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1992-01: Vintage Radio Receiver Design

1992-01: Vintage Radio Receiver Design

When I Think Back...

by Neville Williams

Vintage design - 6: Pentagrid converters, detectors and AGC

4 Once /5-valve superhets, as described in the November issue, had identified and established the prime suburban receiver market, manufacturers sought to devise ways and means of attracting buyer interest to their respective products. Some such measures were mainly cosmetic in the way of cabinetware and controls; others had to do with on-going circuit design and performance.

As indicated in the November article, converter also employed a comparable coil L2 via the usual grid and the single most troublesome aspect of the concentric structure. But in this grid leak. Note that the latter returned 4 first wave of /5-valve superhets was case there were five grids between directly to , so that the only bias probably that of gain — or volume — cathode and — so arranged that would be that resulting from the oscil- control. It came about because the IF they could perform the dual function latory grid current — typically between stage was the only one avail- more flexibly than the existing 0.2 and 0.5mA. able for, gain control, and the range of ad- concept. Grid 2 served as the oscillator anode, justment was simply not sufficient to Fig.1, from an early RCA Receiving and connected through the oscillator embrace both maximum gain for weak Tube Manual, depicts the electrode struc- feedback winding L3 to an HT supply signals and minimum gain for powerful ture and the pin connections of the voltage in the range 100-200V. local stations. To make good the short- original 2A7 (applying also for the 6A7). As I recall from my days in the AW fall, it proved necessary also to attenuate Fig.2, from the same manual, shows Valve Co, grid 2, often described as the the input signal for local stations RCA's typical circuit arrangement 'anode-grid', was a grid in name only, and this led to difficulties, as outlined in with the diagram of Fig.1 adding to the the earlier issue. Oscillator and mixer fiction. In practice, it was nothing more Smoother and more effective gain con- Grid 1, adjacent to the cathode, served than two bare side-rods, with no spiral trol could conceivably have been as the oscillator grid and connected to grid, as such. The rods were simply held achieved by using a variable-mu valve as the active end of the tuned oscillator in place by the mica electrode support a mixer, in conjunction with a separate discs, connected together and wired to oscillator valve. It would then have been the relevant base pin. possible, with one bias control poten- However, being relatively close to grid tiometer, to vary the signal conversion — 1 and cathode, and operating at 100V DC or translation — gain of the mixer, along or more, the anode-grid (or side rods) with the normal stage gain of the IF would typically draw around four mil- amplifier. The catch was that it would lamps, completing an inner that have transformed the receiver into a 5/6- was well able to oscillate in its own right valve set, with a consequent and unac- in conjunction with the associated tuned ceptable price increase. circuit formed by L3, L2 and C. It was left to the valve manufacturers Enclosing the inner triode — to solve the problem, by the release of cathode/G1/G2 — was a screen grid special frequency-changer or frequency designated as G3. Operating typically at `converter' valves which could perform 100V DC and bypassed to earth with an the functions of oscillator and mixer 0.1uF capacitor, it provided an electros- more or less independently. For the tatic shield around the inner Australian radio scene, the most notable and also accelerated towards the anode such valve was the American designed proper those sectors of the beam 2A7 — which was that were not being attracted to the succeeded, in due course, by its 6.3V anode-grid side rods. counterpart the 6A7, and its octal- Immediately beyond this screen was based equivalents the 6A8, 6A8-G and the 'signal grid' G4, connected to the sig- 6A8-GT. STC's model 504E mantel radio of 1939 nal input tuned circuit Ll/C. Beyond this As a logical derivative of existing was fairly typical of sets using the again was another screen grid, 05. Con- and , the pentagrid 6A8-4 a later version of the 2A7/6A7. nected internally to G3, this served the

