POWER GRID TUBES
J. A. McCullough,
Chairman of the Board Eitel-McCullough, Inc.
San Carlos, California
It is interesting to see if the present and future status of power grid tubes may be placed in the proper perspective by looking at the electronic events of the past several decades that have shaped the design of our present products. Before I look into the past, present and future, however, I would like to describe just what kind of devices I will be talking about. At Eimac, we define the classical grid type tube (such as a triode, tetrode, or pentode) as a power grid tube We used to call these devices negative grid tubes, but you know that nowadays no one is supposed to think negatively; hence, we changed the name! My discussion today will consider only those tubes of higher power rating than the types made on receiving tube type automatic equipment. Generally speaking, this classification includes tubes having an anode dissipation rating of 50 watts or greater. I would like to discuss these tubes in the context of time periods during which certain important events took place that decisively influenced the direction and philosophy of power tube development. These time periods are somewhat arbitrary, and were triggered by both scientific advancements and by political events.
The first time period began long ago, certainly before the time of us younger men, prior to World War I. It was emphasized by the advent of the oscillating vacuum tube. I believe it is difficult to conceive any other contribution so significant to the tube art prior to World War I, even though this concept seems so simple today. It may seem incredible today that almost 50 years ago high power vacuum tubes were used during that war!
The second period of tube development started about 1923 or so when two very important events took place - the discovery of the advantages of high frequencies for long distance radio communication, and the perfection of quartz crystal control for radio transmitters. In a period of months, the radio spectrum expanded tenfold, and frequency control could now be defined in terms of kilocycles, rather than wavelength! These were indeed exciting times.
The third period of tube development started in the early "thirties" as a result of the relaxation of the very powerful industrial monopolies that held early radio development in an iron grasp. After the relaxation, many new tube companies sprang into life, and their fresh engineering approaches and manufacturing techniques influenced the directions and philosophies of power tube development. One of the young companies that emerged in 1934 was Eitel-McCullough, Inc., and I am pleased to say it is still here today, in the forefront, I hope, of tube development.
A fourth period of power tube development was occasioned by World War II, and this period seemed to be devoted to improvement of existing tube types, with emphasis on large quantity production. New tube types were also required for work above 30 megacycles, a portion of the radio spectrum that had heretofore been largely unknown territory. Probably the most prolific period of tube development has taken place in the past period; that is, since the end of World War II. No one dramatic event is fixed in my mind, but rather a continuing series of events that have radically changed design concepts and that will exert a sharp impact on future development trends. "The past is prologue," it has been said, and now from a quick look at the early pioneer days, and at the present, it may be possible to extrapolate current trends into the future.
It was only a few years after Mr. DeForest and Mr. Armstrong proved that a vacuum tube could produce oscillations that a need became apparent for generating a fairly large amount of radio frequency power by means of the oscillating tube. In 1915, the largest power tube available had a plate dissipation of about 10 watts. The Bell Telephone Company connected 500 of these tubes in parallel, believe it or not, to create a "very high power" transmitter for that age.
During World War I, radio communication made steady progress, but little tube development work was done. After the War, several breakthroughs in technology permitted significant strides forward in the art of power tube development. One such breakthrough was the development of the thoriated tungsten emitter, or filament, which was much more efficient than the older, pure tungsten filament. The second breakthrough was the "Housekeeper" method of sealing metal to glass. The latter technique permitted the manufacture of high power, external anode water cooled transmitting tubes. There were, in fact, water cooled tubes having plate dissipations in excess of ten kilowatts by the year 1920. An interesting comment is that both the thoriated tungsten and the oxide-of-rare-earth metal type emitters were known well prior to 1920, but the economics of the situation did not see their use until later. In the intervening 44 years, much time, money and effort has been spent in seeking new and better emitters. None have been found to date to take the place of those "old timers". What progress that has been made in emitters has come about by refinement of technique, rather than by development of any fundamentally new emitter material. I wonder if we may draw a parallel in this fact in regards to materials used in solid state devices with the obvious choices in that field, too, discovered real early in the game?
