This thesis, having been approved by the special Faculty Committee, is accepted by the Committee on Graduate Study o f the University o f Wyoming, in partial fulfillm ent o f the requirements fo r the degree o f.c f.^ ^ i^ .^ f.. Chairman o f the Committee on Graduate Study. 1 certity this student received a Master o f Science Degree at the University o f Tammy AagartK Registrar Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Cons ide rat ions in the Design of Broadcast Transmitters. Floyd Willard Wickenkamp Thesis submitted to the Department of Electrical Engineering and the Committee on Graduate Study at the University of Wyoming, in partial fulfillment of the requirements for the degree of Electrical Engineer. LIBRAFY □ F THt UNIVERSITY OF WYOMING LARAMIE Casper, Wyoming 1940 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: EP24885 INFORMATION TO USERS The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleed-through, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. UMI ® UMI Microform EP24885 Copyright 2007 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, Ml 48106-1346 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Habudfott -Contents- page List of Charts and Illustrations............................... ii Acknowledgements...................................... iii Chapter One. Primary 1. 2. 3. Discussion........................................... Reliability....................................... Upkeep............................................. Discussion of available e q u i p m e n t .................. 1. 1. 1. 2. Chapter Two. A Study 1. 2. 3. of Tube Costs in the Low and High PowerFields........ Tubes for low power transmitters.................... Tubes for high power transmitters ........... A comparison of tube c o s t s .......................... 3* 3. 4. 6. Chapter Three. A Discussion of Two and one Half Kilowatt Transmitters....... . . 8. 1. Water-cooled tubes, their advantages and faults . . . . S. 2. Life records of representative tubes............ S. 3. Tube complement costs for various t u b e s ............. 9. 4. Additional cost for auxiliary equipment. . . . . . . . 11. 3. Plate power requirements ............................ 12. 6. Ease of handling various tube t y p e s ................... 13. 7. Filament power requirements.......................... 14. 8. Circuit c o m p a r i s o n s .................................... 16. Chapter Four. A Practical Design Problem . . . . . ........................ 1. Reasons for choice ofcircuit in practical problem . . . 2. Operation conditions for radio frequencycircuit . . . . 3. Conditions of operation for audio circuit. ........... 23. 23. 24. 28. Charts and Illustrations.............................. . . . . 33. Bibliography.......................................... Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 47. -List of Charts and Illustrations- Block diagram of 2.5 kilowattclass "B" modulated transmitter .. page. 33* Characteristics of RCA802 vacuumt u b e ....................... 3^. Characteristics of RCA805 vacuumt u b e ....................... 35. Characteristics of RCA807 vacuumtube ................. 36. Characteristics of RGAS10 vacuumt u b e ........................ 37. Characteristics of BCABl^ vacuumt u b e ........................ 3&. Characteristics of RCA2&5 vacuumt u b e ........................ 39* Characteristics of RCA.1603 vacuum t u b e ...... Uo. Characteristics of Amperex 220C vacuum t u b e ......... M-l. Characteristics of Amperex 228A vacuum t u b e ......... k2. Characteristics of Amperex 892 vacuum t u b e ............... ^3. Characteristics of Eimac 750TH vacuum t u b e ............. . Power ratings of vacuum tubes for plate modulation...... H5. Power ratings of vacuum tubes for linear amplifier u s e .. *+6. Power ratings of vacuum tubes for grid bias modulation use . . . Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ^6. -Ackno wledgement b - The author wishes to take this opportunity to express his appreciation of the courtesies extended, through the prodigality with which the following companies furnished information on their products: Amperex Electric Company. Eitel-McCullogh, Incorporated. Federal Telegraph Company. EGA Manufacturing Company. Lapp Ceramic Manufacturing Company. Thanks also should go to Eadio Stations KDFN and WLW for information concerning tube life. F. W. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CONSIDERATIONS IN THE DESIGN OF A BROADCAST TRANSMITTER. Chapter One. Primary Discussion. In designing a broadcast transmitter, the matters of first im portance are original cost, upkeep, reliability, and approval by the Federal Communications Commission. Of all the above, with the exception of the Commission's approval, the primary interest is reliability. Broadcast transmitters, as well as their tube complement, must be designed for long trouble-free life, since the loss of even a few minutes on the air may mean loss of station prestige plus loss of revenue in case of disruption of a commercial program. Hence, it is usually desireable to purchase the best available tubes and parts. Since, over a period of years, the greater part of the equipment and upkeep cost will be in the tubes, it is the purpose of this paper to endeavor to show how this cost may be kept as low as possible, consistent with the guarantee of reliability and stability. In the case of the smaller transmitters, up to and including 1000 watts, there is little question involved as to the use of air-cooled as opposed to water-cooled tubes. The many new types of air-cooled tubes made available recently by the Radio Corporation of America, Heintz and Kaufman, ltd., and Eitel McCullough, Inc., among others, has made possible very economical design. On the other extreme, above five kilowatts, there is no question but that water-cooled tubes are the only possible solution. At the present time, the largest air-cooled tubes are capable of a class "C" output under conditions of modulation of 2500 watts. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 However, it is in the so-called medium power, regionally allocated class that there is a question as to which type of cooling is the better. In the past, since most of the design problems have been kept closely connected with the larger equipment manufacturers, there has been little incentive to try to reduce cost to the purchaser. Because of the expense connected with building transmitters, there has been very little competition for the three or four large companies in the field. Since the first consideration of the manufacturer is to make a profit, it has been common practice to keep the older style, and usually more costly methods, to arrive at the ultimate setup. This does not mean that the manufacturers are deliberately keeping the cost up; it is merely a matter of habit, and the fact that their products have been used and have proved their reliability for years. In the past year or two, however, the newer items, especially tubes, have been proving their worth in many of the commercial and police transmitters through out the nation. The writer has had some experience also with the new air-cooled tantalum plate tubes in broadcast service and in high frequency service in an amateur transmitter and found them to be excellent. While the original tubes in this group were not as long- lived as the older types, continued improvements have resulted in a series of tubes that compare favorably with tubes of the older classifications. In connection with this paper, much of the work has necessarily been research through tube charts and manuals, with the exoeption of the writer’s own experience gained as chief engineer of broadcasting station KDFN. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter Two. A Study of Tube Costs in the Low and High Power Fields. As stated in chapter one, the most controversial issues so far as the type of cooling is concerned, are in the medium class field. It might be interesting, however, to make a short survey of the more usual lineups in the other two classes, viz., the low-power and the high-power classifications. In the low-power class, that is, 100 and 250 watts, the usual powers for which the Federal Communications Commission issues licenses for local stations, the tube costs are almost low enough to be left out of the picture in comparison with the total cost. In fact, it is usual to use the same type tubes in the more recent transmitters for the final R. F. stage and for the class ”B" modulator stage in both the 100 and 250 watt classifications, merely adding one tube to the final radio frequency stage when increasing power. Since the cost is low, no attempt is made to use class "B" linear finals to reduce the number of tubes necessary. There are many types of tubes suitable for use in the final R. F. stage which are approved by the Commission, among which are the followingi (l) Radio Corporation of America, RCA805, RCA810. Heintz and Kaufman, HK254. Halted Electronics, 905. Taylor, Tl25. (l25 watts per tube) (l25 watts per tube) (Same characteristics as RCA805) (l25 watts per tube) In the case of all the above tubes, two tubes of the same type may be used for Class "B" modulators.