56 ELECTRONICS Australia, January 1992 same purpose as the screen grid in an from the Philips 'octode', the American/- 1930's was commonly run through steel RF or , by reducing the Australian made 2A7/6A7/-6A8 series conduit, which was subject to erratic ear- direct capacitance between signal grid reigned supreme in Australian thing by reason of rust and expan- and anode. receivers until the emergence of sion/contraction effects with variations in multiband receivers called for an up- ambient temperature. Given that Frequency conversion graded converter with better performance receivers were often plugged into lamp In normal operation, the wanted input at the higher frequencies. But that is sockets via 2-way adaptors, extension signal would be fed to G4, being im- another story. leads and/or bodgie power points, it pressed on the anode current much as it added up to a very unstable environment would in an ordinary tetrode or pentode Erratic sound level for incoming radio signals. mixer/amplifier. In the pentagrid struc- Adequate gain control per medium of ture, however, the electron stream had al- variable bias opened the way to the solu- ready passed through 01 and thus been tion of another annoying problem in the While the immediate answer might modulated with the oscillator signal, early 1930's, namely a tendency for the have been installation of a new power deliberately tuned above the signal fre- volume level of receivers to vary spon- circuit and/or a better antenna and earth, quency by (typically) 455kHz. taneously and erratically. Having been an attractive proposition for manufac- Intermodulation — or heterodyne ef- set for comfortable listening, the volume turers was the incorporation of so-called fects — took place such that a multi- level, for no apparent mason, would sud- `AVC' (automatic volume control) whieh plicity of signal components appeared in denly become uncomfortably loud or would hopefully counteract changes in the anode current, including the original drop to a whisper — a situation which signal strength with an automatic and signal and oscillator frequencies plus resulted in numerous complaints and/or complementary readjustment of the direct harmonics of each and resultants at service calls. receiver gain. a variety of sum and differeme frequen- In a few cases the problem turned out It may be helpful to note here that, in cies. Most were rejected by the IF to be a faulty valve, a loose clip on the recent years, technical writers have amplifier system, which was pre-tuned to voltage divider, an intermittent cathode preferred the term AGC (automatic gain the intended intermediate or 'difference' control) to AVC. Not only it is more ac- frequency nominally 455kHz. curate, but it is also more appropriate If this sounds very like what was said what the technique is applied to video about the autodyne or other equipment where the informa- in previous issues, it is, but with one tion being processed is something other vital difference: in the autodyne, the than sound waves. same was directly involved AVC/AGC was not a new idea, having in both functions — oscillator and mixer. already been featured in up-market If a variable negative bias was placed receivers — as, for example, a 9-valve on the grid to reduce the conversion set manufactured in Sydney by Airzone gain of the mixer, it would ultimately for Palings and marketed, by arrange- interrupt the oscillator, rendering the ment, under the Victor label. receiver inoperative. The technique involved the use of a In the case of the pentagrid converter, diode detector, so wired that it would the inner triode was substantially unaf- deliver a demodulated audio signal plus a fected by what was happening in the negative DC voltage proportional to the outer mixer section, so that the receiver strength of the incoming carrier. By ap- designer was free to manipulate conver- Flg.1: Pin connections for the 2A7 plying the negative voltage to the vari- sion or 'translation' gain by applying a pentagrid converter, as viessvd from able-mu stages in lieu of a manually control bias to the signal grid G4. Valve the underside. The cathode, G1 and G2 controlled bias, the front-end gain of the designers made the best of the facility by provide the basic triode oscillator. receiver would diminish automatically givhig G4 a remote cut-off characteristic, with increasing signal strength — and comparable to that of contemporary vari- bypass, or such like. More commonly, no vice versa. able-mu RF pentodes. With increasing fault would be found and, back on the In short, it could obviate front-end bias, the translation gain of the 2A7 fell service bench, the set would perform per- overload by powerful local signals, ftom 520uS (microsiemens, or 11A/V, fectly. In such a case, attention would counteract the effect of abrupt changes in formerly called `mictomhos') at -3V to a focus on the electrical environment in signal strength and, by way of a bonus, mere 2uS at -45V. which the set was being operated. compensate to some extent for night-time Receiver designers breathed a sigh of As distinct from country areas, few fading from distant transmitters. With the relief when the 2A7 became available, receivers in urban homes were provided signal level from the detector thus regu- abandoning the autodyne at the first op- with a regular antenna and earth. There lated, the function of the manual volume portunity, along with local/distant would be no earth, as such, and the an- control knob was simply to adjust the or compound gain control cir- tenna would be a few metres of 'bell sound from the audio system to the ie- cuits. Once again adequate control could ' tacked to the picture rail. In these quired level. be achieved simply by varying simul- circumstances, the amount of signal fed Ironically, while the first-ever ther- taneously the bias of two stages: the through the primary of the antenna coil mionic valve had been a diode, the only mixer and the IF amplifier. could be affected by the household versions readily available around 1930 The success of the American-designed electrical wiring and what lights and ap- were power supply . Small-sig- pentagrid' converter prompted European pliances happened to be switched on or nal detector suitable for use in valve manufacturers to produce their off at any given time. mains receivers were virtually unob- own frequency converters. But apart More subtly, house wiring in the early tainable. As a result, designers of

