the automotive manufacturer's decision to go to a 12-volt electric system. The economics of the situation were such that it appeared to be most advisable to use transistors in only the power output stage and tubes in all the other stages. As a result, the 12-volt hybrid tubes were born. These tubes were not only de signed with 12-volt heaters to match the new 12-volt electric systems, but they were also designed to operate with only 12 volts on their plates and screens. They thus be came an entirely different type of tube from any of their former 12-volt heater types. Once again, specialization had won out over standardization, and the combination automotive and AC-operated tubes were a thing of the past.
There were a number of physical or mechanical changes which also lead to the releasing of large numbers of new tube types. (See Fig. 6-1 for a chart on the evolution of tubes.) Some of these were brought about by technical improvements in manufacturing which lead to reduced costs. Others were the result of attempts to gain control of the replacement tube market through design changes that required costly retooling by other tube manufacturers.
Tubes started off as an extension of the lamp industry and were made in lamp bulbs with lamp-making machinery and lamp-making techniques. Thus, all tubes were originally made in glass on a pressed stem with a tip on the upper end. When the screen-grid tube was invented, permitting much lower grid-to-plate capacitance, it became necessary to keep the grid lead well shielded from the plate. So it was convenient to bring the grid out of the top of the tube, and to mount a grid cap there for making the connection.
Later, when metal tubes were invented and the octal base was introduced, these same grid caps hung on for awhile. It soon became apparent that grids and plates could come out of the same end of a tube if certain pre cautions were observed, so the single-ended tubes came into being. Not at all dismayed by the attempt of one manufacturer to corner the market with the radically new metal tubes, other manufacturers proceeded to duplicate them in glass. Thus were born the GT's, or Bantams, as they were then called. They were cheaper and actually more reliable than their metal counterparts, so they be came the more popular.
1907-DeForest's "Audion" 1920-1.1-volt battery radios; 5.0-volt pure tungsten; 00 and 01 tube types 1923-5.0-volt thoriated tungsten; 01A, 01AA tube types 1925-3.3-volt thoriated tungsten; X99 tube types 1927-2.5-volt indirectly-heated cathode tubes; screen-grid tetrode 1928-2.0-volt oxide filament; 30, 32, 33, 34 tube types 1931-6.3-volt auto sets and transformer radios 1933-300-ma series-string radio with ballast tube 1934-Acorn types for high frequency 1935-Metal envelope tubes 1938-150-ma series-string radios; GT and lock-in tubes; first miniature tubes 1939-TV beginnings, video amplifiers developed 1941--World War II HF radio, radar, special devices 1946-TV first-era small screens and radio tubes 1948-Computer tube developments 1950-TV large deflection ang1e scanner tubes-cascode tuner tubes 1951-Multisection tubes for TV, series-strings with thermistors 1954-TV series-strings, warm-up controlled heaters; TV low-voltage B+ tubes 1956-12-volt hybrid automobile radio tubes
SECONDARY EMISSION TUBES
COLD CATHODES - SPACE CHARGE TUBES
MINIATURES AND SUBMINIATURES
World War II provided the stimulus needed to develop the much smaller subminiature tubes that were used in anti-aircraft shells in the remarkable proximity fuse.
Although some of these tubes had been made for hearing aids, both before and after the war, their greatest contribution was in the techniques they taught, which were later incorporated in the 7- and 9-pin miniatures of the post-war period and the later TV era. These tubes largely replaced all the former GT types, except in a very few instances where the larger bulb and base were essential to the power-handling capabilities of the tube.
The threading of the thin lead wires through the base of a GT-type tube was always a major cost item to the manufacturer. With the ever-rising cost of labor during the great inflation which followed the war, manufacturers were forced to find cheaper methods of doing things. This led to the development of the button stem for octal-based tubes, using stiff wires formed right in the glass, just like the 7- and 9-pin miniatures. The octal base was then slipped directly over these stiff-wire leads, saving much time and effort for the assembler, and thus reducing costs.