After World War I, there emerged two manufacturing groups that were responsible for practically all power tube development in the United States until the early "thirties." These were the RCA-Westinghouse-General Electric group on the one hand, and the Bell Telephone Company on the other. In spite of this apparent two-headed monopoly on tube art, many new and interesting power tubes were developed between 1920 and 1930. In the early "twenties" no consideration at all was given to the high frequencies above 150 meters, and power tube designs showed it! Long leads were the style of the day. To go back in fond memory a bit, the popular RCA-General Electric types of that era included a pure tungsten filament, 5 watt 202; a 50 watt 203; and a 250 watt 204, all with tungsten filaments and all available to experimenters only. The Bell Telephone Company's oxide coated cathode tubes included a 50 watt 211D and a 250 watt 212D, both of which resembled overgrown versions of the "peanut" tubes of that day. All of these "old timers" used the "pinch press" technique to bring the leads to the elements through the vacuum envelope, a carry-over from the lamp industry. Eventually, thoriated tungsten filaments were substituted by RCA for the power consuming tungsten filaments. First, the 204 became the UV-204A, with the new filament; next, the 203 became the UV-203A; and, in about 1925, the UV-202 was abandoned for a new version of the triode - the popular thoriated tungsten UX-210.
Spidery-like water cooled tubes were evolving principally for the then-blossoming broadcast industry. Twenty-five kilowatt plate dissipation water cooled tubes became available to this service. Several important events took place in this era that had a tremendous impact on power tube design. The first event was the discovery by the radio amateur that the frequencies higher than 2000 kilocycles were useful for long distance communication, a fact not compatible with the then-existing theories; and the second event was the perfection of the amplifier following a frequency stabilized oscillator. The successful use of quartz crystals as piezoelectric resonant circuits of extremely high-Q gave unheard-of frequency stability, and made necessary a whole new approach to the technique of amplifying the emission from the low power oscillator. Making these "first generation" power tubes operate on the high frequencies proved to be a tricky and formidable task, surrounded with much "black magic." The "press type" lead seals and the small diameter lead wires brought about the catastrophic demise of many expensive tubes. The failure was caused, of course, by the high capacitive currents flowing within the tube structure at the higher frequencies. A need was obvious for newer, low capacitance power tubes. Tubes of this type, especially designed for high frequency use soon made their appearance. The DeForest "H" tube appeared in 1925, and the UX-852 - a 100 watt plate dissipation tube - was announced in 1927. These radical, new tubes presented new circuit problems as they were essentially high anode voltage, low anode current devices. Output tank circuits having maximum inductance and a minimum of capacitance were the vogue. These "second generation" transmitting tubes, laughable as they may seem to us today, did work better on the new high frequency ranges than did their predecessors and they paved the way for operational gear on frequencies up to 60 megacycles or so.
Concurrently with this progress, the receiving tube manufacturers in 1927, after "flirting" awhile with neutrodynes and neutralized 201-A's and 112-A's, began to use type 222 and 224 screen grid tubes to isolate the input and output circuits to achieve stability. It was only natural that the same scheme would be applied to the larger transmitting tubes. In 1928, the UX-860, a screen grid version of the UX-852, was announced. It was soon followed by a 400 watt plate dissipation version of the UV-860, known as the UV-861.
The rather strong control of the radio industry by two groups of companies (RCA-Westinghouse-General Electric as one group, and the Bell Telephone Company-Western Electric combination as the other), owing to their strong patent position in the previous years, was relaxed to a degree in 1932. This relaxation made it possible for a number of new companies to enter the power grid tube field. Names such as Heintz & Kaufman, Raytheon, Sylvania, Amperex, Taylor, and Eitel-McCullough soon announced their versions of power grid tubes intended for high frequency use. As a result of this broader approach to the power grid tube problems, many new and successful products were forthcoming.
The principal area of opportunity for these new companies was in exploitation of the high frequency communication areas as only a few of the existing power tube types of the early "thirties" were satisfactory in this new role. A 100 kilowatt plate dissipation water cooled type 862 had appeared but the water cooled tubes of the early 1930's were difficult to handle and cranky in operation so that new designs such as the RCA 880 and the Western Electric 298B made their appearance.
