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Valves Were Expensive in the Early Days of Radio and So Designers

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Valves Were Expensive in the Early Days of Radio and So Designers By RODNEY CHAMPNESS, VK3UG would become gassy. Occasionally, even today, a valve with a purple glow inside it will be seen and this is often an indication that the glass to metal pin seal is not perfect and air has leaked VICTORIA into the valve. Incandescent light globes were the first items to have metal pins or wires protruding through a glass envelope. However, this created no real problem, since the vacuum created was satisfac- tory for their operation and the glass- to-metal seals were not as critical. In some cases, the globe was filled with an inert gas such as nitrogen to prevent Above: the Kriesler 11-41 was a popular evaporation of the filament. 4-valve reflex receiver from the 1950s. One problem with valves was that the metals used inside them (ie, for the elements and filaments) had to Valves were expensive in the early be carefully selected, otherwise they days of radio and so designers came up could emit gases when they became hot. These gases could then "poison" with clever techniques to minimise the a valve and adversely affect its per- valve count. One technique was known formance. So early attempts at making valves as "reflexing" and involved using the into viable amplifying devices encoun- same valve to work as both an RF or IF tered many difficulties. However, their potential to revolutionise radio was amplifier and as an audio amplifier. obvious and so a great deal of effort was put into solving these problems. It is for these and other reasons that valves were by far the most expensive nOMPONENTS such as tuning ca- had a small amount of gas left inside, and fragile components in early valve pacitors, inductors (both fixed and due to manufacturing limitations. As a receivers. As a result, the engineers variable), resistors and fixed capaci- result, these valves were rather varia- and experimenters of that era searched tors were in common use during the ble in their performance, even between for ways in which each valve could Spark Era, at the start of last century. supposedly identical types. be made to do more than one job, to However, valves (when they finally Another problem that had to be minimise cost. made an appearance) were initially overcome was how to maintain a extremely expensive and fragile. Suf- good vacuum. This could only be Reflexing fice to say, they didn't have a long life. achieved if the glass and metal leads One of the first to achieve dual Initially, obtaining a high vacuum through the glass envelope had the usage of valves was W. H. Priess, a inside a valve was quite difficult and same coefficient of expansion. If the US Navy worker who patented the most of the early valves were of the coefficient was different, air would principle of reflexing during WWI. "soft" variety. This meant that they eventually leak into the valve and it This technique involves passing a 94 SILICON CHIP siliconchip.com.au signal through the same amplifier twice, at two significantly different V2: 6AD8 6AN7 frequencies — usually once at a radio KATE or intermediate frequency (ie, RF or IF) Fri and then at audio frequency (ie, after 455kHz the modulated signal from the IF stage has been detected). Initially, there was little interest in reflexing for a couple of reasons. First, it was no doubt kept a military secret 1 Onf B TO GRID during the war and so was not widely iHnx..„.. Of 6M5 AUDIO known during those years. Second, the OU TPUT principle, although sound, initially proved tricky to implement. However, valves were still very expensive in the 1920s and this led some manufactur- VOLUME 500k ers and enthusiastic experimenters to FROM refine the technique and this eventu- 3900 SPKR 'OM • VOICE ally yielded good results COIL In theory, reflexing meant that one AGC TO INFB) valve did the job of two. However, as 6AN7 4 there was always some compromise in the operating conditions of the valves Fig.1: the IF (intermediate frequency) amplifier, detector and reflexed audio for each different frequency, the actual stage of the Kriesler 11-41 & 11-49 radio receivers. The detected audio improvement was always somewhat signal at pins 7 & 8 of the 6AD8 are fed back to the grid via the primary less that this. of IFT2, a 47k0 resistor, the volume control, a 4.7nF capacitor and the Reflexing was used for only a rela- secondary of IFT1. tively short time overseas but in Aus- tralia, it was still being used in some receivers as late as the early 1950s. It audio output from the detector was successful and an early example is