__________________ _________________ (l)* See chart, page 37. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 The use of one type tube in both the audio and the radio frequency stages reduces the number of types necessary and the number of spares to be kept on hand. It would be possible to use slightly larger tubes, such as the RCA806, with a rated output of 250 watts per tube, which would necessitate other types for the modulators, or the use of two of the larger tubes, increasing the tube cost and accomplish ing nothing. Since all the tubes listed fall in the price class between ten and fifteen dollars, there is lettle to choose among them, except personal preferences. Any of them will serve the purpose admirably and should give long and trouble-free life. Four of the RCA805 tubes are being used in the KDFAi transmitter to modulate a five hundred watt carrier, and have been in service for nearly five thousand hours with no trouble of any kind. It is interesting to note in passing that as little as three years ago, the cost of tubes for a 250 watt final radio frequency stage was not $27.00 as in the above case, but $60.00 or more. In addition, four tubes were necessary, adding to the complexity of the circuit. Of course, a larger tube could have been used, but then the cost would have amounted to $75.00 or more per tube. This is mentioned merely to show the rapid strides made in lowering tube costs, accomplished through increased tube efficiency and increased production. When we consider the larger transmitter, ten kilowatts or more, the picture is entirely changed. Tubes for such high powers must be liquid cooled, since no manufacturer has so far successfully made a tube which would withstand the tremendous heat generated, or the high Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5 voltages necessary, a requirement for reasonably high efficiency. Thus it is necessary to use some form of cooling. Since water is inexpensive, plentiful and, when free of impurities, non-conducting, it is most commonly used for indirectly cooled tubes. Since there is considerable conductivity in most tap water, distilled water is generally more satisfactory. The water can be cooled by an outdoor spray or fountain arrangement, making use of the cooling properties of the air, after which it is pumped back into the station to be used over again. Tubes must be specially constructed for water-cooling. The cooling chamber takes the form of a jacket around the plate of the tube, which is not glass inclosed as is the air-cooled tube, but is, itself, the outside of the tube. The distilled water is pumped through the jacket and past the cylindrical plate of the tube, then the water is cooled as described in the previous paragraph, and returned to the tube jacket by means of centrifugal pumps. The cost of the water- cooling equipment is high, and requires extra precautions to minimize leakage. There is a choice in water-cooled high power transmitters between class "B" linear amplification or class "C" amplification. Either method is perfectly satisfactory, but, as their operation is quite different, many engineers have marked preference for one or the other. Tube cost in any event will be found the most important single item. We will consider as an example, a five kilowatt set. In this case, there is a choice between high and low level modulation. Tubes readily available includes Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Amperex 892, rated at 5. fcw. modulated class "C". 2.5 kw. class "B" linear. Amperex 220C (ratings same as 892). KCA892 (same as Amperex 892). BCA892R, rated at 2.9 kw. modulated class nCn. The RCAS92E is a special tube of the water-cooled type, but using air-cooling by means of large hollow aluminum fins fitted on the socket, through and around which air is forced. Since the 892R is rated at only 1 kw. for linear service, it would not be suitable except as a class "C" amplifier, for a five kilowatt transmitter. The cost of each of the tubes listed, less sockets, runs between $250. and $325. There is little choice among them since they should all give long life operation and have been proven in operation. For five kilowatts, then, class "C", the approximate cost for the final stages would be summed up thus: Final radio frequency stage, one 892 t u b e .... $285. Final class "B" modulator, two 892 t u b e s ..... 570. t o t a l .... $855. For five kilowatts obtained by means of a class "B" linear stage: Fihal linear amplifier stage, two 892 tubes . . . . $570. To the cost of the linear stage would be added complications in the driver stage, so the final cost for the two transmitters would be very nearly the same. Similar figures could be given for 25 kw. orother sizes, except fifty kilowatts, in which case it is customary to usetwo type 862 tubes in class HBn linear, since thera’is no tube manufactured which carries a fifty kilowatt rating in class "C". Two 862 tubes Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. operating at 25,000 volts will give 50 kilowatts output. The 862 costs $1650. It is beyond the scope of this paper to discuss completely the desigji of such large transmitters. However, more information on water-cooled tubes and their operation will be given later, as well as methods used in considering the different items of design applicable to all sizes of transmitters. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter Three. Discussion of Two and a Half Kilowatt Transmitter. As was mentioned previously, it is the outputs between one and five kilowatts in which there is a question as to the choice of the best type of circuit and tubes. First, a dedision must be made as to whether air-cooled or watercooled tubes will be used. Then we can choose the circuit. Water-cooled tubes, for years commonly used in the larger trans mitters, have been found to have good life expectancy, are fairly reasonable in cost, and are generally well known and understood by the average engineer. Their disadvantages include: 1. They require complicated cooling systems. 2. Extremely high voltages are necessary for reasonable efficiency. 3. They are difficult to handle. 4. The filament power required is much higher than for air-cooled tubes. 5. A specially built, well insulated socket and water jacket are needed. Following is a list of typical tube3 with their service records, taken from the files of WLW and KDFN. The water-cooled tubes are from WLW's records, the air-cooled are from the records of KDFN. Type no. Cooling. Serial no. Life in hours. 862 862 870 870 204a 204a 204a 852 Water 10725 10736 10012 10117 34616 19357 13700 6128 16,633 9,663 15,689 0 (Defective 9,882 M52 14,000 4,000 II n 11 Air 11 11 n Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. From this chart it may be observed that life expectancy for both air-cooled and water-cooled tubes is high. Most manufacturers guarantee their tubes for a period of not more than six months or 1000 hours service, depending on which occurs first. However, the tubes are usually designed to last for a much longer period, and the guarantee is merely to guard against defective tubes. In taking up the matter of tube costs, we can most easily get a complete picture by making a chart that shows tube costs under differ ent conditions of operation. Since, under certain conditions the cost of the driver stages is quite a factor, we should include the cost of both the drivers and final stages. Water-cooled* Class "C" Final, Class "B" Modulator. Final R F stage, one 228A tube ....... Final class "B” audio, two 228A’s Plate Voltage. $ 225. 6000 volts. .... 450. 6000 " RF driver, two 810 tubes -............ 27. 1250 " Audio driver, four 845*s ............. 40. 1250 " Total Class "B" Linear Final. Final R Fstage, one892 .........$ Modulate R F driver, two 810*s Class m AB" ......$ 742. modulator, four 845’s Total 285. 10000 volts. ...... 27. 1250 " .... 40. 1250 " ......$ 352. Air-cooled. Class MBW Linear Final. Wot practical with present tubes, since eight 750TH's would be required for two and one-half kilowatt output. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Air-cooledj Class "C" Final, Class "B” Modulator. Final R F stage, two 750TH*s ...... Plate voltage $ 350. Final class nB w modulator, two 750TH*s RF driver, two 810' s 350. 4000 " 27. 1250 n .......... 40. 1250 " .... $ 7b7 . ........ ...... Audio driver, four 845's 4000 volts. Total When, air-cooled tubes are compared with water-cooled tubes, it L is seen that the former have a great advantage. In most of the m o d e m transmitters, it is common practice to add a certain amount of forced draft to keep the ambient temperature as low as possible. By drawing air from the top of the transmitter enclosure, and with an intake at the bottom, it is possible to reduce the temperature considerably and thus provide a greater margin of safety for condensers and other parts which are affected by high temperatures. Water-cooling, however, is not such a simple matter. As mentioned previously, it is best to use distilled water or some non-corrosive, non-conducting liquid, since a conductive cooling medium such as tap water will allow high leakage losses from anode to ground and encourage formation of scale on the plate, and so reduce the cooling effectiveness. The lower cooling efficiency brought about by scale may cause hot spots on the plate, which in turn will allow gas to be released internally, and destroy the filament emission. For these reasons, the extra cost of distilled water seems to be very much worthwhile. It might be noted in passing that the majority ofwater-cooled tubes are unsuitable for the higher frequencies, because of their Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. high, interelectrode capacities. As it is possible to get more effective cooling with water, the hulk of the tube is much less than for air-cooled tubes for the same plate dissipation rating. However, the added space required for pumps, cooling coils and other cooling equipment will offset the advantage of size in the tube itself. Since it is impossible to remove all conducting materials from water, even distilled water, it is best to reduce the leakage current through the water by the use of a porcelain cooling unit. This porcelain coil is built in a helical form to give a length of fifteen feet or more. The unit should be installed in close proximity to the tube socket, and the tube socket is often mounted inside the coil. (2) The cost of the auxiliary equipment for the cooling system is normally a one cost item, that is, there should be little upkeep required. However, the first cost is considerable. Following are ap proximate prices on representative items required: Belays for shutting off high voltage in case of failure of water circulating system (two required)..................... $40.00 Porcelain cooling and isolating coil ............... 95*00 later pumps (two required) ......................... 150.00 In addition, the following will be needed, one for each water-cooled tube in use: Water jacket ...................................... Mounting insulator and clamp for water jacket .... $35.00 22.00 Filament transformer, one for each tube except in the case of two-phase filaments, for which two are required.50.00 (2). long, "Porcelain Cooling Unit". Electronics, October, ’38. P. 24. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Cooling equipment, then, costs from $400. to $500. for the average set. To be completely impartial, we must consider the cost of sockets for the air-cooled tubes. In most cases, ten dollars per socket will cover the cost of a socket for any of the air-cooled tubes in common use. The only exception to this rule is in the case of the new forced-air-cooled RCA tubes, RCA891R and RCA892R, the sockets for which are $60.00. However, these two types are essentially water-cooled tubes RCA891 and RCA892, in which the water jacket has been replaced by a large finned socket and cooling shell through which air is forced. (3). Characteristics are similar to the comparable water-cooled tubes, except that the plate dissipation is reduced by about fifty percent. Plate voltage requirements are of extreme importance in any transmitter, since the cost of transformers, rectifier tubes, filter condensers, and other parts increase rapidly with increased voltage. For voltages up to about six thousand, regulation pole transformers are satisfactory, used in three-phase circuits with outputs up to twenty kilowatts. designed output. For higher voltages or for greater power, specially transformers will be required at much higher cost per unit of Since pole transformers are made in quite large quantities, their cost is not excessive, as no special designing is necessary. •*ti choosing rectifier tubes, the commonly used 872A is suitable up to 3500 volts in single-phase full-wave circuits. A three-phase connection will allow an output of 8700 volts* (3). RCA bulletin, "Air-cooled RCA 5 kw Transmitter, type 5D. RCA Manufacturing Co., Camden, N.Y. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. For voltages in excess of 8700, it will be necessary to use the 669 or similar tubes. The 869 tube is suitable for voltages up to 7000 in a single-phase full-wave circuit, or 20,000 volts in three-phase full wave circuits. The cost of the 872A is $10.00, while the 869 is priced at $125.00. Filter condensers are also an important item especially at the higher voltages. For example, a 5000 volt, 2 mfd. condenser sells for « about $35*00 while a 10,000 volt, 2 mfd. condenser costs more than $120.00. Insofar as the voltage rating of the filter chokes is concerned, the cost is only fractionally more for 10,000 volts than for 5>000 volts. In addition, if higher voltages are used, the current rating could be lower for the same power output from the filter. In the final analysis, the cost would be very nearly the same. From the above, it is obvious that the lower voltages have many marked advantages. As regards ease of handling, air-cooled tubes seem to have the advantage. Unless water-cooled tubes are cooled with distilled water, it is necessary to dismount the tubes and clean the plates at least once a month to keep down scale formation. Even if distilled water is used, it is necessary to check and replace gaskets periodically; pumps must be repacked, and the water lines must be watched closely for leaks. It is no small matter to replace a water-cooled tube because it is a part of a necessarily leak-proof water line, and must be carefully installed and checked for water leaks before operation can be resumed. This may often mean a loss of valuable air time. Air-cooled tubes may be replaced with comparatively little Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. trouble or lost time, since ail that is necessary is to remove the tube from its socket, replace with another, make the necessary connections to plate and grid, after which operation can be resumed. Since both air-cooled and water-cooled tubes become very hot during operation, most stations keep a pair of heavy leather gloves handy for grappling with the tubes. The gloves together with a heavy cloth or chamois pad, will allow removal of even the hottest of tubes in a few seconds without danger of dropping the tube or burning the fingers. If this is not done, it may be necessary to w a i t many precious minutes before handling the tube. A check of the filament voltages and wattage ratings of the various air and water-cooled tubes shows that, on the average, watercooled tubes with equivalent maximum electronic space current. example, the Eimas 750TH with a maximum space current For ( d. c. plate current plus d. c. grid current) rating of 1.25 amperes consumes lbO watts in heating the filament. For the Amperex or Western Electric 228A water-cooled tube, the maximum space current is 1.25 amperes, the same as the 750TH, but the filament power is 880 watts. The RCA892 with a filament power of 1320 watts, has a rated space current of only 2.0 amperes. A factor whioh offsets partially the seemingly great difference in filament power is that air-cooled tubes operating at lower plate voltages require higher space currents for the same output. However, tubes such as the 750TH with an output rating of 2500 watts for two tubes, uses 320 watts of filament power while the 228A tube with the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. same output, 2500 watts, requires 8S0 watts filament power. The reason is that while the air-cooled tubes use an efficient thoriated filament operating at fairly low temperatures, the water-cooled tubes use tungsten filaments which must be operated at much higher temperatures. The use of tungsten