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1992-01: Vintage Radio Receiver Design

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1992-01: Vintage Radio Receiver Design When I Think Back... by Neville Williams Vintage radio receiver design - 6: Pentagrid converters, diode 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 capacitor and the single most troublesome aspect of the concentric electrode 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 cathode, so that the only bias probably that of gain — or volume — cathode and anode — 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 amplifier stage was the only one avail- more flexibly than the existing autodyne 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 antenna 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 triode 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 electrodes Australian radio scene, the most notable and also accelerated towards the anode such valve was the American designed proper those sectors of the electron beam 2A7 pentagrid converter — 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- tetrodes and pentodes, 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 tetrode or pentode, 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- Automatic gain control 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 frequency changer 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 control grid 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.
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