1990-06: the Rise and Fall of Thermionic

When I Think Back.. by Neville Williams The rise and fall of thermionic valves or `tubes' - 2 The initial 25-odd years of the present century saw 'wireless' sleeve or by a heat tolerant slag (e.g., emerge as a practical means of communication and aluminium silicate). In fact, the idea had been patented by entertainment, but widespread demand from about 1930 Messrs Freeman and Wade of Westing- onwards triggered a virtual explosion in technology, requiring house as early as 1921, but not commer- valves far removed from the lamp-like diodes and triodes 'of cially developed. It was taken up later Fleming and De Forest. by the McCullough and Kellogg compa- nies in the US circa 1925, and used to In the late 1920's came a big industry winding, the 26 triode worked reason- produce double-ended, indirectly heated push to get away from the tedium bat- ably well in all stages of typical contem- replacements for the 01-A. teries, wherever possible, and to pro- porary TRF (tuned radio frequency) re- The idea was that a set of the new duce receivers for full AC mains opera- ceivers — except the detector, where McCullough/Kellogg tubes could be tion. It called for a complete retiolution the hum problem demanded. a more plugged directly into an existing battery in valve design and manufacture. radical solution. set, their original function being pre- It was easy enough to rectify and fil- Fortunately one was forthcoming, in served by having the cathode sleeve ter AC for the HT (plate) supply, but the form of the indirectly heated cath- connected internally to the now other- not for the valve filaments. Nor could ode: a thin metal tube coated externally wise dormant filament pins. The new the filaments of existing valves be oper- with an emissive oxide (Fig.3). A sepa- heater, brought out through the top of ated satisfactorily from AC. Several rate heater element was located inside the bulb was energised via •a supplemen- volts of AC applied across the filament the cathode sleeve, but electrically insu- tary overhead harness, fed from an ex- would be much the same as having a lated from it by ceramic end bushes or ternal low voltage 'filament' transform- large 50Hz voltage superimposed on the grid/filament bias. It would be treated and amplified by the valve as an input signal. While the effect could be cancelled, in part, by suitably centre-tapping and ear- thing the filament supply, the temperature of the delicate valve filament — and therefore its emission — would still vary over each individual half-cycle. This caused the electron stream to be modulated at 100Hz, creating an intol- Two classic AC erable hum in the loudspeaker. powered receiving valves AC powered valves from the late The industry's initial response to this 1920's — early problem, circa 1927, was a notably con- 1930's era. At left servative one: the American type 26, a is a 24-A RF triode much like the traditional 01-A amplifier tetrode, except that it was fitted with a rugged with a type 47 power output filament rated at 1.5V and 1.05A. The tetrode at right. purpose of the low operating voltage Together with the was to minimise signal injection into the type 80 rectifier, grid/filament circuit; secondly, the stout, they formed the heavy current filament was meant to ex- heart of many hibit high thermal inertia, thereby Australian radio reducing the temperature variation over sets of that era. each successive half-cycle. With critical setting of the earthed centre-tap on the transformer filament 42 ELECTRONICS Australia, June 1990 ica had produced battery valves with an extra grid, placed between the control grid and the plate. Operated at a fixed positive potential somewhat below that on the plate and bypassed to ground, it served as a passive electrostatic shield between the two critical electrodes. (See Figs. 4 and 5). By bringing one of the connections — normally the grid — out through the top of the bulb, the capacitance between the connecting leads could also he minimised, reducing the overall grid-plate capacitance by a hun- dred times or more. Reacting to this demand, valve manu- facturers released a number of mains type 'screen-grid' (or tetrode) RF amplifiers, the best known in Australia being Examples of the first 'all metal' 8-pin octal based valves which first appeared probably the 24-A (or 224) released in in America around 1935. Later octal valves had a glass envelope. 