filaments in water-cooled tubes is made necessary by the difficulty of obtaining a high vacuum. Even a reasonably small amount of gas will greatly reduce the efficiency of a thoriated type of filament, and the amounts left in water-cooled tubes will tend to permanently harm the filament. High gas content in water-cooled tubes is due in part to the large metal surface exposed to the internal vacuum, and to the fact that it is difficult to get a good seal between the copper plat& and the glass envelope. Air-cooled tubes, on the other hand, are much more easily pumped to a high vacuum because there is much less metal in the tubes, the metals used are more easily de-gassed than is copper, and the tube elements are completely enclosed in a glass envelope, removing the problem of sealing metal to glass except for the relatively small element leads. As stated above, since thoriated filaments operate at lower temperatures, it is plain to see that the power required to raise the filament to operation temperature is much less than in the case of a tungsten filament. A filament starter is adviseable with all the larger tubes includihg the air-cooled, although in the case of air-cooled tubes, it is permissable to start the filament heating at only a slightly reduced voltage to reduce the initial rush of current. As the temperature of tungsten increases, the resistance increases quite rapidly and the current is reduced accordingly. At room temperature, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the current may be several times the operating current. A reduction of 25 or 30# for about 30 seconds will serve to protect the tube filament. When considering water-cooled tubes it must be kept in mind that the starting voltage has to be low since the cold resistance is very low compared to the filament resistance at the high operating temperature necessary with water-cooled tubes. It is best with watercooled tubes to apply not more than 25$ voltage to start with and increase the voltage slowly by means of rheostats or a series of time delay relays, so that two or three minutes are consumed to bring the filaments to operating temperature. Also, when the filament of a water-cooled tube, which may be as much as an eighth of an inch in diameter, is turned off, it should be reduced slowly to prevent straining and buckling the filament. Since the filaments in air-cooled tubes are much smaller in diameter and operate at lower temperatures, there is less expansion and contraction, and a smaller change in resistance. It is usually not necessary to cool air-cooled filaments slowly; they may be turned off at once after the conclusion of the days operation. A very complete discussion of the care and operation of water-cooled tubes may be obtained from the different manufacturers, notably RCA, Western Electric, Amperex, and Federal Telegraph companies. The foregoing is not to prove the superiority of one type of cooling over another, but to give the engineer an idea as to what to expect from different tubes. How that we can make a decision as to the choice between aircooling and water-cooling, the next important step is to discuss the type of circuit to use. First, a short discussion of the more common Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. methods of modulation is in order. These methods are as follows: 1. Class "£n linear amplifier in connection with a modulated B. F. driver. The linear amplifier operates at approximately cutoff "bias so that the plate current is zero with no excitation. 2. Class "C" amplifier, plate modulated by a class "B" modulator. The class ”C" amplifier operates with high grid bias so that a considerable amount of grid drive is required before the plate current commences to flow. The class "£n modulator operates so that only a small plate current flows without audio input. 3. Grid bias modulation. The audio signal is mixed with the incoming radio frequency signal on the grid of the final stage. Grid modulated amplifiers operate in class "£" or class "A" and are relatively low in efficiency. 4. Heising modulation, a class "C" amplifier is plate modulated by a class nA n modulator. Since class HA" modulators are very low in efficiency, Heising modulation is fast becoming obsolete. Class "B" linear simplifiers are used for amplifying a modulated radio frequency signal as stated above. Since we want distortionless output, it is important that the output voltage be proportional to the input voltage. Linear amplifiers are also known as distortionless amplifiers, because of their property of magnifying the signal fed to the grid so that the output signal is increased in voltage and identical in shape to the grid input signal. One of the chief dis advantages of the linear amplifier is its low efficiency, which is between 23 and 35 percent. This means that the plate must dissipate at least twice the power output obtainable. The big advantage of the linear class n£" amplifier, aside from its distortionless character, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. is the reduction in size of the final audio stage. Since the radio frequency signal is audio modulated in a stage previous to the final stage, much less audio power is required. The usual proceedure in medium power transmitters is to modulate the driver stage (the radio frequency stage just preceding the final stage). The driving power required for a linear stage is much lower than that required for a class "C" stage, and will vary with different tubes, hut is usually from two to five percent of the input to the filial stage. Since the output must he free from distortion and phase shift, it is necessary that the driver tuhe he capable of delivering considerably more power than the peak requirements, with nearly perfect regulation. That is, with an increase in the audio signal, the radio frequency power delivered to the grid must remain practically constant. It is often adviseable to load the driver stage with a constant non-inductive load so that the regulation will he improved from no load to full load, since the more heavily loaded amplifier will give more nearly constant voltage output than one that is almost completely unloaded during certain parts of the cycle. A satisfactory load will usually he one that loads the driver stage about as much as the peak power requirements for the class "B" stage. Then a modulated driver with a rating of three times the maximum power delivery to the final stage would he suitable. At first thought, it would seem that, since a class "Bn amplifier is biased to cutoff, there would he no current flowing in the plate circuit without audio modulation, hut it should he remembered that the radio frequency carrier is being applied to the grid at all times; thus the plate current is nearly constant and the instantaneous current will Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. vary at an audio rate above and below the average plate current. Since the plate must dissipate a constant amount of energy, with or without modulation, the efficiency is low and large tubes must be used to obtain appreciable output. Since a class "Cn amplifier is biased beyond cutoff, so that with low grid driving power there is no plate current flowing, the class "C" amplifier is sometimes called an efficiency amplifier, because the efficiency of the final is governed to a certain extent by the value of bias and by the grid driving power. Most class "C" amplifiers are biased so that plate current flows about 120 to 150 degrees of the cycle, a value which gives optimum efficiency commensurate with a reasonable amount of driving power. While it is possible to bias the stage to four or five times its cutoff voltage, so much extra grid driving power is required for only slightly higher efficiency that it is not considered good engineering practice. A class "C" modulated amplifier, since it is itself being modulated, will require an audio signal sufficiently large to give 100 percent modulation, or a value equal to half the input power to the radio frequency final stage. Thus, the input to the audio modulator will be required to be about 80 percent as much as the input to the final amplifier. Two tubes are required in class "B" audio circuits for good quality since one tube will operate only during the half cycle that its grid is positive. Thus, to get a complete cycle of audio output, two tubes operating 180 degrees out of phase with each other are required. Driving power for the E. F. stage will run much higher than for the linear amplifier, since the grids are biased highly negative. The Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. "bias