ELECTRONICS Australia, January 1992 57 There was an urgent need for a high- The subjective effect was due purely to WHEN I THINK BACK gain triode, which valve manufacturers the interaction of AGC with the tuning subsequently met with the 2A6, followed routine. In the case of manual gain con- receivers such as the Victor, mentioned in order by its 6.3V equivalents the 75 trol, detuning the receiver to either side above, resorted to the use of like and the octal-based 6B6-G. With an of resonance caused the sound volume to the 27 or 56, with the grid serving as the amplification factor of 100, these offered fall away at the same decibel rate as the diode anode. The anode was simply ear- a stage gain as a resistance-coupled slope of the selectivity curve. But with thed, serving only as an impromptu amplifier of around 56, which just about AGC, detuning the receiver reduced the shield around the diode elements. restored the status quo. strength of the incoming carrier — yet at In an up-market receiver, an extra I remember with lingering dismay the the same time the receiver gain was auto- valve provided just another reason for the first prototype we cobbled together at matically increased, thereby partially of- higher price. But at a competitive budget Reliance Radio of a 4/5-valve superhet fsetting the loss of sound volume. level, an extra valve wired as a diode was with AGC. Based on an existing model To the user, the set appeared to be less no more acceptable than the same valve with a pentagrid converter and routine selective. In fact, it may not have been so serving as a separate oscillator. The prob- coils and IF , the third socket because, when an adjacent signal was en- lem, in short, was to translate AGC into 4 was rewired to accommodate a duo- countered, the consequent reduction in mass-produced /5-valve superhets — diode-triode instead of the anode-bend gain could well be sufficient to render the without adding to the cost. detector. An AGC circuit replaced the original signal inaudible. variable cathode bias system, and an But real or subjective, possible con- Duo-diode triodes audio volume control was inserted be- sumer dissatisfaction caused manufac- Once again, valve manufacturers came tween the detector output and the turers to take a long, hard look at IF to the rescue, aided by the fact that detec- triode grid. channel design before committing them- tor diodes could be very small — by selves to diode detectors and AGC. The reason of the relatively low voltage and Selectivity problem immediate result was that IF trans- current that they were required to handle. The receiver certainly worked smooth- formers wound with multi-strand By fitting an otherwise ordinary valve ly enough, but gave the impression of Clitzendrahtl wire became a necessity with a slightly shorter grid/plate assemb- being atrociously broad in terms of selec- rather then an option. ly and a slightly longer cathode, enough tivity — with stations seeming to overlap Instead of single-strand wire, the wind- of the cathode could be exposed to serve one into the other. We all agreed that, ings were wound from so-called `litz' one or two tiny circular or semicircular even if such a receiver eliminated com- wire comprising (typically) seven or , accessed