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Marconi and Osram X21 new economical battery heptode frequency changer.
TheX21 is a new battery-operated heptode valve for use as a frequency changer, its characteristics permitting the stage to be included in the AVC system. Economy is one of its chief features, for the filament current is 0.1 A only at 2 Volts, while the average HT current under working conditions 1.7 mA. It is rated for 150 Volts on the detector anode, and between 40 and 50 Volts on both the screen and the oscillator anode.
A seven pin base is fitted, and the pins arranged as is now standard for this type of valve, and having the control grid connection located at the top. The total HT current is divided up approximately as follows with zero grid bias on the control grid the detector anode circuit passes 0.45 mA., the anode circuit of the oscillator sections accounts for 0.6 mA., and the screen 0.6 mA also.
These low current values are not obtained by cutting down the voltages to the lowest that will give results, but the valve is designed for economical working; and no improvement can be effected by applying higher voltages. The efficiency definitely falls off when the oscillator and screen potentials are raised, and our tests show that the valve functions at its best with voltages nearer 40 than 50 in each case.
The output from the detector to the first IF stage is controllable by the negative bias applied to its grid, since the tetrode section has variable-μ characteristics, and a sufficiently wide variation for normal purposes is provided by a change of the order of 10 Volts.
There is every advantage in using an efficient IF transformer following the X21, as it has a high AC resistance, this being about 1.4 megohms at zero bias and increasing with negative bias on the control grid.. With zero bias the conversion conductance is 0.2 mA/V.
The X21 is included in both the Osram and the Marconi lists, and costs 18s 6d.
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