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A double pentode assembly in one bulb for economical battery operation.
New Hivac QP240 double pentode output valve for QPP operation.
The High Vacuum Valve Co, Ltd., 113-117, Farringdon Road, London, EC1, have now added to their range of battery valves a new type for QPP operation. Known as the QP240, it consists of a double pentode assembly in one bulb and is rated to give a power output of the order of 1.4 Watts with 150 Volts HT. Owing to the high efficiency of the two elements in the valve - their mutual conductance is given as 2.3 mA/V - this output is obtained with a grid swing per system of 14 Volts RMS, so that about -19.5 Volts grid bias is needed to avoid swinging the grids into the region of positive grid bias.
With the specimen tested the measured total anode current in the quiescent state, with 150 volts HT and -19.5 Volts grid bias, was 6.9 mA., while 1.2 mA. flowed in the auxiliary grids circuit.
An output adequate to meet most requirements with battery operation can be obtained, however, with a considerably lower HT voltage, for if operated with only 125 Volts on the anodes just over one Watt may be expected. Under these conditions the total quiescent HT current amounts to 5.8 mA., of which 4.9 mA. is passed by the anodes and 0.9 mA. by the auxiliary grids. These figures were obtained with a grid bias of -16.5 Volts.
With output systems of this type, and more especially if the valves are of high efficiency, any tendency towards self-oscillation must be watched and, if suspected, counteracted, as not only will it spoil the quality, but also give rise to an excessive HT consumption.
Instability of this nature gave actual warning during our tests by a high-pitched whistle, but should the oscillations be outside the audible range their presence might conceivably be overlooked. But poor quality and an unduly high anode current reading will, in most cases, be an almost certain indication of their presence.
A 250,000 Ω resistor connected between the centre tap on the input transformer secondary and the grid bias battery was found efficacious in suppressing such oscillations. The makers recommend joining a resistance of between 5,000 and 10,000 Ω in series with the auxiliary grids supply, and this might be adopted as a further precaution.
With the maximum HT voltage the valve requires to work into a load of 14,500 Ω, while with 125 Volts on the anodes and on the auxiliary grids the optimum load is 16,500 Ω. The filament consumption is 0.4 Amp. at two Volts, and the valve is fitted with a seven-pin base of standard pattern and the connections are as in other QPP valves of the same type.
An output filter consisting of a 10,000 Ω resistance and a capacitor of between 0.005 and 0.01 μF joined across the two anodes could be used as a frequency corrector, though we found this unnecessary, as the quality was particularly good with a correctly adjusted transformer ratio to suit the loud speaker in use. There is ample power for normal needs, and as an economical power stage for battery operation the QP240 can be confidently recommended. Its price is 19s. 6d.
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