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New Tungsram RF Pentodes

Wireless World, June 23, 1938.
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Battery valves for use on short and ultra-short waves.

Two battery RF pentodes just introduced by the Tungsram Electric Lamp Works (Great Britain), Ltd., will interest battery set users, as these new valves have been designed to give a good performance on the short- as well as on the ultra-short wave-lengths

The control grid is brought out to the top cap in both valves, while the other electrodes connect to a seven-pin base of standard pattern.

Particularly good for battery valves is the mutual conductance of these new pentodes, it being 1.7 mA/V for the SP2D, and 2 mA/V for the VP2D. The first mentioned is a straight RF pentode, and the other is a variable-μ type.

The screen grid will take the same potential as the anode, and 150 Volts is the maximum for these electrodes. However, the mutual conductance values just given are for operating voltage of 120 only, which is the usual amount of HT supplied for a battery set. The filament current is 0.1 Amp at 2 Volts in both cases, and the valve will stand a maximum of 10 mA cathode current, by which is meant the sum of the anode and screen currents. The valves are, of course, directly heated.

With 120 Volts on anode and screen grid, the SP2D requires -1.5 V grid bias, under which conditions the anode current is 1 mA. The VP2D would in the quiescent state be giving a grid bias of -3 Volts when the anode current is about 3.5 mA. The anode current cut-off point for this valve is approximately -10 Volts grid bias.

Even when the valves are operated with lower screen potentials, quite good mutual conductance values are obtained; the VP2D, for example, with 75 Volts on the screen and -1.5 Volts GB, has a slope of 1.05 mA/V. The anode current then falls to 1.3 mA, and the screen current is 0.6 mA. Thus quite economical operation is possible it necessary, yet still maintaining quite high efficiency in the valve.

Some tests were made with these valves in the detector stage of a short-waive receiver, regeneration being obtained by joining the negative of the filament to a tap on the coil and inserting an RF choke in the positive filament lead.

There was little to choose between the two valves on wavelengths down to about 12 metres, but below this the VP2D was definitely the better, as oscillation was much easier to obtain. Indeed, the valve oscillated readily down to just below 5 metres using quite an orthodox circuit.

The top grid connection contributes to its efficiency on the ultra-short waves, as the input capacity is reasonably small. The valves are not particularly small physically, for they measure 4⅞ in high overall, and the bulb diameter is 1¾ in at the widest part. They cost 11/- each.

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