voltage most be overcome and a large positive grid swing obtained in addition to obtain large output. About ten percent of the output power must be supplied by the driver. For the class "B" audio stage, an audio signal of very high quality must be available, necessitating a class "A" or class "AB" audio driver. Audio driving power equal to about seven percent of the audio output of the modulator will suffice. For grid modulation, the efficiency is very low, only 20 to 25 percent. However, the audio signal required is very small, on the order of two to four percent of the output from the modulated stage. Grid modulated amplifiers are difficult to tune and, because they require such large tubes, are little used in broadcast amplifiers, except at times as a modulator for a linear amplifier circuit. Up to thiB point, it will be seen that the linear amplifier seems to have the advantage insofar as complicated circuits and driving power are concerned. However, there are two points on which the linear amplifier is inferior to the class "C" modulated amplifier. The first is comparative stability and ease of tuning; the other is that under most conditions, larger tubes and much higher plate voltage must be used in linear circuits to obtain an output equal to that of the class nC" circuit which operates with comparatively small tubes and at more reasonable plate voltages. In the adjustment of a class "C" stage, plate circuit tuning, bias adjustment, driving power adjustment, antenna loading and audio loading are not especially difficult, and when once adjusted, will usually give little or no trouble. Stability in class HC" modulated amplifiers is limited only by the care used in building and in design. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. la linear amplifiers, the plate impedance must be carefullyadjusted so that the tube impedance and that of the tank circuit are very nearly the same. The grid bias must be adjusted so that the tube is operating exactly at cutoff -without excitation, since carrier shift will result with incorrect bias, as well as the possibility of having excessive or too little available modulation depending on whether bias is too low or too high. In other words, if the linear amplifier is operated beyond cutoff, it is possible to have complete modulation of the positive peaks even though the class "C" amplifier were operating at only 60 or 70 percent modulation, the class "C” amplifier in this case being the driver. Driving power is also a critical point in linear amplifiers. .Excessive radio frequency excitation will often result in a negative carrier shift, while too little R. F. excitation will reduce the output and cause overheating of the linear stag©. The loading to the antenna must be adjusted carefully to match the impedance of the plate and tank. Excessive distortion and reduced output are two common results of incorrect antenna loading. The other class "B" linear adjustments are routine, as in the case of class "C" amplifiers. However, linear amplifiers are more subject to parasitics and any small change in loading due to any one of several causes, may give trouble. Hence, linear amplifiers are usually in need of closer supervision and must be more thoroughly and carefully tuned for high quality operation. Grid bias modulated amplifiers require careful adjustment of the bias, the plate load and antenna load, and the R. F. excitation must Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. be closely adjusted for satisfactory operation. Grid bias modulated amplifiers are probably slightly more stable than linear amplifiers but not nearly so trouble-free as a class "C" modulated amplifier. In summing up the comparison in tuning ease of the three systems, we find that the class "C" stage has no especially critical adjust ments, the linear and grid modulated amplifiers have four each. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter Pour A Practical Design Example, laving taken up the advantages and disadvantages of the various tubes and circuits used in m o d e m broadcast work, let us put our conclusions to practical use by designing a transmitter rated at 2% kilowatts output. While a one or a five kilowatt transmitter could have been chosen, it was felt that the 2g kilowatt size offers the most for the money invested and is probably not so well known as it should be. That size operates at little more expense than the one kilowatt size, the original cost is only slightly more and gives a power gain of four decibels. On the other hand, 5 kilowatts is a power gain of only three decibels over 2% kilowatts, and 5 kilowatts output requires higher voltage with all its attendant difficulties, as well as much higher cost. As we have seen, while linear amplification is less expensive as far as tube cost is concerned, it is more complicated than and not as stable as a class "C" amplifier. Air-cooled tubes seem to offer less complications and probably would cost less over a period of time than water-cooled tubes, for these reasons, a class HC" final, class nB" modulated transmitter was chosen in this particular case. Under some conditions it might be better to use one of the other combinations. It is best to start with the final stage and work back through the exciter stages to the oscillator in the case of the radio frequency portion, or to the audio input in the audio section. Since the Eimae 750TH tube answers all the requirements for the final radio frequency Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. stage, and is approved by the Commission (see F. C. C. approved tubes in chart section), tvo 750TH's will be used, operating under the following conditions: Plate voltage . ..................... 4000. volts. Plate current....................... 900- milliamperes. Efficiency set by F. C. C 70. percent. Power input ....................... 3^00. watts. Plate dissipation ................. 1100, watts. Plate power output 2500. watts. ............... Incidentally, commencing July First, 19^0> the Federal Communi cations Commission is requiring that all broadcast stations use the direct method of measuring transmitter output, that is, the antenna current and the antenna resistance at the point of coupling from the transmission line are measured and the power into the antenna is computed from the formula: power equals current squared times antenna resistance. However, usually it will be found that the efficiency factor set by the commission is not far wrong and the current, voltage and power figures given above will be quite accurate. Since the driving stage must be capable of an output of eight to ten percent of the input to the final stage, a power output of about 3^0 watts should be available at good regulation. The RCA810 triode is a new tube just recently released by the Radio Corporation of America with characteristics similar to the widely used RCA805 but with a heavy duty filament and capable of considerably more output than the 805 tube. The 810 tube is capable of an output of more than 200 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ■watts operating at low plate voltages. Low voltage is desirable at this point since the audio drivers as well as several other tubes can then operate at t h e same voltage, keeping to a minimum the number of power supplies. Then the choice for this stage will be two RCA810 tubes tinder the following conditions; ■Plate voltage ..................... Plate current ...................... Estimated plate efficiency Plate input power 500. volts. milliamperes. 55• percent. ................ 625. watts. ............ 215. watts. ............... 410. watts. Plate dissipation power Plate power output .... 1250. In order to reduce the amount of grid driving power requirements on this stage, and since plate voltage is of less importance than in the final stage, the driver stage is not driven as hard as in the final stage. Bias amounting to about 1-gr times grid cutoff voltage is sufficient for the driver stage. In the voltage amplifier stage preceding the RCA810 tubes, output equal to about six percent of the input to the driver stage is re quired. An output of 625 x .06 or 37.5 watts is sufficient. To obtain 40 watts of radio frequency power an 810 tube could be used, although a beam type tube such as the RCA814 is probably more suitable. The advantage of a tetrode or pentode type tube in the lower stages is the additional isolation afforded the crystal stage from the final stage. Because of its high amplification factor, a large change in load on the plate would have little effect on the grid impedance* Another advantage of the type 814 is its low driving power requirement and a maximum plate voltage rating of 1250 volts, which allows the same Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2b. plate voltage to be used on both the 810 and 814 tubes. Since the type 814 does not require neutralization, it is possible to simplify the circuit somewhat. The type 814 will operate under the following conditionsj Plate voltage ........ 