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Build an Old Time One-Valve Radio T •~ a.. e e Ifito .ega'3a, . tr Ssc~ . a~:r"..,`'r"'~g~~ t.~.= ~ y~~..i•'`'E~;sr,:~ .?-~`.r ;~'~I:i'Y ~~":'-:2i:5'-~'r:" *: ap~ i d -.t e -•i' e'•,y`. z'a--'~ ~a.r •~`,`'t ,'-yerk',t"3-c .rLt ~`u"r. ,~,`.~f'F' 06***104r"' G: _•ad2r. r,`r,*•r--,.x-00-fti;,s -,yr-4.o o-+J-0,0_4 03-c?'rv7-F * Special `nostalgia' project: BUILD AN OLD TIME ONE-VALVE RADIO T There's a lot of interest nowadays in building simple valve-based radio sets, of the type that were popular from the 1920s right up until the 1960s. Here's the first of two articles which give all the information you'll need to construct an authentic one-valver starting with a basic `grid-leak' set and progressing to a regenerative circuit with surprisingly good performance. Enough information is given to allow you to use almost any old valve or other components to hand. by PETER LAUGHTON Cleaning out my radio `shack' (read had, and how much I'd learned. This, cussion is the following project. mess) the other day, I came across a along with several recent Letters to The circuits described can be built number of radio receivers that I con- the Editor asking for more vintage using almost any combination of com- structed years ago based on valves, radio projects, resulted in me talking ponents, even from junked valve TV and remembered how much fun I'd to Jim Rowe, and the result of that dis- sets. But for convenience and safety, 62 ELECTRONICS Australia, May 1997 they will be described using a 1.4 volt battery valve that is still available: the HEADPHONES 20000 OR 1T4. The filament of this valve is sup- GREATER plied from a single 1.5V alkaline bat- tery, whilst the 'high tension' comes ANODE (PLATE) from two or three 216-type 9V batter- +45 VOLTS ies connected in series. The parts to construct the radio are also available SHORT from the Vintage Wireless Radio ANT 1nF Company in Sydney if your junk box doesn't have enough 'bits and pieces' JV~M- LONG 1.5M oro o + FILAMENT in it. (See end of article for prices etc). ANT L1 oft 500pF SWITCH VOLTAGE Understanding feedback Before we start, a few words about EARTH 0 0 COMMON feedback. To many people this is usu- ally associated with a rude, loud noise from a PA amp at a concert, etc. But L1: 60T OF #28g ECW V1: A609, 26, technically the term means something TAPPED AT 30T, ON 1/2 12AT7, ETC. 50mm DIAMETER FORMER FIG.1 much more useful, which can exist in two forms: positive and negative. Basically, feedback is achieved by A basic `leaky grid' receiver circuit, using either a triode valve or a multi-grid valve feeding back energy from the output connected as a triode. This type of set uses the valve's grid and cathode as a of an amplifier to its input. If the sig- detector diode, and then the valve as a whole to amplify the resulting audio. nal fed back is in phase or in step with the original signal, then it adds to it Conversely, if the feedback is out of components in a circuit as well as the and this is called positive feedback. phase or out of step with the original effects of temperature, especially with This type of feedback tends to exag- input signal, so that it subtracts from solid state components. Negative feed- gerate or enhance any frequency it, then it is said to be negative feed- back is commonly found in audio ampli- selectivity; hence the effect with PA back. In this case the total amplifica- fiers, and can be applied to several amps, where the sound begins to tion is reduced. stages of an amplifier at the same time. `ring' at a particular frequency before Commonly called degeneration, this a condition is reached where the type of feedback tends to smooth out Grid detectors whole set-up becomes unstable i.e., any frequency selectivity, and also The basic circuit for a grid or cumu- begins oscillating. reduce the amount of distortion pro- lative type of detector is shown in The effect of positive feedback is to duced by an amplifier as the output Fig. 1. As can be seen, apart from a increase the signal available at the out- distortion is present in the signal fed crystal set, nothing could be simpler. put over what was there before i.e., back to the input. The amplified fed- Interestingly, there is no solid state the effective gain increases, as well as back distortion is in opposition to the equivalent for the 'grid leak' type of the selectivity. If there's too much pos- output distortion (as it's out of phase) detector shown here. Even