Neutralization of the new high power tubes was still troublesome and tricky at the high frequencies so there was much interest in a new circuit originally called the "Inverted Ultra Audion Amplifier." The ultra-audion was a popular oscillator circuit of this era, and the new amplifier circuit resembled it, and was named after it. Today, we know the "Inverted Ultra Audion" circuit for what it really is the grounded grid, or cathode driven amplifier. The "Inverted Ultra Audion" was an early attempt to circumvent the problem of triode neutralization. Even so, with the apparent interest in unusual circuits and the use of medium power tetrodes and pentodes to eliminate neutralization problems, the pre-war tube industry did not bring out "screen grid" tubes for power levels higher than 400 watts.
The demand apparently, was not acute for larger tetrodes and pentodes. In 1932, an article appearing in QST, an amateur radio magazine, described how the author was able to obtain plate efficiencies of 80 to 90 percent in a kilowatt amplifier using two, 100 watt plate dissipation tubes of modest cost. His technique was simple. He more than doubled the plate voltage and used high grid bias and drive, a condition giving large output power without exceeding the plate dissipation limitations. This provocative and unique approach caused at least one tube company to design tubes which could provide similar efficient performance with a degree of reserve capacity, and under reasonable operating conditions. At the same time, the impact of the new Class B audio system was providing economic opportunities for the redesign of older equipment. The popular, pre-war communication system adopted by the airlines substituted two, seventy-five dollar 450 watt triode tubes, capable of providing 2,500 watts of carrier power with high-level modulation, in place of a single three hundred dollar tube that gave only 400 watts of carrier power in low level modulation service. The new broadcast transmitters of this period were going to high efficiency Class C amplifiers, high level modulated Class B modulators. This was a period of ferment - new tubes and new ideas were in the air and on the air. As a sidelight, experimental television appeared during this period, but had no apparent influence on tube design. The same could be said for the experimental work being done with F.M.
The requirement of the military for vast quantities of transmitting tubes during World War II was enormous. Surprisingly, very few new tube types were required for high frequency military communications. As the war progressed, much of the military communication efforts moved up into the very high frequency region where few previous tube requirements had existed. As a consequence, new tube types were developed for communication at VHF. Special tubes appeared to produce the large amounts of pulsed power necessary to supply the VHF radar oscillators. The planar-type triode tube quickly became the most effective tube to be used in the rapidly expanding VHF communications circuits. Planar tubes were manufactured up to several hundred watts plate dissipation, but none of the various versions proved as popular as the famous 2C39, 100 watt plate dissipation triode. The popularity still exists today for the newer ceramic version of this tube - the 3CXlOOA5.
One of the most important contributions to the communication art of this period was the wide use of cavity-type resonant circuits. No longer could the tube designer ignore the associated circuitry that plagued the equipment designer Because of the demand for very high orders of peak emission for radar work, cathode powers and areas were dramatically increased. A major concern of the power grid tube industry during the early part of World War II was how to cope with the primary grid emission problem created by these large cathodes overheating a closely spaced grid structure. One effective, but expensive, solution to the grid emission problem in tubes using a thoriated tungsten emitter was to make the grid out of pure platinum. Practical coatings applied to molybdenum grids were developed before the end of the War, and the parallel oxide cathode grid emission problem was resolved by plating the grid wires with gold. In summary, then, while much work was done in microwave during the War era, the chief contributions to the power grid tube art were development of planar-type tubes, the understanding and use of cavity resonant circuits, grid emission problems, and development of high peak power tubes for radar.
By the end of World War II, a complete change in philosophy covering power grid tubes took place. The multi-grid concept tended to give way to the "beam tetrode" configuration which had appeared in the late "thirties" as a receiving tube type known as the 6L6. At a given plate voltage, this new audio tube design provided much more output power than the then existing pentode types of the same general power class. A transmitting version of the 6L6 known as the 807 appeared about 1938. With the beam tetrode, several problems were handled simultaneously. First, proper placing of the beam forming structure minimized the effects of secondary emission, making the pentode grid unnecessary; second, the extra structure performed the role of screening the input from the output circuit in a reasonable manner, but more importantly, it helped to accelerate the electrons from the cathode to the anode. Large beam tetrodes having anode dissipations up to a kilowatt appeared in the late post-war "forties." These high gain, glass tetrodes were much more compact and more efficient than earlier glass triodes, tetrodes or pentodes and proved to be an almost universal replacement for the hard-to-drive triode tubes so popular just before World War II. The new glass tetrodes, moreover, proved quite effective up to, and including, the new post-war FM broadcast band. Newer and improved versions of various pre-war high power triodes both air and water cooled also made their post-war appearance and worked well up to the FM frequencies. These developments proved just how rapidly the power grid tube industry met the challenge of providing all the power required for good performance in the 100 megacycle FM band.
