New ETA indirectly heated screen grid valve, Type DW7.
The range of ETA valves, distributed by the Electrical Trading Association, Ltd., Aldwych House, Aldwych, London, WC2, has been augmented recently by the addition of a new indirectly heated screen-grid valve for AC operation, and styled the DW7. Its nominal characteristics are:- AC resistance 200,000 Ω. Amplification factor 600. Mutual conductance 3 mA/V when measured with 100 Volts on the anode, 75 Volts on the screen and zero grid bias.
The heater requires 4 Volts at 1 Amp, and under normal conditions it operates with 200 Volts on the anode, 80 Volts on the screen, and a grid bias of between -1.5 and -3 Volts; the average anode current being of the order of 5 mA.
Tested in a typical three valve receiver fitted with one HF stage and a tuned-grid circuit the valve gave exceptionally, good results, and in view of its high amplification the precaution was taken of enclosing it in a metal screen.
A slight reduction of the screen voltage may be necessary in certain cases to obtain perfect stability at the lower end of the Medium Wave band, and if a potentiometer is used for this purpose, it will serve, also, as a volume control. Incidentally, volume can be controlled satisfactorily by over-biasing the valve, using a variable resistance in series with a 500 Ω fixed resistance to provide the optimum grid bias when the control is set to the position for maximum volume.
The anode-to-grid capacity of the valve is given as 0.005 pF and the price is 15s. 6d.
Anode current-anode voltage curves of ETA DW7 indirectly heated screen-grid valve.