through extra base pins. plaints about erratic changes in volume more strands of 41 B&S enamelled wire, The first such valve to become readily level, there would be at least as many spun together to form a single silk- available in Australia was the 55, a other complaints to do with poor ap- covered conductor. Because high fre- general-purpose triode with a 2.5V parent selectivity. quency currents tend to flow on the sur- heater, a 6-pin base, top-cap grid connec- It transpired that the problem was the face of conductors, litz wire exhibits a tion and two small-signal diodes suitable result of two effects — one real and the lower RF resistance than a single wire of for detection and automatic volume other subjective. The reality was that, the same overall dimension, yielding a (gain) control. whereas an anode-bend detector winding with a significantly higher 'Q'. While it made possible a 4/5-valve su- responded purely to the voltage across (This assumes, by the way, that perhet with AGC, the 55 proved a disap- the associated input circuit, diodes were the strands are all tinned and pointment for another reason: with an power operated, responding to the signal soldered together at each end of, the amplification factor of 8.3, it offered a input voltage but at the same time draw- winding. Fractured strands reduce wind- stage gain, as a resistance coupled ing current from the source. In effect, a ing efficiency). amplifier of just under six times. As a diode detector shunted the input circuit While this was not the end of the story, detector/amplifier, this would have been with a resistance about half that of the as- the use of litz wire for IF transformers roughly a tenth that of a 57 as an anode sociated diode load. The end result was and the secondary of the antenna coil bend detector — resulting in a serious an immediate loss of both gain and selec- showed the way to more practical loss of receiver sensitivity. tivity in the associated IF . designs. The art of tuning TYPICAL PENTAGRID CONVERTER CIRCUIT TYPES 2A7 i 6A7 Even so, consumers had to become ac- C = GANGED TUNING CONDENSER customed to receivers equipped with (40 TO 350 Kir) CI,C2,C5,Ce,G7 = 0.1 AGC. Instead of just tuning for the Cs= 0.00025 tlf loudest signal, they had also to learn to c48 see TABLE BELOW RI 250 000 OHMS, 0.1 WATT tune for the 'deepest' sound, with good R2s 10000-50 000 OHMS, 0.1 WATT bass and an absence of carrier 'swish' R3= OSCILLATOR-ANODE (GRID N112) and/or sibilants on speech. It became al- VOLTAGE -DROPPING Roe 150-300 OHMS, 0.1 WATT most routine to walk into a house and Rye SCREEN (GRIDS NI 311.5)FILTER RESISTOR hear an 'edgy' voice or distorted I = 60-MILLIHENRY R-F CHOKE music emitting from the new radio — T = 4854- I-F TRANSFORMER GRID N!4 GRID N12 GRIDS NS 355 PLATE a clear indication that it had not been 1145 SUPPLY SUPPLY SUPPLY SUPPLY correctly tuned. One answer to the problem was the Flg.2: RCA's typical circuit for the 247 pentagrki converter, from an early RCA provision of a small back-lit tuning receiving tube manual. The oscillator circuit (bottom), the RF signal Input circuit meter, visible through a cut-out in the (left) and the IF output circuit (right) can be readily identified. dial or cabinet front. With a full-scale