Because many of these types also went to straight-sided bulbs when this change was made, they usually were given new type designations, consisting of the letter G following the former designation. If they were also given new electrical ratings, they sometimes got the letters GA or GB added behind their original identifying numbers.
The latest cause for the release of additional new tubes is a new group of miniatures similar to the 7- and 9-pin varieties, but this time having a 12-pin base. These slightly larger miniatures have been introduced in response to the ever-present need to reduce the production cost of equipment. They permit even more functions within one envelope than the dual-purpose tubes formerly available. These new tubes will be known, no doubt, as "triple-purpose tubes."
The multipurpose tube is quite old, dating as it does from the first commercial AC types, back in the 2.5-volt heater days. The first multipurpose tubes had a common cathode with two sets of electrodes around it. In one in stance, as with the diode-triodes, one section was built above the other and the common cathode merely extended down through both. In the other instance, the two sections were concentric about the same cathode with a simple triode being assembled inside a conventional pentode. These were the pentagrid mixers.
More recently, a new method of building a multi element tube with only one cathode was developed, typified by the dual triode 6J6. This method consists of building the structure with half plates and grids and locating them on one side of the cathode. The mating section is similarly constructed on the other side. The 6X8 is a triode-pentode having this type of construction.
Many designers wanted independent cathodes in these multi-element tubes to permit greater flexibility of circuit design. For this reason, the dual-diodes, dual-triodes, and later the triode-pentodes, all with individual cathodes, became very common. It is a mathematical law that the more elements you have to work with, the more combi nations you can develop. In the case of the multi-element tube, the number of combinations which have come out of the three or four elements is truly remarkable. With the addition of the new 12-pin miniatures, which will allow as many as three triodes in one envelope, the number of combinations becomes enormous. Their offspring should be very numerous.
Aside from the simple filament voltage and mechanical size or shape variables which have been mentioned, and which have served to increase the number of available types many fold, there are those types which were added because of improved electrical characteristics. The individual instances are too numerous to mention, but they fall into certain broad classes that are repeated each time a new family is introduced.
Some of the earliest reasons for introducing new tubes were changes in amplification factor, transconductance, and transfer characteristics or cutoff. These are still being juggled. Each time a new type is released, someone will find that a similar type having a higher or lower rating can also be used to some advantage.
Many similar audio-power output types exist, each varying somewhat in drive requirements or, in other words, efficiency. Some are the result of a real or an imagined need; others are the result of a special selection which ultimately became so widely used that the manufacturer chose to register its characteristics, thus making it legitimate and available to all.
Another factor influencing new tube releases is the simple fact of competition between equipment manufacturers who frequently require a new type in order to permit them to have something "original" in their annual product redesign. Some of these innovations are actually technological improvements, while others are pure whimsy. Cost reduction and circuit simplification are strong driving forces that have resulted in many new tube types. Once in a while, some will be developed that do not have what it takes to do the job, and result in poor field records and a damaged reputation for the equipment manufacturer. When this occurs, there is a reaction period and new tubes are brought out to cover up for the weak nesses in the earlier models. Sometimes these are true improvements; more often they are simply a distraction intended to take the minds of their field people off their unsavory experience with older types that were simply misapplied.
Recently, a new factor has been added to the competitive aspect of new tube developments. That is the com petition of foreign equipment and tube manufacturers.
Starting largely in the high-quality audio component field, this influence has been spreading into the TV and industrial field. Tubes made in Europe, having different design concepts, are gaining favor with some domestic equipment manufacturers. When this happens, the domestic tube supplier loses business, so he immediately counters by making a similar or an identical type.
There are now quite a number of these foreign-originated tubes being made in this country under EIA type numbers. As the foreign electronics industries develop further, we must expect even more of their ideas to affect our own industry. This means more tube types. Many of these are real contributions to the technology of our times.
Some are merely different ways of doing the same thing that something else could have done just as well. But ideas move in fads. Right now there is a popular fad among equipment designers to regard any foreign introduced tube as a "must" in his new design. Perhaps it is the result of the popular preoccupation with foreign cars.