1250. volts. Plate current 100. milliamperes. Plate input power ................. Estimated plate efficiency Plate output power ........ ............... 125. watts. 60. percent. 75. watts. The output is much more than required, but it will do no harm and may allow the efficiency of the 810 driver stage to be increased. Any increase in driver efficiency will allow delivery of more power to the final grids, resulting in increased efficiency from the power amplifier stage and greater power output. Since the 814 is a beam power tube, and as such, has very high power sensitivity, the grid drive required for 75 watts output is only about a watt. The 814 stage could be operated directly from the crystal stage, but, to keep the heating of the crystal as low as possible, it is usually advisable to run it at very low plate voltage and reduced power input. For this reason, and since another lower power stage adds little to the oost, it is best to include an additional stage between the oscillator and the 814 stage. Output from this additional stage is not important, so an 814 driver stage consisting of an RCA802 pentode tube is seleoted. This type, like the 814 tube, has high power sensitivity and requires less than a quarter watt driving power. type 802 can operate at a plate voltage of 400 volts, which same voltage will also be used on various other tubes. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The The 802 stage will operate under the following conditions* Plate voltage 400• volts. Screen voltage 200. volts. Plate current 30. mil H a m p e r es . Screen current 7. milliamperes. Plate power input 12. w a tts. Estimate;- efficiency 50. percent. Plate power output 6. watts. For the oscillator an voltage and with its power RCA802 tube isused, operating at low output limited to about two watts, which is more than enough to drive the 802 first buffer stage. Since the plate current is very low and nearly constant, it is better to use a series dropping resistor or voltage divider to lower the plate voltage, rather than use a separate power supply. The plate power may be obtained from the same source as the first buffer stage. The 802 oscillator will operate under the following conditions* Plate voltage ..................... 250. volts. Screen voltage ..................... 150. volts. Plate current Plate power input 20. milliamperes. ................... Oscillator plate efficiency Plate power output ....... 5. watts. 40. percent. 2. watts. The above lineup of tubes should give very satisfactory operation, and should be extremely stable, since a surplus of grid power is always available. Pentode and beam tubes are used wherever possible, so that only two stages need neutralizing. This makes for simpler construction, since single ended coils may be utilized, the voltage across the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. coils is less, and condensers with lower voltage ratings may he used. The audio portion of the transmitter will he considered in the same way as the radio frequency, that is, from the high power stages and progressing hack to the input. For the class "B" audio modulator stage, the amount of high quality audio power required is equal to one half the input power to the final power amplifier stage, for 100 percent modulation. In other words, an audio power equal to fifty percent of the radio frequency input power will completely modulate the radio frequency carrier so that the carrier envelope will vary from zero to twice the unmodulated carrier intensity. Then, since the input to the power amplifier stage is 36OO watts, an audio power output of 1800 watts will he necessary for 100 percent modulation. To obtain 1800 watts of audio power, and always remembering that two tubes must he used in class "B" for distortionless output, two 750TH tubes can he used. A glance at the tube chart shows that two 750TH's operating with JQOO volts on the plates, will give 2000 watts output. However, operation at 3000 volts would necessitate another power supply, since the radio frequency power amplifier operates at 1+000 volts. 4000 volts may he used on the class "B" stage also by a reduction in the peak plate current. An additional advantage is that with reduced current, higher quality is more easily obtained than if the tubes were operating near their peak output. At UOOO volts, the two tubes could supply up to 3000 watts without exceeding any of their ratings. The higher voltage also tends to simplify the matching of the tubes to their modulation transformer, since higher impedance circuits give better audio regulation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The 75 OTH tubes operate tuider the following conditions: Plate voltage . 4000. volts Plate current- maximum signal- . . . 750. milliamperes. Plate to plate load impedance . . . . 8000. ohms. Plate power input 3000. watts. Plate efficiency- maximum signalPlate power output 60. percent. 1800. watts. Maximum audio grid driving power for the modulator should he from five to seven percent of the plate input power to the modulator under conditions of maximum modulator output, hut under the present conditions, where the output is only ahout sixty percent of maximum of which the modulator is capable, four percent audio grid driving power is more than enough. This reasoning is h o m e out hy the tuhe manuals of the different manufacturers. To obtain 120 watts of distortionless audio, it is best to use class "A" or class nABn amplification, and there should he an excess of audio for best quality, or rather, the power available should he considerably more than the operating power. The most logical choice seems to he push-pull-parallel RCA845 tubes which are almost uni versally used for the audio driving stage in transmitters from 500 watts to five kilowatts. The four 8^5 tubes will operate in class HAB" under the following conditions: Plate voltage ..................... Zero signal plate current Maximum signal plate current Maximum signal power input 1250. volts. SO. milliamperes. .... ....... HOO. milliamperes. 500. watts. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Maximum signal efficiency 42. percent. Maximum signal power output.......... 210. watts. Since only 120 watts are required, and since distortion increases with an increase in plate input power and audio grid driving power, the four 845 tubes should supply very high quality audio, with little or no measurable distortion. Little driving power is requited for the 845 stage since the grids are driven into positive or grid current region only on maxinnxn peaks. A voltage amplifier stage will suit the purpose admirably. For this purpose, two SOJ tubes have been chosen, operating at a plate voltage of 400 volts. The 807 tubes are sturdy, have long life, are easily obtainable, and will put out a good amount of power if required. In addition, they have an available amplification of around a hundred, and require little audio grid driving power. They may be operated in class "A" with an output of 16 watts. In class "ABH they are capable of 80 watts output per pair. The push-pull SOJ stage will operate as follows: Plate voltage..................... Plate current 400. volts. .......... 120. milliamperes. Plate power input ................. 48. watts. Efficiency (class "A") 30. percent. Power o u t p u t ..................... 