the field itive feedback, the amplifier will and thus tends to cancel it out. effect transistor, whose characteristics become an oscillator. Another word for Negative feedback also tends to com- approach that of valves, doesn't work positive feedback is regeneration. pensate for changing characteristics of as well as a valve does. Therefore this circuit arrangement is unique to valve technology. In this type of detector, the grid cor- responds to the plate of a diode, which RFC HI-Z CRYSTAL works together with the valve's fila- (IF USED) 0.1 uF EARPHONE ment or cathode to detect the incom- I-0 ing RF signals fed to it from the TO AUDIO 4.7k tuned circuit formed by L l and the AMP 500pF capacitor. As a result of this B diode action the rectified RF produces a varying DC voltage across the 1.5M +45V 'grid leak' load resistor, with the 220pF capacitor acting as a low impedance to the RF signals. The - „ 80 HEADPHONES, >2k - 0-8S7: OR EVEN A SPEAKER varying DC voltage then causes the FIG.2 (GOOD SIGNAL AREAS) grid to vary the electron flow to the plate, giving some audio amplification at the same time. Note that at this stage we haven't applied any regener- ation or feedback. Three different alternative ways to use the output from the basic receiver of Typical component values are given Fig.1 — or those of Fig.3, 5 or 6 — if you don't have a pair of high-impedance and almost any triode valve, or any headphones. valve wired as a triode, can be used. ELECTRONICS Australia, May 1997 63 For example, the 12AT7 and 12AU7 range of double-triode valves, etc., are all satisfactory (using only one half of BRACKETS the valve, of course). With these 25mm HIGH & valves, filament voltage can be either 25mm LONG 12.6V at 150mA or 6.3V at 300mA. High tension with these TV type of valves needs to be around 60 to 90 volts for satisfactory operation. Battery operation is not really practical as the plate current drain will be around 20mA or so, and the 10 or so 9V tran- sistor radio batteries that would be required to obtain the necessary high tension voltage are expensive! Other triodes, such as the battery How to make your own very simple adjustable series capacitor, to allow a types, including the 2V and 6V series longer antenna wire to be connected to the receiver's tuning circuit without of battery valves like the A609 can also loading it down too much. A fixed mica or ceramic capacitor of about 200pF can be used. Pin connections for some typ- be used instead, if you wish. ical valves that can be used in this cir- cuit are at the end of this project which a multi-element valve, such as The grid leak type of detector is sen- (Fig.9). Just make sure that you use the the 1T4 pentode, is now used as a pen- sitive, but it `loads' the tuned circuit, correct filament voltages and currents. tode rather than wired as a triode. The resulting in relatively poor selectivity A 6U7 pentode from the octal era sensitivity of this type of detector is (the ability to resolve different signals can also be used. Just connect the considerably increased by using a pen- close to each other). The linearity is screen grid to the anode to convert it tode valve. As before no feedback or poor, as is the signal handling capaci- into a triode and supply the filament regeneration is applied as yet. ty. This means that it is easily over- with its 6.3V at 300mA. Filament voltage is still 1.5V DC, loaded by a strong signal, which can The valve used in the prototype was whilst 18 to 27 volts DC (two or three completely drown out a weaker signal a 1 T4 miniature seven-pin battery pen- 9V radio batteries) are satisfactory for adjacent to it. Note also that if the tode, wired as a triode, (grid 2 on pin 3 the high tension (HT) requirements. plate voltage is made too high, the no- connected to the anode). The output is Battery life will be several months of signal current can increase beyond the to a crystal earpiece, or high impedance intermittent use, with the filament ratings of the RFC, headphones, or headphones, or some low impedance drawing 5 OmA at 1.5V and the anode transformer and they can be perma- stereo type headphones (using a match- current about 3mA at 27V. nently damaged. - ing transformer), or even to an external Fig.3 also shows how a radio fre- Try experimenting with lower val- amplifier using any of the circuit quency choke (RFC) can be used, giv- ues for the grid leak resistor (down to arrangements shown in Fig.2.
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