Just the job for a caravan is this sensitive medium and long wave receiver whose only power supply requirement is 100mA from a 12 volt car battery! Our contributor displays his customary ingenuity in producing yet another cleverly designed circuit which offers maximum performance from a minimum of components
THIS ARTICLE DESCRIBES A SIMPLE hybrid circuit which will give good results on medium and long waves from a 12 volt supply and a normal car-type aerial. Current consumption is only of the order of 0.1 amp so that the receiver may be used for long periods without running down the battery. It is particularly suitable for use in a caravan when a 12 volt supply is available from the car battery. It can also be used as a car radio when the car is stationary, but the output is not sufficiently high for use on the open road except in a quiet motor car. It is ample for occasions when the car is stationary.
Components List: Resistors (All fixed resistors 1/4 watt 10% unless otherwise stated) RI 2.7k
R2 47k
R3 1.2k
R4 2.2M
R5 220k
R6 1M
R7 150k
R8 1M
R9 330
R10 l00 ohm, 2 watts, 5%
VR1 25k potentiometer, linear track
VR2 5k potentiometer, pre-set Capacitors C1 0.01 uF, paper
C2 0.1 uF, paper
C3 0.0l uF, paper
C4 l00 pF, silver-mica or ceramic
C5 330 pF, silver-mica or ceramic
C6 0.01 uF, paper
C7 27 pF, silver-mica or ceramic
C8 l.000pF, paper or ceramic |
C9 l00uF, electrolytic, 12V wkg.
VC1 800pF variable, solid di-electric Inductors L1, 2, 3 Coil unit (see text) L4 2.5mH choke, Repanco type CH1 L5 Primary (red and blue leads) of Repanco transformer type TT53 T1 Output transformer, Repanco type TT5 Valves V1/2 3Q5GT (DL33) Transistors TR1 MAT101 or MAT121 TR2 G.E.C. S5 or S6 (see text) Switches
S1 (a), (b), fc), 3-pole 2-way
S2 s.p.s.t. (may be ganged with VR1) Loudspeaker 3ohm impedance Miscellaneous
2 International Octal valve holders, Ferrite rod 3 x 3/8in Cabinet, etc.
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The Circuit
The circuit is shown in Fig. 1. TR1 is a common emitter high frequency amplifier with the input untuned. A good transistor will give higher amplification, in these circumstances, than a valve, as its input impedance is low and it is reasonably well matched to the fairly low impedance of the aerial-earth system. No extra selectivity is offered by an untuned stage, but with a circuit designed to be used with an aerial only about 3 or 4 feet long, and incorporating a highly selective radio frequency transformer for medium waves, this is of little importance.
The tuning coil arrangements are unusual. The coil unit consists of LJ1 L2 and L3. These are wound to the dimensions shown in Fig. 2. L1 has 300 turns of 38 s.w.g. enameled wire wound in a single pile and with a tapping at 100 turns from the end remote from L2. L2 and L3 use 32 s.w.g. wire. L2 has 50 turns, close wound, and L3 has 25 turns wound in a narrow pile. The coils are wound on a sleeve made of "contact" or similar paper-backed adhesive plastic on a piece of 3/8in ferrite rod, 3in long. Coverage will be from about 190 to 550 metres on the medium waveband, and from about 1,250 metres to well over 2,000 on the long waveband. Turns should not be removed from L1 in order to lower the wavelength coverage as this will result in L1 resonating with its self-capacitance within the medium waveband, and will render the receiver inoperative on the long wavelength end of that band.
If Fig. 1 is studied it will be seen that, with S1(a), (b), (c) in the lower position, L1 forms the collector load for TR1 and couples, on the "large primary" principle, to L2, which is the medium wave tuning coil.* This arrangement provides good selectivity and uniform coupling throughout the band. LS is the reaction coil for medium waves. The earthy end -
* It is conventional practice, on medium and long waves, for an aerial coupling coil to have a considerably larger inductance than the tuned coil. The aerial coupling coil, in company with the aerial-earth capacitance, then becomes broadly resonant below the low frequency end of the band covered, thereby boosting sensitivity at this end.—EDITOR.
- of LI is taken to the positive filament pin of V2 to reduce the voltage available for TR1 to about 9 volts. La and L3 have their earthy ends taken to positive, and not negative, battery line, to facilitate the special reaction-cum-volume control method used. C3 completes the r.f. circuit and provides a short r.f. path for VC1. With S1 (a) (b) (c) in the upper position, L1 is still the collector load for TR1, but is now also the long wave tuning coil. The collector is tapped into it to reduce damping by TR1. The reaction coil is now L2
V1 and V2 are pentode output valves of a type which are frequently advertised in this magazine. Filament voltage is supplied via RI0. V1 is a leaky-grid detector. It will be found to work very efficiently with the small h.t. voltage available for it. R.F. for reaction purposes is taken from the anode through 5 to the slider of VRi. As the slider of VR1 is moved in an upwards direction, the part of the track between C5 and the reaction coil is reduced and reaction increases. At the same time the d.c. voltage available for V1 increases, since VR1 is across the battery. At the minimum position, V1 is deprived of anode and screen-grid voltage, and no signals are passed. As may be seen, V1 is a triode so far as reaction is concerned, its anode and screen-grid being joined together for r.f. signals. It will be found that a smooth control from zero to oscillation point results.