Perhaps some of you will remember the gloom and dark predictions of the "forties" of how impossible it would be to resolve the tube problems that arose at 100 megacycles During this period, too, television moved out of experimental status and into the living room. The tube problems "Uncle Miltie" and "I Love Lucy" provided were more severe than those of the FM service because the TV transmitter tubes had to produce more power at higher frequencies, and, most important, the output power of the TV transmitter had to vary linearly with the exciting voltage applied to the grid. Some very interesting and exotic tube designs appeared which, while working in a satisfactory manner, were not to retain popularity. Special power grid tubes incorporating low inductance terminations and close electrode spacing - variations of conventional designs - proved to be the economic and reliable answer to the H.F. TV tube problem. Along other lines, there appeared about 1948 a compact external-anode type beam power tetrode - the 4X150A - that, with its numerous variants and offsprings has remained popular over the years. Employing forced air cooling of the anode, the plate dissipation of this compact tetrode was 150 watts, about twenty times the dissipation of the old UX-210 in about half the space! These physically compact tetrodes were the answer to the new trend for more compact and more efficient circuits and components. Possibly the major technological breakthrough of the 1950's was the mastering of the use of ceramic materials in the manufacture of power grid tubes. The many advantages of ceramic over glass, both from a processing and a stability standpoint, led to the direct substitution of ceramic for glass in many existing tube types. The new material really became important when the tube designers took full advantage of the superior characteristics of ceramic for the design and construction of completely new tube types. Power grid tubes that use ceramic materials pretty well dominate the new product development area today. The UHF TV assignments found the tube industry split on whether to use power grid tubes or various other microwave devices at these frequencies.
Today, we find that both the klystron and the multi-grid power tube are doing a good job in this frequency region. Because of circuit considerations, and also because of the need for very high transconductance, some of the tubes designed for UHF TV service and other UHF applications have the anode in the center of the structure with the cathode-grid assembly surrounding the anode. There seems to be no limit as to what a power grid can achieve, both as to power and to frequency, when there is enough incentive for the manufacturer and the engineer to do the job. In the 1933 to 1940 period, high level modulation of Class C amplifiers was the accepted technique for frequencies below 30 megacycles or so. Since World War II, single sideband, suppressed carrier transmission has come into wide usage. This communication system requires the amplifier tubes to be operated in a linear Class AB or Class B mode. Tubes designed for this stringent linear service are much more sophisticated than those designed prior to World War II, with the result that only a handful of pre-war tubes find use in new sideband equipments today. The return to popularity of the linear amplifier is interesting, because many broadcast transmitters of the early 1923 to 1932 period used this design concept to escape the prohibitive cost of large Class A modulators. The advent of the Class B modulator in the early "thirties," it seems, eclipsed the linear amplifier for a few decades
The technique of disposing of the energy dissipated at the anode of a power grid tube has passed through some radical changes since 1945. Prior to World War II, tubes usually disposed of their anode energy by direct thermal radiation or by water cooling. After the War, forced air cooling became quite popular, even with tubes of rather high plate dissipation capabilities. Today, thermal radiation cooling is usually employed in tubes having a plate dissipation rating of one kilowatt or less, and even in this power range, radiation cooling is being challenged by the compact, forced-air cooled tubes. At higher power levels, forced-air cooling and water cooling remain quite popular, but vapor-phase cooling is fast catching on. Vapor-phase cooling takes advantage of one of Nature's laws which stipulates that a great deal of energy must be expended or absorbed to change a substance from one physical state to another. To change a quantity of water at a temperature of 1000C to steam at the same temperature, for instance, requires the expenditure of 540 times more energy than that necessary to heat the same quantity of water from 990C to 1000C. If a water cooled system is employed which allows a ten degree differential between inlet and outlet water temperature, you can readily understand that the same quantity of water converted to steam is 54 more times as effective in expending energy than the same quantity of water raised 100C in temperature. Because of the economics involved, vapor-phase cooling is here to stay.
