NEW RECEIVING TUBE

Radio Record, Volume I, Issue 28, 27 January 1928, Page 5

NEW RECEIVING TUBE

GREAT FUTURE PREDICTED TESTS UNDER WEIGH.

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A new type of receiving tube has just been placed on the market which, if the prophets are correct, is destined to perform wouders in the field of radio frequency amplification. This tube is called the VX 222 and differs from other usual types of tubes in so far as it has four electrodes. {fhe usual grid filament and plate are present but in addition there 1s a double grid which entirely shields the plate. On account of this arrangement it is called a "shield grid" tube. It should be noted that the term to be used is not "shielded grid."? It is shield grid on shielded plate. The tube fits the conventional VX Socket but the terminal connections to the grid in the 201A type connects to the shield grid in the 222 type. The control grid in the case of the new tube is brought out to a metal cap on the top of the glass bulb. The main feature of the new tube is the low grid-plate capacity which renders it highly suitable for use as a tadio frequency amplifier. The 201A tube has a capacity of 10-15 tmicro-microfards which, if oscillation is to be prevented, requires neutralisation. The 222 tube, however, has a capacity of only 0278 micro-microfards, or about one five-hundredth of the capacity of the 201A, When used as a radio frequenecy amplifier, a voltage gain of 200-250 has been obtained on the longer wave lengths, and 40-45 on broadcast wavelengths, as against 8-10 with a YOLA. | The filament voltage required is 3.8v at a current of .1382 amperes. A a negative bias is required on the control grid it is convenient to light the filament from a 6-volt supply through a rheostat and utilise the voltage drop over portion of the rheostat to obtain this necessary bias. It is stated that one 922 inbe used in a suitably designed circuit should give results as good as a superlieterodyne using old type tubes. _In order to take full advantage of the properties of the new tube it 1 necessary for it to be well screened from outside influences. For this rurpose metal cups can be cbtained which slip entirely over the tube leaving only ' the metal cap at the tep ‘of the tube protruding. One of these tubes was used as a Bhort wave radio frequency amplifier on the recent cccasion when WGY, Schenectady, rebroadcast the short wave transmission of 2FC, Sydney, and on short wave the new tube is said to bring in long distance stations like locals. The writer is the prowl possessor of one of these tubes-probably the first and only one in New Zealand-and is at present making up a stage of radio frequency amplification for use with his short wave timer. If results come up to expectations, we shall probably arrange for a further description of the apparatus.