58 ELECTRONICS Australia, January 1992 sensitivity of about 10mA, it would nor- mally be wired into the anode or cathode FLUORESCENT circuit of the IF amplifier. Under no-sig- COATING nal conditions, the meter would read full scale. When tuned to a station, AGC RAY -CONTROL would reduce the anode or cathode cur- ELECTRODE rent and the pointer would swing back towards its rest position. On the scale be- hind the pointer was an arrow and the words 'Tune for the greatest swing'. It was a useful fitment, but one that be- cause of its cost was largely confined to up-market models. Rather than becoming involved with a mechanical tuning meter, some manufac- turers released receivers with tuning in- Fig.S: The electrode structure of the 6E5, taken from RCA literature. It was the dicators contrived from low-current first electron ray Indicator to be widely adopted in Australia, and was followed by filament lamps or neon devices — none quite a few variants released by RCA and other manufacturers. of them all that impressive. Once again valve manufacturers came anode current, voltage drop across the pentagrid converter, with automatic up with a practical answer, in the form of anode resistor 'R' would result in a rela- gain control. an 'electron ray' tuning indicator, sub- tively low voltage on the anode and the • IF amplifier 6D6 (61(7, 6U7-G) vari- sequently dubbed a 'magic eye'. The first ray control electrode. able-mu pentode also with automatic of these, the 6E5, was released in Under these conditions, the ray control gain control. Australia around 1935, by which time electrode would repel the adjacent • Detector/amplifier: 75 (6Q7, 6B6-G) most manufacturers had swung over to electrode stream, creating a triangular duo-diode high-mu triode providing 63V valves. I understand that a 2.5V 'shadow' extending on either side by diode detection, delayed AGC feed version was also released, but I cannot about ±45°. With the cathode and ray voltage and audio voltage amplifica- recall ever having encountered one. control electrode hidden by a small inter- tion, with provision in some cases for European manufacturers came out with nal shield, the user was aware only of a phono input. their own configurations and type num- conical electrode, glowing bright green • Output valve: 42 (6F6, 6F6-G) pen- bers, which appeared on the local market except for a 90° triangular shadow. tode, with treble limiting and, in most in limited numbers. Tuning the receiver to a station would cases, top-cut tone control. generate a negative voltage on the AGC • . 80 (5Y3-G) with field coil How the 6E5 worked line, therefore on the indicator valve grid. filter system. As illustrated in Fig.3, the 6E5 was The anode current would fall, the anode • Tuning indicator 6E5. based on a small general purpose triode voltage would rise and the shadow angle Fig.5 shows a typical circuit using the in an ST-12 valve envelope, with the would be reduced — the edges of the il- above valve complement. It is not based cathode extending into a display assemb- luminated area appearing to move on any one specific receiver but, like ear- ly occupying the domed top of the bulb. together. The user was instructed to tune lier circuits in this series, is typical of the This involved a shallow cone-shaped tar- for the 'smallest shadow'. em — while also providing a basis for get electrode about 20mm in diameter, 'Magic eye' tuning indicators were less relevant comment, beginning with the with a phosphor coating similar to that 'clinical' and more visible than small frequency changer. used for green screen cathode-ray tubes. milliamp meters and, with their gim- Unlike the autodyne, discussed in ear- A small metal vane — the ray control micky name, became a strong promo- lier articles, the configuration of a pen- electrode attached to the triode anode, tional feature in the mid 1930's. They tagrid converter did not lend itself to protruding on one side into the space be- gradually disappeared, however, as lis- much variation, apart from minor dif- tween the cathode and target. teners learned to do without them and ferences in the choice of component hi use (Fig.4) the cathode was returned especially when they realised that they values. Grids 1 and 2 were simply wired to earth directly or via a cathode bias had to be replaced from time to time as a triode oscillator, with the usual grid circuit. The grid was connected to the when the 'magic glow' dimmed. isolating capacitor and a resistor ('grid AGC (AVC) line and the anode fed leak') returning direct to cathode. from the HT supply through a suitable Grids 4 and 5 provided a separate vari- load resistor. The target was connected Mid-1930's receiver able-mu tetrode function, accepting the direct to B-plus. In operation, Prompted by a stream of application wanted signal from the antenna coil, attracted from the cathode would strike data from the respective valve manufac- mixing it with the oscillator signal per the surface of the target electrode, caus- turers, a style of domestic urban receiver medium of the internal electron stream, ing the coating to glow a bright green. gradually emerged that reflected and delivering the required difference — The indicator was normally mounted so Australian technology of the mid-1930's. or 'intermediate' — frequency to the IF that the top of the bulb was visible It could be summarised as follows. system at 455kHz or thereabouts. through the dial scale, or through a small (The valve types shown in brackets are The 300-ohm resistor and bypass hooded escutcheon set into the adjacent octal-based alternatives, which were capacitor in the cathode circuit ensured front panel. either available as imports in all-metal the minimum specified bias of -3 volts With no signal input, there would be, at construction or in view as octal-based for the signal grid (G4) under no-signal most, only a small negative potential on glass types). conditions. With a very strong signal the triode grid. With a consequently high • Frequency changer: 6A7 (6A8-G) input, the AGC voltage might apply an