1929 as a companion valve of the 27 triode. Two years later, the 'variable- er. By also substituting a commercial Tetrodes & pentodes mu' type 35 (235) appeared, with very 'eliminator' for the B-batteries, the old similar characteristics except that a vari- battery set could thus become all-AC Apart- from the line-up of 26's as RF able-pitch control grid made it possible operated! and AF amplifiers and a 27 as detector, to adjust the signal amplification over a Taking the longer view and faced with the initial wave of mains powered re- very wide range by varying the bias the need for an indirectly heated detec- ceivers likewise departed little from the (Fig.5). tor to go with their type 26 triodes, circuit concepts that had previously RCA produced the 27 — an otherwise been developed around battery valves. From type to type traditional triode with insulated cathode But that phase was shortlived, and the During 1931/32 a flood of locally pro- and a 2.5V/1.75A heater — fitting it only receivers of that type I encoun- duced mains receivers was released onto with a 5-pin plug-in base. In so doing, tered personally were those that subse- the Australian market using one or two they signalled the mass production of quently accumulated in the factory base- type 35 valves as RF amplifiers, a 24 or AC mains powered receivers. ment as trade-ins. 27 as detector, a 45 power output triode Fairly obviously, the evolution of In the present climate of nostalgia, or a 47 power tetrode, and an 80 recti- valves to this point had hinged largely those hum-prone 26/27 discards would fier. About the same time, however, a on the incandescent filament/cathode probably be worth a lot more now than highly practical version of the superhet- and the problem of ensuring a suffi- they were -then! erodyne type circuit began to gain fa- ciently copious electron stream for a Designers soon made it clear that they vour and appeared on the market with a manageable current drain. Less lamp- preferred indirectly heated valves for all comparable valve complement. like constructional methods had also sockets and, what's more, that they'd By the time I entered the industry in emerged but, in terms of amplification had enough of the chronic instability of early 1933, those valves, too, had been factor, transconductance and plate triodes as RF amplifiers — a by-product superseded and replaced by the '50' resistance, triodes like the 26 and 27 of their high internal grid-plate capaci- series, also using 2.5V heaters but fitted (226 and 227) differed little from the tance. with slimmer envelopes and boasting original 201. Research in both Europe and Amer- improved performance. At the same reducing the grid/plate capacitance, it makes it possible to PLATE TETRODES, (ANODE) achieve high gain in RF amplifier stages without instability SUPPRESSOR and oscillation. PENTODES GRID ~y SCREEN GRID The signal handling capability of a tetrode valve can be & DIODE CONTROL further improved by inserting a suppressor grid between TRIODES GRID the screen and plate. Operated at earth or cathode DIODE potential, it prevents slow-moving 'secondary' electrons, PLATE dislodged from the plate by the incoming 'primary' electron stream, from being attracted back to the screen. CATHODE PILAHERT Some multi-element valves also provide one or two small OR HEATER diode plates, adjacent to one end of the filament or Fig.4: A basic triode, as in Fig.2, contains a filament or a cathode sleeve, so that signal detection can be provided heater/cathode combination, a control grid and a plate (or within the same envelope, along with a signal-dependent anode). In a screen-grid (or tetrode) valve, an extra grid is voltage to provide automatic gain control. placed between the control grid and plate. Operated at a Other typical multi-element valves contain twin triodes, fixed positive potential, usually somewhat lower than the separate triode and pentode sections, or complex electrode plate, and bypassed to earth, it serves as a passive structures intended for specialised roles such as frequency electrostatic shield between grid and plate. By thus changers in superheterodyne receivers. . ELECTRONICS Australia, June 1990 43 WHEN I THINK BACK CATHODE INSULATED HEATER time, the tetrodes had become pentodes, with a third 'suppressor' grid in- DIRECTLY HEATED CATHODES INDIRECTLY HEATED CATHODES serted between plate and screen to re- (FILAMENT TYPE) , (HEATER TYPE) fine the overall dynamic characteristics (Fig.5). Fig.3: To minimise current drain, battery valves normally use a directly heated As it happened, the 2.5V series had coated filament (left). But most mains powered valves used an indirectly one serious limitation: their unsuitability heated cathode (right), to minimise hum injection into the signal path. for use in automotive receivers — operating commonly, in those days, from 6- bought large numbers of replace- cially, the valves were being manufac- volt systems. RCA and others had pro- ment valves from radio shops. tured to RCA specifications, covering vided for this market with the 36-39 • Light-fingered employees in radio such things as the number of turns and series, but in the mid 1930's they factories showed a special affinity the winding pitch of the grids for the released 6.3V versions of the 2.5V for valves, because of their unit various valve types, the nature of the series, suitable for either mains or au- value and the fact that they were side rods to which they had to be tomotive use.

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