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Some Popular Output Triodes — 1 a Special Group of Valves Has Always Been Held in High Regard by High Quality Audio Enthusiasts

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Some Popular Output Triodes — 1 a Special Group of Valves Has Always Been Held in High Regard by High Quality Audio Enthusiasts Some popular output triodes — 1 A special group of valves has always been held in high regard by high quality audio enthusiasts. These are the big output triodes, some of which are still in production. This month we look at the first generation of these valves. Before 1923 and the advent of broad- tories, was and still is a remarkable or- was estimated that replacement of a casting, the concept of fidelity of audio ganisation. Their prime purpose was to valve in a submarine repeater could cost reproduction hardly existed. Head- provide the vast Bell Telephone network $40,000. Obviously, low initial cost was phones were in universal use, and they with its equipment. As well, they are in- not significant if it compromised quality were judged mainly by their sensitivity. volved in outside commercial undertak- and reliability! Transmissions were practically all in ings. In Britain and Australasia, they are It is on record that every one of the 306 Morse code, and the prime requirement represented by their affiliates Standard Western Electric type 175HQ valves was audibility. Often receiver audio Telephones and Cables (now Alcatel used for the first trans-Atlantic telephone transformers were tuned to around 1kHz, Australia), an organisation I imagine cable operated continuously for 22 years, to improve intelligibility of telegraph with which most readers are familiar. without a single failure. The later type signals. In 1920, the only service where 455A-F valves were equally reliable. By audio quality had any meaning was not Reliability paramount 1978 there were 5674 of these valves in in radio, but the rapidly expanding tele- Not being concerned so much with use, each with an average continuous phone repeater technology. high volume, low initial cost equipment service of nearly 15 years. There had America's Western Electric Bell Labo- as with extreme reliability, Western been only two probable failures and in ratories had taken Lee de Forest's erratic Electric's philosophy for valves has al- any event, redundancy of amplifier de- Auction triode and by 1915 had tamed it, ways been very different from the do- sign meant that service was not affected. to become a practical commercial device mestic consumer market. Even premium semiconductors would capable of factory production to close To put this into perspective, in 1978 it find it hard to beat that sort of record. standards. In Europe valve development This then is the background to the type was accelerated by wartime demands, of organisation that in 1924 produced the one very significant result being the fa- 205D. I guess that few readers have han- mous `R' valve. dled a 205D, but as we shall see, many will The advent of broadcasting soon cre- have encountered the 205D in operation. ated a demand for loudspeakers and, to drive them, valves capable of more than Favoured by hams the few milliwatts required by head- Meanwhile, American General Elec- phones. In Britain, Marconi-Osram had tric and Westinghouse were developing developed from the R valve their type valves and equipment for RCA. As we 25, a large valve used by the British Post related in this column for May 1990, the Office for telephone service. From the production of the first practical moving type 25, Marconi-Osram produced a coil loudspeaker depended on the avail- family of large triodes, their 'LS' series ability of an audio amplifier capable of for loudspeaker work. The LS5A, intro- producing a genuine one watt. This be- duced in 1924, was used by the newly came possible when, in 1924, RCA's established BBC and became popular UV210, essentially a scaled up UV201 A with affluent amateurs interested in im- valve, became available. The following proving sound quality. year it received the new long-pin base, to Meanwhile Western Electric had taken become the UX210 and proved to be one of their wartime valves, the VT2 and popular for audio amplifiers and espe- in 1917 further refined it to become the Fig.1: Although the spherical Western cially with radio amateurs as an oscilla- 205A, a `miniature' transmitting valve, Electric 205S and the S-bulb RCA tor and RF amplifier. capable of handling 300 anode volts. The UX210 were very different in Transmitting versions, the 10Y and 205 series underwent several develop- construction, their characteristics 801 were produced. Only with the ad- ments, and by 1924 