L4 is an anti-breakthrough choke to prevent any possibility of a powerful medium wave station forcing its way through on to the long waveband.
The low frequency anode load is L5, which is the primary of an inexpensive transformer designed for coupling a crystal pick-up to the input of a common emitter amplifier. The winding has a very high inductance, this being well maintained with the small current passing through it in the present circuit. It is shunted.by a 220k ohm resistor, R5, to prevent threshold howling. Far greater amplification is possible by this method than by using resistance coupling, especially when only 12 volts are available for V1.
The amplified signal appearing across L5 is fed to the grid of V2 by means of C6. R7 and C7 maintain stability. Negative feedback is provided by R6.
Valve-Transistor Coupling
The arrangements for coupling V2 to TR2 involve a new circuit developed by the author. V2 will have a very high output impedance. Provided it is found possible not to damp this output impedance, V2 will offer a very high degree of amplification despite the limitation of less than 12 volts high tension. TR2 is connected as a common collector large signal amplifier. In this configuration it is capable of providing good current amplification. No voltage amplification is offered, as there is 100% negative feedback of voltage inherent in the operation of a common collector amplifier which, incidentally, results in unusually good quality. Quite considerable voltage amplification is, provided by the two pentodes so plenty of power is available from TR2. A common collector amplifier must have an output load which is not too low, to maintain its current amplification and high input impedance which, in favourable circumstances, is similar to the output impedance of a pentode. It follows, therefore, that direct coupling from a pentode valve to a common collector transistor power amplifier is both practicable and highly efficient, provided a transistor is chosen which gives satisfactory results with a base current equal to the anode current passed by the pentode.
It will be seen that the input of TR2 forms the output load for V2. This is a most satisfactory load as it has a high impedance to a.c. (the signal) and a low resistance to direct current. A snag, however, remains to be overcome. Although the resistance to d.c. of V2 forms the bias arm from the negative supply line to base, the path from base to the positive line remains open. There is, therefore, a state of affairs which can lead to thermal runaway, and the normal solution of a resistor between base and positive arm is ruled out as it would cause hopeless damping of the load offered to V2. The answer to the problem is to abandon any idea of current stabilisation and adopt, instead, a form of power stabilisation. That is to say, we allow the current passed by TR2 to vary but arrange that any increase in current is accompanied by a corresponding decrease in voltage available, the power remaining substantially constant and within the limits allowed for the transistor. This effect is produced by R9/C9 is a bypass for the signal.
TR2 must be a high amplification output transistor which will tolerate l00mW and, in order not to be damaged while VR2 is being adjusted, up to 30mA current. The author recommends the G.E.C. S5 or S6.* For maximum power, bearing in mind the function of R9 and the impedance of the output load, TR2 should pass about 10mA. For this it will require a base current of from about 50 to 150uA depending on its amplification factor. It is necessary, therefore, for V2 to be set up to pass this current, and the necessary adjustment is made by VR2, which controls grid bias. If a milliammeter is available it should be inserted between T1 and the positive line, and VR2 adjusted so that 10mA is registered. If no milliammeter is available adjustment can be done by ear, using a powerful station and adjusting for maximum volume without distortion. If grid bias is too great, both V2 and TR2 will be short of current and there will be distortion. If grid bias is too small a heavy current will pass through TR2 and,
* The S5 and S6 (or GET.S5 and GET.S6) transistors do not appear in the normal lists, but are available as surplus items at very low cost. The author obtained his from Lasky's Radio, 207 Edgeware Road, London, W.2. A possible alternative in the present circuit would be the ACY18 or ACY21.— EDITOR.
because of R9, it will be short of voltage and again there will be distortion. R9 will prevent damage to TR2 whatever the setting of VR2.
Loudspeaker Requirements
It is recommended that a good loudspeaker be used and that it be mounted in a separate box to avoid any possibility of microphonic howling due to the large amount of audio frequency amplification.
Nothing longer than a normal car aerial should be used. Earthing is automatic through the battery. The receiver should be insulated from the metalwork of the caravan or car and may then be used whether the negative or positive side of the battery is earthed. There is some advantage in using a metal case for the receiver, since this prevents direct pick-up of a very powerful signal by the coil unit. The lead to the aerial should not be screened, since this will reduce input as a result of losses due to self-capacitance which cannot be "tuned out" with an untuned input stage. There is no need for screening, either, with a stationary engine.
The circuit will be found surprisingly sensitive. In South Devon, the Home, Light and Third programmes, together with four Continental stations, can be received in daylight at good volume, and about 30 alternative programmes are offered after dark.
Radio Constructor Vol.10, No.9 (1966)
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