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Triode Hexode and duo-diode.
The X41 triode hexode.
Interesting new valves are announced by Marconi and Osram. The first of these is the X41 triode-hexode, which is the combination of a hexode and a triode in a single glass envelope and is intended to function as a frequency-changer in a superheterodyne. A common cathode is used, and the heater consumes 1.2 A at 4 Volts. The triode section oi the valve functions as an oscillator with the tuned circuit in the grid circuit of the valve. A grid leak of some 50,000 Ω returned to the cathode should be used for biasing purposes, and with this a grid capacitor of 0.0001 μF is suitable. The anode supply should be obtained from the HT supply through a dropping resistance of some 30,000 Ω. The optimum operating conditions are secured when the oscillator voltage is about -25 V peak, and this is most readily determined by checking the grid current. A milliammeter inserted in series with the grid leak should show a current of some 0.4 mA.
The hexode section of the valve consists of a control grid which surrounds the cathode, two screen grids, between which a coupling grid is inserted, and an anode. The coupling grid is connected internally to the triode grid, and so has applied to it the full oscillator voltage. The anode is rated for some 200-250 Volts, with 70 Volts on the screen grids and -1.5 Volts grid bias on the control grid, and under these conditions a conversion conductance of 0.55 mA/V is obtained, while the AC resistance is as high as 2 MΩ. The valve is fitted with a B7 7-pin base, to which the connections are the same as in the case of the more familiar heptode.
Short-wave Reception
The valve possesses several advantages over the heptode. In the first place, although the conversion conductance is of the same order, the AC resistance is higher, and so the first IF circuit is more lightly damped, thus making for higher selectivity. Secondly, the mutual conductance of the oscillator section is higher, with the result that reliable oscillation can be secured even at wavelengths as low as 4 metres, where a single heptode usually fails. Thirdly, unwanted couplings between the two sections are greatly reduced, so that there is much less interaction between the signal and oscillator frequency circuits, and this is very important in short-wave reception.
This new valve is listed at 20s, and it is understood that a universal model with a heater rated tor 13 Volts at 0.3 A will shortly be available at the same price and under the type number of X31.
The second of the new valves is a duo-diode, the D41, which can be used either in ordinary AC sets or in universal receivers, since its heater consumes 0.3 A at 4 Volts. It is advised that the diode load resistance should not be less than 0.25 MΩ, and that the peak HF input should not be greater than some 40 Volts. The valve is fitted with a B5 5-pin base and is priced at 5s. 6d.
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