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Wireless Set No.19: the "B" set.
(a) General.
The "B" set is a U.H.F. Transmitter and receiver operating on R/T only, with a frequency range of approximately 229-241 Mc/s. Four indirectly heated valves are employed and power is derived from the common supply unit. On "receive" a four-valve supper-regenerative circuit is used embodying a separate quench oscillator, the latter operating on a frequency between 158 and 228 Kc/s. When used as a sender, three valves only are connected in an anode-modulated oscillator circuit.
(b) Controls
(1) On-off switch
A double-pole on-off switch S10B in the H.T.+ and heater leads enables the "B" set to be switched off while leaving the remainder of the equipment operative.(2) Tuning
This control adjusts the split-stator condenser C25A and varies the frequency of the set between 229 Mc/s and 241 Mc/s. It is not calibrated, but the dial is marked with 10 divisions enabling resetting to be carried out.(3) Gain
Operates a 100 ohm potentiometer R35A controlling the A.F. gain of the "B" set.(4) Quench
This is used on "receive" only and is a permeability tuning adjustment associated with L24A which determined the quench frequency. It enables this frequency to be adjusted to avoid interference between sets in a net due to the beating of their quench frequencies of harmonics of these frequencies.
(c) Special note on super-regenerative receivers.
In this type of receiver the incoming signal is applied to a cumulative grid detector with reaction, maintained in a state of high sensitivity by increasing the reaction beyond oscillation point. The heterodyning of the carrier which normally results from this condition is prevented by varying the anode voltage of the detector at supersonic frequency, thus the detector ceases to oscillate every negative half-cycle of the latter frequency. This action is accomplished by means of another valve known as the "quench oscillator".
(d) Detailed description of receiver
(1) The super-regenerative detector stage.
The oscillating detector valve V7A is a special low capacity triode, type CV6 (E1148), the grid and anode leads being brought out to top caps on the valve envelope. This stage forms a self-excited Colpitts oscillator, the oscillatory circuit comprising L11A and the split-stator variable condenser C25A. The latter enables the value of the split capacity to be varied for the purpose of tuning the circuit to the signal frequency. Cumulative grid detection is obtained by means of C27A and the grid leak R32A and R18C.
The R.F. input developed across L12A is fed to the detector via C46A and a tapping on L11A. The inclusion of the cathode choke L13A prevents negative feedback, thus increasing the oscillator efficiency on the high frequencies employed.
H.T. voltage applied to the anode is picked up from the H.T.1+ line via switch S10B, relay S5B, resistance R11B, decoupled by C31A, load resistance R33A, the quench anode choke L15A, and the resistance R31A. A.F. voltages developed across R33A are applied to the A.F. stages through the coupling condenser C29A. L12A and C46A form a filter to keep spurious oscillations out of the aerial which would cause interference on the "A" set.
(2) The quench oscillator
In this stage a variable-mu R.F. pentode V1D type 6K7G is employed in a parallel-fed Colpitts oscillator circuit tuned to the quench frequency by L14A, C28A and C37A. This frequency may be varied between the limits of 158 Kc/s and 228 Kc/s by means of the variable inductance L14A.H.T. to the anode of V1D is applied via its anode load impedance L15A which is also included in the anode circuit of the detector, V7A. Variations in potential at the quench frequency are tgus applied to the anode of the detector, the average potential of which has been adjusted so that oscillation ceases every other half cycle.
Screen-grid H.T. is fed via R34A, decoupled by C54M, from the resistor R11B.
(3) A.F. amplifier and output stages.
The A.F. output of the detector is coupled by R33A, C29A, and the gain control R35A to the grid of the pentode V1E. C30A, R6H, and C30B form a filter to keep quench signals out of the A.F. circuits. V1E is the first A.F. amplifier and it is a 6K7G R.F. pentode. H.T. is fed through R23C and R7J. R1A decoupled by C45N feeds the screen grid. Grid bias is obtained in the normal manner by means of a resistor in the cathode lead. This resistor is not decoupled and so current negative feedback is provided. The A,.F. output of V1E is taken via C29B to the grid of V8A which is a beam tetrode, type 6V6G. This valve is over-biased, for economy reasons, by means of R49A and R9D in series, decoupled by C16B. H.T. is fed to the anode via the primary of transformer T5A, the secondary of which is connected to the 'phones.
(e) Detailed description of Sender
(1) Modulation amplifier
Due to the low output from the moving coil microphone it is necessary o use a pre-amplifier between the microphone and the modulating vsalve. Output from the microphone is taken via the transformer T4A to the control grid of V1E which functions in the same manner as on "receive". The output is again fed through C29B to the control grid of V8A which now acts as the modulator valve.(2) Modulator
V8A is now biased by R49A only so that the valve can deliver maximum power output. Its output is again taken through T5A which has a third winding which is now connected in the anode circuit of V7A, sidetone is obtained from the "phone" winding of T5A. Level frequency response is ensured by providing negative feedback from the phone winding of T5A, through R36A, to the primary of T4A.(3) Oscillator
V7A acts as a U.H.F. oscillator producing the sender carrier. H.T. is supplied through the modulation winding of T5A so enabling anode modulation to take place. The anode voltage is increased and the gird leak is reduced compared with the values used on "receive" so asto obtain maximum power output. The Quench oscillator is rendered inoperative by disconnecting its screen H.T. feed. A half-wave aerial is connected to a tap on the inductance L11A by means of a screened lead in series with C46A.
WS No.19 contents "B" set circuit diagram Low power wirelesses contents Radio Pages Home Page
Rod Beavon 17 Dean's Yard London SW1P 3PB
e-mail: rod.beavon@clara.net