But whatever its cause is, the result will be many new tube types having a slightly foreign accent in the next few years.
The tube industry has been one of those industries that appears to have sprung into being, completely developed.
Of course it wasn't really that way and much development and research went into the very early tubes. But it is true that since their appearance in commercial form, very few major technological advancements have been made. Al most all new tubes have been of the "rework last year's model" type. Thus, there have been many changes in heaters, as well as in bulb sizes and shapes, and bases and caps have come and gone. But these changes were not fundamental, and they permitted no new uses or major improvements in performance. This is why we waited until this point to discuss those really significant developments which, while they meant the introduction of some new tube types, were more than justified because of their effect in moving the art ahead by gigantic steps.
The first of these was the introduction of the indirectly heated cathode with its special low-temperature, high activity coatings. Besides making direct AC filament operation practical, these cathodes provided the necessary peak powers needed in many later applications where pulse operation was essential. To this day there have been no major improvements in cathodes or in their coatings, even though a great deal of research has been devoted to this subject. Although some success is being reported with cold cathode techniques, they are far from being practical at this time.
The second major advancement in tube structures was the invention of the screen grid. The improvement in gain and efficiency permitted by this development over the original triode has not been duplicated by any single advancement since that time.
The beam power tube ranks as a major development because it permitted the evolution of the most efficient pentodes ever produced. Certain classic designs, developed soon after this principle was discovered, have never really been improved upon. They have been scaled up to meet newer requirements, but their fundamental excellence remains unchallenged.
The hybrid tubes for use in conjunction with transistors seem to have been overlooked by many designers in their feverish attempt to transistorize everything today.
Perhaps in a more sober era of re-examination, their true potentialities will be recognized. Their gain, efficiency, economy, and stability are really far out in front of most transistors available today. Their continued use in auto motive equipment is pretty well assured, but why they have not seen expanded use in many other forms of equipment is still something of a mystery to many who are familiar with their potentialities.
Space-charge tubes have always held great theoretical interest for most tube designers. Until recently, very few practical examples of this type of tube had been produced.
In the hybrid automotive line, there is one type--the 12K5--which is a good example of what can be accomplished using this principle. It is well suited to transistor and tube combinations. If this form of circuitry becomes more popular, the space-charge tube should produce many variations and contribute its share to the ever-expanding family of tubes.
Another major contribution of fairly recent origin is the frame-grid tube. Unlike some of the sales-invented slogans of the past, describing such standard production techniques as "gold grids," the frame grid is a real step ahead. The grid windings for very close-spaced tubes, such as are required for extremely high transconductance, or for very high frequency performance, had about reached its limit some years ago. The frame grid permits a much finer winding, or closer spacing to be achieved with production type tubes. These tubes are finding an immediate use in TV tuners as well as in various industrial and military applications. It is expected that their numbers will increase rapidly as their field benefits are recognized.
One final development shows promise of being the fore runner of many new types in the next few years. This is the secondary emission tube. This principle has long been known and has been used in photomultiplier type tubes.
But the use of this principle in a successful Class-A amplifier is a major development of great significance. The transconductance of tubes using this principle can be very high. In addition, their high-frequency characteristics make them very attractive in the wide-band amplifier field and in such applications as computers where very high frequency switching is a requirement.
WHY SO MANY TUBE TYPES?
The answer to "why there are so many tube types"--is found from our discussion to be the result of many factors, involving technological evolution and competition (both foreign and domestic), but most of all a trend away from standard tubes for more or less standard applications, toward the development of highly specialized functions. As this trend is certain to continue with the ever more specialized nature of the different segments of this industry, the number of tube types available is likewise certain to go on expanding at an accelerated rate, unless, or until, some other device replaces them entirely. But in that case, the problem won't be ended. It will just have a new name. In case you weren't aware of it, there are approximately 5000 semiconductor devices registered al ready, and they've only been with us about one-fifth the time that vacuum tubes have!