15. watts. The voltage amplification factor under the above conditions will approach fifty. At this point it is best to stop and see how much voltage amplifi cation will be required to bring the audio level up enough to drive the grids of the final audio stage to the required output. The usual transmitter is designed to allow 100 percent modulation with input to Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the audio channel at zero level, or a power of .006 watts across a 500 ohm line. The power required to drive the final audio stage is 120 watts, a level of plus 44.5 db., where dbs equal 10 x logqo (output power/input power). Then a total gain from the input to the grids of the 750TH tubes of 44.5 db. should be sufficient. Additional gain amounting to ten db. should be allowed if a regenerative feedback cirouit. is used, an addition much to be desired. Feedback is used to reduce distortion, hum and tube noises, as well as increasing the audio stability b y feeding a portion of the output signal back into the input stage or one of the intermediate amplifier stages in an out of phase relation with the original signal. If the feedback voltage is correctly applied it will be in opposition to the tube noise and distortion voltages generated within the tubes themselves, and will act to attenuate them. This may be more fully studied in several very fine articles appearing in periodicals. (4). Then allowing 10 db. for inverse feedback, and another 10 db. for losses in the circuits, it is found that there is a total gain of 44.5 plus 10 plus 10 db., or 64.5 db. Since the power gain is zero, the voltage gain through a transformer will be nearly equal to the ratio of the transformer. Then the power gain for each step is as followsi Class MB" driver transformer (4)". ........... minus 5 db. Bell, W. A., ^Inverse Feedback. pp. 47-49. (step down) Radio Magazine, Oct., 1938. Nalley, "Negative Feedback Applied to Class ’B ’ Audio. Radio Magazine, July, 1937. pp. 54-57. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8^5 driver stage . , plus 8^5 input network minus 2 db. (resistance loss), . 7 db. 807 voltage amplifier plus 34 db. 807 input network minus 2 db. (resistance loss). . total — plus 32 db Since we require a total of 64.5 db., 32»5 db. are still necessary for full output. By adding a push-pull stage using a pair of 1603 pentodes, it is possible to realize a gain of 100 or more or at least 4o db. voltage gain. In addition there is a voltage gain in the input transformer of around 10 db., so that there will be an excess of voltage gain. The 1603 tubes may be operated as follows: Plate voltage . . . . . . . . . . . . Plate current per t u b e ........... 25O. volts. 2. milliamperes. It is also possible, when necessary, to operate the two 1603 tubes to obtain a quarter watt or more output, at a sacrifice in voltage gain. While no claims are made for the superiority of this design over others, it is felt that it foims a nice balance between original cost, maintenance and operation cost, and reliability. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 75 watts OF 2.5 KILOWATT 8l4 louffer 2 watt; osc. 750TH final DIAGRAM 802 BLOCK 810 driver Audio innut zero db. 25 watt Sb-5 81+5 driver driver 750TH mod. 210 watts watts 807 voltage f amp. 8l+5 driver driver Figures above line show approximate output from preceding stage. A zero level of .006 watts is used. 1800 watts TRANSMITTER. r 1603 > voltage amp. 807 voltage amp. r "B" MODULATED f 1603 > voltage amp. CLASS Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. S10 driver TUBE CHARTS. (condensed, from RCA Technical Manual TT3). RCAS02 Characteristics: Heater voltage ..................... 6.3 volts. Heater current..................... 0.9 amperes. Grid-plate c a p a c i t y ............... 0.15<gnfd. Input capacity..................... 12.0«(mfd. Output c a p a c i t y ................... 8.5«(pifd. Maximum ratings and typical operating conditions: Plate v o l t a g e ..................... 500. volts. 250. volts. Screen voltage Suppresor voltage 40. volts. Plate c u r r e n t ....................... ^5. milliamperes. Driving power (approximate) ......... 0.25 watts. Power output (approximate maximum) . . 16.0 watts. RCA802 is a pentode transmitting tube of the heater-cathode type for use as an R. F. amplifier. The plate connection is Brought out through the top to maintain low grid-plate capacitance. Neutralization is usually unnecesary. The 802 tube is especially good as ah R. F. oscillator or amplifier. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. RCA805 Characteristics: Filament voltage 10.0 volts. Filament current ........... Grid-plate capacity .... 3.25 amperes. ............... 6.5^mfd. Input capacity ................... 8.5^nfd. Output capacity ................... 10.5%mfd. Maximum ratings: Plate voltage ................... 1500. volts. Plate c u r r e n t ............. 210. milliamperes. Plate dissipation 125. watts. ............... Typical operating conditions: Power output, class "B*1audio (2 tubes)370. watts. Power output, class "CM B. F. ... Power output, class "B" E. F. .... 215. watts. 57.5 watts. BCA805 is a high-mu, three electrode transmitting tube of the thoriated-tungsten filament type for use as a radio-frequency amplifier, oscillator and class "Bn audio-frequency amplifier. The plate connection is brought out through a separate seal atthe top of the tube. The grid is designed so that the amplification of the tube varies with the amplitude of the input signal. (condensed from BCA. Technical manual TT3). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. RCA807 Characteristics: Heater voltage..................... 6.3 volte. Heater current ..................... 0.9 amperes. Grid-plate capacity 0.2 mmf. ............... Input capacity ...................... 11.0mmf. Output c a p a c i t y ................... 7.0 mmf. Maximum ratings: Plate v o l t a g e 600. volts. Plate c u r r e n t 100.milliamperes. 25. vatts. Plate d i s s i p a t i o n Typical operating conditions: Plate voltage 400. 600. volts. Power output,class "AB" audio (2 tubes)60. 80. watts. Power output, class 9.0 12.5 watts. Power output, class "CH E. F ........... 15-0 (telephony) 25.0 watts. Power output, class nCHE. F ......... 25.0 (telegraphy) 37.5 watts. "B" R. F ......... BCAS07 is a heater type transmitting tube incorporating design principles involving the use of directed electron beams. This tube may be classed as a tetrode, although its operation is more nearly like the pentodes. The exceptionally high power sensitivity makes this tube excellent for use as an R. F. or A. F. amplifier, frequencymultiplier, oscillator and plate modulated amplifier. In R. F. applications, the 807 may usually be operated without neutralization. (condensed from RCA Technical Manual TT3) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. RGA810 Characteristics: filament voltage .................... 10.0 filament current................... 4.5 amperes. volts. .Amplificationfactor.................. 35. Grid-plate capacity ................ 4.Smmf. Input capacity ................... 8.7 mmf. Output capacity .................... 12.0 mmf. Maximum ratings: Plate voltage... .................. 2000. volts. Plate current 250. milliamperes. Plate dissipation 125. watts. Typical operating conditions: Plate voltage...................... 1500. 2000. volts. Power output, class nB n audio(2 tubes) 510. 590. watts. Power output, class "B" E. f ........ 60. 60. watts. Power output, classHC" fi. f ........ (telephony) 180. 250. watts. Power output, class "C" E. f ........ (telegraphy) 275. 375. watts. The BCAS10 is a high mu triode transmitting tube with high perveance, capable of high efficiency at low plate voltages and with comparatively small amounts of grid driving power. This tube has a heavy duty filament shielded at both ends, which tends to conserve filament input power, and eliminates bulb bombardment and stray electrons. (condensed from EGA. Technical manual TT3) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. KCASlU Characteristics: Filament voltage .................... 10.0 volts. Filament current................... 3*25 amperes. Grid-plate c a p a c i t y 0.1 mmf. Input capacity....................... 13.5 vsnf. Output c a p a c i t y ..................... 13.5 Maximum ratings: Plate v o l t a g e 1250. volts. Plate c u r r e n t 150. milliamperes. Plate dissipation 50. watts. Typical operating conditions: Plate voltage...................... 1000. volts 1250. volts. Power output, class "B" R. F ............. 20. 25. watts. Power output, class "C" R. F............. 70. jS . Power output, class "C" R. F........... 100. (telegraphy or buffer service) watts. 130. watts. RCA£>l4 is a filament type transmitting tetrode incorporating design involving the use of directed electron beams. Type Sl4 requires no neutralization when used in R. F. applications. The high power sensitivity makes this tube especially suitable for use as and R. F. amplifier, frequency multiplier and plate-modulated amplifier. (condensed from RCA. Technical Manual TT3.) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. BCAS45 Characteristics: Filament voltage ................... 10.0 volts. Filament current................... 3.25 amperes. Amplification factor ................ 5-3 Grid-plate c a p a c i t y ..................13.5 Input capacity...................... 6,0 mnf. Output c a p a c i t y .................... 6.5 nmf. Uaxinrum ratings: Plate voltage 1250. volts. Plate c u r r e n t Plate dissipation 120. milliamperes. . 