the 205D was in full were comparable. Globular envelopes vent of the equally ubiquitous 807, im- production. were a distinctive feature of many WE mediately prior to World War II was the Western Electric with its research or- valves, which had oxide-coated UX210 superseded. ganisation, the Bell Telephone Labora- filaments or cathodes. At this stage I would remind readers 98 ELECTRONICS Australia, September 1993 JTUSteps 233-J 239-,4 21Cb 52,500") 239-A 216.6, 233-H 205-0 127-0 T' put Trans. Output Trans. State of the art audio in 2, • 1 Ilzf 38S _ _ &f i, Input Trans. ► -1 7 ?'s the late 1920's: Western C2 2 1. I s 'i Co l' 3&, •1 Electric 46A amplifiers 37,600 R 38 using 205D triodes as cö 2 ck a ,T R1~ ~ 3 21240' rectifiers, as well as out- 38P , put valves, were used in 2400' thousands of cinemas 4 S 388 51onf /5000 ,' 2 for early 'talkie' sound in- '38C 205-D 2, f 2-21-Cb stallations. As WE had 24;000") not developed an indi- ' 3 38N 3 3 3 rectly heated valve, low ö~ aQ 575"/ microphony type 239A V-3' /GG and later the 264 series with 1.5W300mA fila- R11 R12 R13 R14 ments were used for the R15 R16 R17 OS- voltage amplifiers. The 20WRheostaf V.7: 46A could produce only 3 3T M 0-300 Milamp. 4~ S ST about four watts, but this Gnd-12 V Ks-6/82 Metet QQO4 4, 'Oa k° 303-8 303-C Trans: was adequate for horn- Trans. o V .2 loaded speaker drivers 4-12V. 4 such as the WE555. Ks-6224 /1L~ V A. C. I1 Dp,~ Door Switcb F -Two 3-amp. 8ryanl Plug Fuses that the original RCA numbering system for valves was clumsy. The prefix UV, UX and UY referred to the base, and the first digit identified the supplier 1 for Arcturus, 2 for RCA and 3 for Cunning- ham etc. Only the second and third digits had any real significance, and before long the rest was dropped. Thus the UX210 was known as the 10, the UX250 as the 50 and the UX245 became the 45. This abbreviation did not apply to West- ern Electric valves, which generally had their own system of identification. Two major developments influenced high quality sound research in the mid 1920's. These were electrical gramo- phone recording and sound movies, both of which set new standards in reproduc- tion and sound generation. Both the Western Electric and RCA organisations were major players, and good use was made of both the 205D and the 210. 50 years of service Western Electric designed new ampli- fiers, including the 42A and 46A, for the Fig 2: For the best part of three years, `talkies'. Both used a push-pull pair of the UX245 practically monopolised 205D valvesa es in the final stage, and were American receiver output stages. capable of producing three to four watts Fig.3: With the largest bulb ever used of audio power. Used with WE 555 horn- for American non-transmitting valves, loaded speaker drivers, this was suffi- and construction that some were still in the impressive UX250 was used in cient power for small movie theatres. But service in the early 1980's! There can some `top of the line' receivers. for larger auditoriums, additional power be few other types of electronic equip- was provided by the type 43A amplifier, ment with continuous service spanning beginning to be widely used, in many which used a push-pull pair of 211 trans- more than half a century, and there cases installed in console cabinets with mitting valves coasting at a modest 12 must be few readers who have not at their improved baffling. More audio watts output! some time attended a WE-equipped power could be used, and in February Many thousands of the 42A and 46A theatre and heard one of these amplifi- RCA announced the Westinghouse de- amplifiers were in use world wide, and ers in operation. veloped UX250. With the biggest enve- such was the quality of their design By 1928, moving coil speakers were lope ever used on an American ELECTRONICS Australia, September 1993 99 VINTAGE RADIO VALVE CHARACTERISTICS non-transmitting valve, the mighty 50 VALVE AMPL ANODE MUTUAL ANODE ANODE POWER could develop nearly three times the TYPE FCTR RESIS CONDUCT VOLTS mA WATTS power output of the 10. Some were used in prestige receivers, but the 50 really belonged in public address and theatre LS5A 2.5 2750 1.0 400 33 2.6 sound systems. 205D 7.3 3800 1.9 350 30 1.8 For some years, the 50 was unrivalled 10 8.0 5000 1.6 425 18 1.6 in sound systems, but it was really too 50 3.8 1800 2.1 450 55 4.6 large and over-powered for most domes- 45 3.5 1700 2.0 275 35 2.0 tic service. A much more suitable valve appeared in 1929. In many respects the The characteristics of the valves described, in typical single-ended operation. junior brother of the 50, the UX245 was Two in push-pull would produce something more than twice the power output.
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