the was initially used in high-end receiv- then fed back to the input of the RF am- Radiolette model 31/32. ers in the early 1930s, then in receiv- plifier valve. From there, the resulting There were others, as I quickly ers at the bottom end of the market to amplified audio signal was typically discovered when I looked through reduce valve count (and thus cost). applied to an audio output stage. the 1938 edition of the Australian Of- This is not to say that reflex receivers In a well-designed receiver, this ficial Radio Service Manual (AORSM). performed poorly because they were system could be made to work quite There were at least eight mainstream aimed at the bottom end of the mar- well. However, most of these early manufacturers that had at least one re- ket. Some sets did leave something to TRF sets were built breadboard style, flex model: Aristocrat, Astor, Croyden, be desired but others were very good so layout could be (and usually was) HMV, Hotpoint-Bandmaster, National, receivers. quite critical. Unfortunately, due to Fisk-Radiola and Westinghouse. In incorrect wiring techniques and the fact, Fisk-Radiola and its badged sta- How reflexing works inevitable stability problems that fol- blemate Hotpoint-Bandmaster lead The mere thought of restoring a lowed, many people soon came to the the way with quite a few models, both radio receiver with a reflex stage has conclusion that reflex circuits were mains and battery operated. sent cold shivers down the backs of "cranky" and best left alone. many enthusiasts. As a result, such That wasn't to be the end of reflex Kriesler 11-41/11-49 receiver sets have either been shunned or used receivers, however. When superhet The final volume of the AORSM has simply as non-working show items. receivers were subsequently devel- no manufacturers with reflex receiv- I must confess that during my early oped in the 1930s, the breadboard ers. In fact, the last reflex receiver days as a serviceman I wasn't all that style of construction was quickly featured in the AORSM is the Kriesler keen on dealing with reflex sets. abandoned. Instead, the parts were 11-41/11-49 (in Volume 12, 1953), so There were just so many wires and mounted on a metal chassis, with the Kriesler appears to be the last manu- components going here, there and critical components shielded. This facturer of these sets in Australia. everywhere and a real mix of signal was necessary to ensure stability and The Kriesler 11-41 & 11-49 were leads. However, most of the reluc- consistent performance and this style 4-valve mains-operated mantel receiv- tance to service reflex sets (as with of construction quickly became the ers, the two models being almost iden- AGC stages) was due to the fact that standard technique for Australian tical. So let's take a closer look at the servicemen lacked the test equipment manufacturers. circuit to see how reflexing worked. that we have today. As a result, some manufacturers Fig.1 shows the IF (intermediate Originally, reflexed stages were in- decided to see if more stable high- frequency) amplifier, detector and cluded in tuned radio frequency (TRF) performance reflex receivers could be reflexed audio circuitry stages of these receivers. These sets typically had an developed using metal chassis and im- models. The front-end uses a 6AN7 RF stage followed by a detector. The proved shielding. Their efforts proved converter valve and this feeds a signal siliconchip.com.au JULY 2010 95 speaker's voice coil to the bottom of 6G8G +50V the volume control, to improve audio RF INPUT FROM quality. ANTENNA TUNED CIRCUIT 100PF Full AGC It's not all that common to see the 72 1.75M 500k full AGC voltage applied to a reflexed 1 IF/audio stage but Kriesler has done 50nF 10nF AUDIO TO I 104 11-0- 6V6GT this here. In this circuit, AGC is taken 101-11 OUTPUT from the top of the 47kO resistor (at the I I 70k top of the volume control) and applied via two 1MS2 resistors to the grid of the +30V 200k 6AD8 reflex stage. Because its plate YM +170V HT load resistor is only 15ka the 6AD8 look does not have high audio gain — only about 10-12 times. As a result, the IF amplifier con- -2 TO -22V BIAS & VOLUME ditions are not far from normal and CONTROL LINE the stage is not likely to overload on Fig.2: the reflexed circuitry in the Astor GR (Football) TRF receiver. In this strong signals. By contrast, Astor reflex circuit, the detected signal on pM 4 of the 6G8G is fed back to the grid via receivers often used a 70k0 plate load a 5001d2 resistor, a 10nF capacitor and a 1.75M0 resistor. A variable bias resistor in the reflex stage. (-2V to -22V) line controls the volume (the lower the bias, the greater the gain).
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