ELECTRONICS Australia, January 1992 59 bias applied when there was no signal control, and provide drive voltage for the WHEN I THINK BACK present to activate the AGC. magic eye indicator. By requiring the detector to work into a so-called `AC' extra negative voltage to G4 of anything The diode detector load of much lower impedance than its up to -40V, at which point the conver- direct 'DC' load, there would be a sion gain would be reduced from Turning to the duo-diode triode, the circuitry to do with detection, AGC and proportionate reduction in the modula- 500uS to a mere 2uS — enormously tion depth of the incoming signal which simplifying the one-time problem of the magic eye function commanded a great deal of attention during the mid- it could handle without distortion. front-end gain control. In a 'worst case' situation, a designer Incidentally, to measure the AGC volt- 1930's, as I well remember from my in- volvement in the A.W. Valve Co might choose a 1M diode load with the age in such a circuit calls for an idea of minimising the damping on the electronic voltmeter, with an internal laboratory and technical publications. Valve manufacturers' recommendations input circuit. For audio take-off, he might resistance of several megohms. Using an shunt this with a 1M volume control, fed ordinary multimeter, minor deflection of were treated with considerable respect by the engineering fraternity. through a coupling capacitor. A 1M the pointer may usefully indicate that a resistor might also be added to feed the negative control voltage is present but When first introduced — or re-invo- AGC system, with a similar resistor to the actual reading is meaningless, be- duced — to the domestic receiver scene the magic eye grid — both bypassed at cause of the shunting effect of the instru- in the 1930's, diode detectors came in for the remote end by a 0.1uF capacitor. As ment on the very high impedance circuit. a fair amount of criticism both for their a result, the nett AC load would be only Operation of the oscillator section can effect on selectivity, as already men- one quarter of the DC load, with be checked by simply unsoldering the tioned, and for reputedly exhibiting severe consequent distortion on cathode end of the 50k resistor and bridg- higher distortion than the hitherto widely waveforms involving more than about ing the gap with a DC milliammeter, used anode bend detector. 25% modulation. positive connection to cathode. Normal The damping effect of a diode rectifier In Fig.5, the direct or DC load for the grid (01) current over the broadcast band on the associated tuned circuit was inar- diode detector is 0.55M, made up of a was usually in the range 025 to 0.5mA. guable, and had to be offset by the use of 50k resistor forming part of an RF filter No measurable grid current would in- litz windings — and in due course, by the network and a 0.5M the dicate that the valve is not oscillating, introduction of ferrite cores. audio volume control. Signal for the calling for possible valve replacement But analysis showed that distortion audio amplifier is picked off from the and/or inspection of the circuit to identify was not a problem in a basic diode detec- sliding contact and, having in mind the some other possible fault. tor, provided that the design of the tapered element in most volume controls, The IF stage is essentially similar to receiver was such that the detector the audio circuitry may well be shunting those shown in earlier circuits, except operated with an RF input of at least 10V only a few thousand ohms of the diode that the gain is controlled by a negative peak — as would normally be the case load at typical settings. As a result, its ef- potential from the AGC circuitry reach- with automatic gain control. fect on the operation of the diode would ing the grid via the secondary of the first Where the difficulty arose was in the be negligible. IF transformer. As in the case of the 6A7, ill-considered addition of supplementary a cathode resistor and bypass ensured circuitry to feed the audio amplifier, to The AGC circuit that the 6D6 had the required minimum derive AGC voltage for front-end gain If the AGC voltage were to be derived from the diode end of this same network — so-called `simple AGC' — it would TYPICAL ELECTRON-RAY TUBE CIRCUITS obviously impose an undesirable load DIODE SECTION OF DIODE SECTION Or on the detector circuit. It would also SECOND DETECTOR I—F TRANS. SECOND DETECTOR have the effect of feeding a negative bias to the converter and IF valves in the presence of even a very small signal, thereby marginally reducing the effec- tive sensitivity. To preserve the sensitivity to very weak signals, it was/is desirable that a threshold be established such that no AGC voltage would be applied until in- coming signals reached a predetermined 5* 154 TO SELF—BIAS TO SELF—BIAS level. This is achieved in Fig.5 by using a RESISTOR (RC) RESISTOR (RC) OR GROUND separate diode as the AGC source, fed OR GROUND from the IF amplifier anode via a 100pF capacitor. Since its load resistor returns to earth, current can only flow when the 101 .0 ME6OHM FOR 1+ = 250 VOLTS RI RA 0.5 IMEGOHM FOR II+ = 100 VOLTS + r.-- 0.2 MEGOHM signal peaks are sufficient to overcome = 100 TO 200 ispf = .05 IfIEGON1.4 the sum of the diode's own = 0.2 MESONA11 Cs = AVG FILTER CONDENSER RR DETERMINED SY TEST. SEE TEKT. Cs = 0.05 TO 1.0 and the volt or so of cathode bias. R4 = AVC FILTER RESISTOR C4 = CO The technique was/is commonly described as 'delayed' AGC — a rather Fig.41: Typical early tuning indicator circuits published by RCA. The circuits were misleading description, because the word later refined in various ways, and the 6E5 itself was displaced by other types wrongly suggests a time delay rather than which offered improved display characteristics. a voltage threshold.