In the early "thirties," the grounded grid, or cathode driven circuit first became appreciated and used. This circuit is today even more popular than ever and is widely used in the high frequency spectrum. Multi-grid tubes may be operated in this circuit, with all the grids connected to r.f. ground even though each electrode is operated at its proper d.c. potential. The triode tube requires even a less complicated circuit. In either case, the cathode is above ground by the amount of the driving voltage. The grounded grid circuit is simple and quite stable even in the VHF region, although the power gain is somewhat lower than when the tube is in an equivalent grid-driven circuit. About 1935 or so, a series of power grid tubes appeared that required no bias to limit the plate current under zero-signal conditions. I'm sure you remember the 838 and the 805. These old-style tubes were used for grid-driven Class B audio work. In the 1960's the zero-bias idea was revived and is proving to be a popular design as an r.f. linear amplifier tube, particularly in the grounded grid circuit. In yet another aspect, power grid tubes are finding increased use as high power switch tubes. Sophisticated control of megawatts of power by "hard tube" modulators is quite common today. Radars have been continually upgraded in capability. Much higher peak and average power than dreamed of in World War II research laboratories is an operational fact today. Power grid tubes and other type microwave tubes have both contributed their part to this advance in radar capability. The dividing line between power grid tubes and other type microwave tubes, by the way, for single tube generators of megawatts peak and tens of kilowatts of average power seems to fall in the 300 to 400 megacycle range today. A concluding glance at today's capabilities shows that from 1945 to the present the commercial use of TV and FM in the 100 to 200 megacycle range, with output powers of 20 to 50 kilowatts, has become commonplace. The use of UHF TV equipment in the 400 to 900 megacycle range, with output powers of up to 25 kilowatts is becoming widespread. Single sideband in the high frequency spectrum is common and its use is growing at VHF and UHF. Planar tubes and cavity circuitry are in general use. Ceramic has become standard material in power grid tube fabrication.
Multi- grid tubes have reached the 250 kilowatt dissipation level. A single power grid tube can develop several megawatts of peak power, and tens of kilowatts of average power, at frequencies up to 400 megacycles. Hard tube modulators can easily control many megawatts of peak power for radar and industrial application. Power grid tubes are used in industrial applications from a few kilocycles to hundreds of megacycles, and at power levels into the megawatt region. No one may deny that the post-war period has seen phenomenal changes and advances in power grid tubes and in their performance and reliability The latter subject - reliability - is an often overlooked accomplishment during all of this development activity. Some of you can remember when 1000 hours of tube life was considered satisfactory, and the user was lucky to get that. Tube life of many thousand hours is now routine and is expected. The customer is certainly getting a better, and longer-life product for his money than at any time in the past 50-odd years of vacuum tube production.
And so, just what may the future hold for this industry of ours? Can we look ahead through time and see where we'll be in a decade or two? I think we'll see many interesting design changes creeping into the power grid tube of the future. As a result of increasing demand for high density communications circuits, we will expect and find that much more rigid technical requirements for system linearity will be imposed on equipments. By linearity, I mean that the output of the system is a rather faithful reproduction of the exciting signal. There are few, if any, tubes in existence today that will meet tomorrow's requirements for linearity. Over the past several years, much work has been done in trying to determine those parameters that influence tube linearity. More work will assuredly be done tomorrow. Many interesting things have been discovered in the past year or so that may improve tube linearity which some of you "power grid" people might consider exotic for our industry. These discoveries will lead us into interesting areas in the near future. There is no question that much more linear tubes must be designed and built. Power tubes will be designed having better than 60 decibels isolation between input and output circuits, without the troublesome problem of neutralization. The use of tetrodes, pentodes, and tubes with even more elements to manipulate the electron stream will become increasingly popular. The multi-grid advantages will be used in power grid tubes having plate dissipation capabilities several times the present 230 kilowatt limit of the modern pentode. Power grid tubes of special design may well be the economic answer to phased-array radar systems, even beyond L-band frequencies. Sophisticated automatic processes will make greater use of power grid tubes in industrial work where extreme precisions of time and temperature are required.
The power grid tube industry has travelled a long and fascinating path from the exciting concept of 550 tubes connected in parallel in a 1915 model transmitter. That was only 50 years ago. I am sure the next 50 years will be as exciting and progressive as the last 50 years have been.
J. A. McCullough
Chairman of the Board Eitel-McCullough, Inc.
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17th September 2001