75. watts. Typical operating conditions: Plate voltage 1000. Power output, class" A " Power output, class nAB"(two tubes). 21. . 75. 1250. volts. 2k. watts. 105. watts. ECA245 is a three electrode power amplifier tube of the thoriatedttingsten filament type. Especially suitable in class "A" audio service, (condensed from RCA Technical manual TT3) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. e c a i 6o 3 Characteristics: Heater voltage . ................. 6.3 volts. Heater current..................... 0.3 amperes. Maximum ratings: ^ Plate voltage 250. volts. Plate c u r r e n t ........................ 6.5 milliamperes. Screen voltage 100. volts. As class "A"amplifier, pentode connection: Plate v o l t a g e 250. volts. Screen voltage 100. volts. Plate current ....................... 2.0 milliamperes. ^Amplification factor 1500. or more. ECA1603 is a triple-grid tube of the heater-cathode type, especially suited as a voltage amplifier for audio frequencies. Voltage gains of several hundred times are obtainable, (condensed from ECA. Technical Manual TT3) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Amperex 220C Characteristics: Filament voltage ............. Filament current ............. Amplification factor . . . . . . Grid-plate capacity ... 35. ......... Input capacity ............... Output capacity ............. Maximum ratings: 15,000. volts. Plate voltage ................ Plate current ............... Plate dissipation ........... Typical operating conditions: Plate voltage Power output, ............... class"A" audio . .10,000. . . . 325* 7,500. volts. 250. watts. Power output, classHB" audio. . .15,000. (2 tubes) 9,000. watts. Power output, class"B" R. F. ... 2,500. 2,500. watts. Power output, class"C" R . F. (telephony) ... 5,000. 5>000. watts, Power output, class "C" R. F. (telegraphy) . . .10,000. 7,500. watts, Amperex 220C is a water-cooled tube widely used for audio and radio frequency applications in transmitters from 2 .5 to 5 kilowatts output under conditions of modulation. (condensed from Amperex tube chart #220C) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Amperex 22SA Characteristics: Filament voltage ......... 21, 5 volts. Filament c u r r e n t ................. Ul, 0 amperes. Amplification factor 17,0 ............. Grid-plate capacity ............... 23,4 mmf. Input c a p a c i t y ................... 15,0 mmf. Output capacity................... 3. 0 mmf. Maximum ratings: Plate voltage .................. 6,000. volts. Plate current .................. 1,500. milliamperes. Plate dissipation.............. 5,000 watts. Typical operating conditions: Plate voltage.................. 5,000 6.000, volts. Power output, class"B"audio (2 tubes) Power output, . . . 3,750 9.000. watts. class“B" R. F. . . . 1,000 watts. 2,500 watts. Power output, class "C" R. (telephony) F. . . . Power output, class "Cn E. F ............ (telegraphy) 5,100. watts. Amperex 228A is a water-cooled tube with a power output of 1 to 2.5 kilowatts for broadcast service. (condensed from Amperex tube chart #228A. ) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Amperex 892 Characteristics: Filament- two unit type for single-phase or two-phase operation. Filament voltage ................... 11.0 volts. Filament current..................... 60.0 amperes. Amplification factor ............... Grid-plate capacity 50. 32. mmf. Input c a p a c i t y .................... .17. mmf. Output capacity....................... 1.8 mmf. Maximum ratings: Plate voltage 15,000. volts. Plate current 2,000. milliamperes. Plate dissipation 10,000. watts. Typical operating conditions: Plate voltage ................. 6.000. 10,000. volts. Power output, class "B" audio . . . (two tubes) 8.000. 22,000. watts. Power output, class "B” £. F. . . . 1.000. 2 ,500. watts. Power output, class "C" E. F. . . . (telephony) 5,000. watts. Power output, class "CM E. F. . . . (telegraphy) 10,500. watts. Amperex and EGA. 892 tubes are similar in characteristics and ratings, water-cooled, and with a power output of 2 .5 to 5 kilowatts in broadcast service. (condensed from Amperex tube chart #892.) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Eimac 750TH Characteristics: Filament voltage ................... 7-5 volts. Filament current ................... 21.0 amperes. Amplification factor................. 30. G-rid-plate c a p a c i t y ............... 4.5 mmf. Input capacity ..................... 6.0 mmf. Output c a p a c i t y ........................ 8 mmf. Maximum ratings: Plate voltage 6,000. Plate current 1,000.milliamperes. Plate dissipation volts. 750. watts. 2,000. ^,000. Typical operating conditions: Plate voltage Power output, class"B" E. F .... volts. 35O. watts. Power output, class"C" £. F. (telephony) ... 750. 2,100. watts, Power output, class"C" E. F. (telegraphy) ... 1,000. 2,250. watts, Power output, class"B" audio (2 tubes) . . . 1,000. 3>000. watts. Eimac 750TH is an air-cooled vacuum triode with an output of 2 .5 kilowatts for two tubes in radio frequency broadcast service. The grid and plate are made of tantalum, allowing high temperatures to be attained by the elements with negligible release of gas. The type 75OTL is the same as the 750TH except for the amplification factor, which is 13 times for the 750TL. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Approved power ratings of vacuum tubes for operation in the last radio frequency stage of broadcast transmitters, abridged from the Federal Communications Commission charts. -High Level or Plate Modulation* Power rating (watts) Amperex ECA Eitel Federal McCullogh Telegraph 50 ------ 50T ------ 75 203A 838 ------ 100 ------ 125 Western Electric 814 211D F303A 203A 838 242A-B-C 100TH-TL F102A ------ ------ 805 150T ------ 250 2o 4a ------ F204A 204a 212D 350 849 250TH-TL F349A 849 270A 500 — 450TH-TL ------ 833 251A 750 851 500T F351A. 851 279A 1000 ------ 750TH-TL F346a 846 -- 2500 228A 750TH(2) 750TL(2) F3652A 1652 228A 5000 220C -- F307A F392A 207 891 892 220B-C 10000 -- -- F101B 858 F332A-B-C 232A-B 25000 -- 40000 ------ ------ ------ 50000 ------ ------ 100000 ------ ------ F117B 803 805 810 ------ — ------ 898 862 298A ------ — ------ ------ ------ 320A Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Power ratings for linear amplifiers. Power rating (watts) Amperex Eitel Federal McCullogh EGA. Western Electric 203A 25 50 HF200 100TH-TL — 803 2^2 C 75 204a -- F3o4a 20Ha 212D 125 ----- 250T U5OT F10QA 8U9 270A 250 Sks 750T F351A 85I 25U 500 -- -- F31+6a &ke 279A 1000 228A ----- F328A 1652 228A 2500 220C ----- F307A 892 220C 5000 ----- ----- F358A 858 ----- S500 ----- F110A ----- 232A 25,000 -- -- 862 898 298A ECA Weste Elect -- Power rating for grid-bias modulation. Power rating (watts) Amperex Eitel Federal McCullogh 50 212E 250TH-TL -- 2o 4a 212E 270A 100 -- ^50T -- -- -- 125 -- U50T -- -- -- 250 -- 750T -- -- -- 1000 -- — F328A -- 22SA 2500 S92A — F307A 892 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Bibliography Amperex Tube Manual, Amperex'Manufacturing Co., hew York City. Bell, W. A., "Inverse Feedback," Radio Magazine, Oct., 1938, pp. 47-49. Broadcasting Yearbook, Broadcasting Publishing Co., Mew York City, 1939. Doherty, W. H., "A Mew Linear Amplifier of High Efficiency, " Radio Magazine, June, 1936, pp. 18-21. (condensed from Proceedings of Institute of Radio Engineers) Eitel McCullogh Tube Charts, Eitel McCullogh, Inc., San Bruno, Calif. Electronics Engineering Manual, Editorial staff, Electronics Magazine. McGraw Hill Publishing Co., 1938. Hawkins, J. M. A., "Care of Transmitting Tubes," Radio Magazine, Feb., 1936, pp. 66-84. Hawkins, J. M. A., "The Class C Amplifier," Radio Magazine, Feb., 1936, pp. 58-82. Long, J. J., "Porcelain Cooling System at WHAM," Electronics Magazine, Oct., 1938, pp. 24-25. Marsden, C. P., "Thermionic Snission," Electronics Magazine, Dec., 1938, pp. 22-32. national Association of Broadcasters Engineering Manual, .National Association of Broadcasters, Vfashington, D. C., 1938. Radio Amateur*s Handbook, American Radio Relay League, West Hartford, Conn., 1938, 1939. RCA Tube Manuals, RCA Manufacturing Co., Camden, N.J. (released at irregular intervals) Smith, W. W., The Radio Handbook, Radio, Ltd., Los Angeles, Calif., 1938. Sterling, George E., The Radio Manual, D. Van Nostrand Publishing Co., New York City, 1939"! (third edition) Terman, F. E. Radio Engineering, McGraw Hill Publishing Co., New York 1932. "Tubes, Inc.," Editorial staff, Electronics Magazine, Nov., 1938, pp. 13-15. Tubes, Federal Telegraph Co., Newark, N. J., 1938. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.