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New American output valve for low-voltage system.
RK-100. Image courtesy The Board of Trustees of the Science Museum.
Details of a new valve of unusual interest which would appear to solve the problem of obtaining a, large undistorted output when-only a low HT voltage is available are given in the June issue of QST by J R Nelson and James D Le Van. The data shows that for an HT supply of 110 Volts an output of over 7 Watts can be obtained for less than 5% third harmonic distortion!
The valve is fitted with an indirectly heated cathode, the heater consuming 0.6 A at 6.3 Volts, which is surrounded by two grids and an anode. The inner grid is termed the cathanode, for it serves the dual purpose of anode and cathode to the two halves of the valve. It acts as the cathode of a conventional triode, of which the other grid and the anode are the remaining electrodes, and it also functions with the true cathode as the anode of a gas discharge tube. The valve is filled with mercury vapour and the cathanode is maintained at a positive potential of some 10 Volts. The discharge, therefore, occurs between the true cathode and the cathanode. The velocity of the electrons, however, is great enough for many to shoot through the cathanode, which may consequently be regarded as a cathode emitting electrons with an initial. velocity equivalent to about 1 Volt. The control grid and anode then function in the manner of an ordinary triode.
Two points result from this construction. It is readily possible to make the grid and anode physically large and so capable of dissipating a considerable amount of heat, and a very high mutual conductance with a low AC resistance is obtainable. With the RK-100 valve the mutual conductance is 12.0 mA/V with 100 Volts anode potential and 2.5 Volts negative grid bias when the ionising current to the inner grid is 150 mA, and it rises to no less than 20 mA/V when this current is increased to 250 mA. The AC resistances for these conditions are of the order of 4,100Ω and 2,500Ω.
Unlike the conventional triode, the control grid takes an appreciable current under all conditions. It is, however, small, with a negative grid bias, so that the input resistance of the valve is high, although lower than that of an ordinary evacuated valve. The valve is not confined to the output stage of a receiver, for it may also be used as an HF amplifier in transmitters and as an oscillator, in which role it will function at as low a wavelength as 2 metres.
For receiver use the valve will undoubtedly find its chief function in the output stages of sets designed for operating from 100 Volts DC supplies, for the provision of an adequate output in such sets has hitherto been a matter of extreme difficulty. There is no doubt, however, that it would prove extremely useful in small transmitters also.
The Raytheon data-sheet can be found here
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