60 ELECTRONICS Australia, January 1992 6A7, 6A8, 606. 6K7, 75, 507 42, 6F6, 6A8 - G 607-G 100pF 686 -G 6F6-0

001,uF 0-02pF 455kHz 455k z F

LOAD 7000.n. 0.05M 0.5M

100pk • 10uF

100pF

/100pF uF

FC2000n 50k TO.1FF 0.1)JF

0.051 8uF

240VAC X 6-3V

Fig.5: The incorporation of a pentagrid converter and AGC called for basic changes In the design of 4/5-valve superhet receivers, This circuit is not based on any particular model, but is nevertheless typical of overall Australian practice In the mid 1930's, With rare exceptions the 2.5V valves had been superseded and relegated to a 'replacement' role.

That aside, delayed AGC removed one biased to a similar voltage to that of the tor to the live PU connection, the other of the potential shunts from the diode detector cathode. Fig.4 notwithstanding, PU connection being earthed. Some load — although it did have its own simple self-bias was not recommended makers included the phono facility, separate effect on selectivity, by shunting because of variations in target (therefore others didn't. For the rest, the 6B6-G the IF transformer primary. cathode) current between individual in- triode amplifier and the 6F6-G output Ironically, while delayed AGC was dicators. AWV's preferred approach was pentode are straightforward, with a top- desirable in terms of receiver sensitivity, to derive the requisite voltage from a cut tone control and a treble limiting the resulting AGC control voltage was relatively stable source — for example, capacitor across the loudspeaker output less suitable for activating the tuning in- the cathode circuit of the output valve. transformer — fitted for the reasons out- dicator. Even though a transmission may One other difficulty had arisen with the lined in earlier articles. have been audible in the loudspeaker, it 6E5, in that the shadow angle reached 0° The need to provide a stable cathode might not have registered on the tuning with a grid bias of -8 volts. A further in- bias for the provided suf- indidator, leading to the impression that crease in signal strength (and AGC volt- ficient reason to favour full cathode bias the indicator was faulty. age) could have little further effect, for the output valve and this is depicted To avoid this threshold effect, the A.W. beyond causing the illuminated edges to in Fig.5, as distinct from back-bias Valve Co suggested in Radiotronics overlap. Efforts to overcome this limita- shown in the earlier circuit. No.70 '(November 1936) that the tuning tion by using only portion of the AGC Built along these lines, receivers in the indicator be fed from the detector diode voltage were compromised by the fact mid-1930's were eminently satisfactory — taking care to minimise the loading by that such measures also affected the sen- for listeners in urban or other well served using a 2M feed resistor, as shown. sitivity to weak signals. areas. Servicemen had access to service The problem was overcome by releas- data for individual models, but the Potentially flawed ing the 6G5 with a remote cut-off charac- designs were sufficiently consistent from They warned, however, that with the teristic, being otherwise a plug-in one model to 'the next for a good ser- indicator cathode simply earthed, there replacement for the 6E5. It retained es- vicemen not to rely overmuch on the was a potential path for electron migra- sentially the same low-level sensitivity, literature. But the process of evolution tion from the indicator cathode, by emis- but extended the cut-off voltage by about was not by any means complete. With the sion to the grid and thence through the three times. success of the 4/5-valve configuration isolating resistor and detector diode load, came the challenge to shed yet another to the positively charged cathode of the Audio system valve — with the idea of offering a detector/audio stage. Thus, instead of the Now equipped with its own in-built `second set' in every home. One that detector diode being simply referenced to volume control, the audio system shown would be cheaper, smaller and more port- its own cathode, it could be exposed to in Fig.5 was ready for operation from a able than the existing console. an external bias current. This could phono pickup. This involved the On the other hand, the shortwave fad prejudice its behaviour in the troughs of provision of a suitable 2-pin socket or was just around the corner, along with weak or heavily modulated signals, twin terminals at the rear of the chassis, the hi-fi revolution, which was to have a where the instantaneous carrier level fell plus a 2-way `radio/PU' on the significant impact on the audio systems to near zero. front panel. This was so wired that it of ordinary receivers. These matters will As a precautionary measure, it was would transfer the active end of the be the subject of future articles. suggested that the indicator cathode be volume control from the 50k filter resis- (To be continued)

ELECTRONICS Australia, January 1992 61

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