Screened Grid Valve

Sun (Auckland), Volume I, Issue 297, 7 March 1928, Page 14

Screened Grid Valve

For Radio Amplification

THROUGH the courtesy of the British General Electric Company The Sun has one of the 5625 valves on test, and with rough make-up, a Browning-Drake set was altered to take the new valve. Volume on IYA was emphasised in a way that had never been heard from a stage of H.F. amplification. Wellington was almost as loud, Dunedin also almost as loud. The Australians all at good speaker strength by 9 p.m.

The usual B.D. transformer was used, and the effect of the reaction coil without oscillation was

more nearly approaching the ideal than is usually obtained. To be able to build the volume up so simply and so effectively bears out the claim of the managing director of one of the biggest set and accessory manufacturers in England, that the British set for 1928 will be. a two-valve set, with no audio stages, a remarkable claim, but which this remarkable valve, yet little beyond the experimental stage, seems capable of establishing. A point noticed was that with the much higher R.F. amplification and sensitivity, the old selectivity went completely by the board, which, without completely screening the coils, etc., per stage, was only what was to be expected. With these special valves, special methods become necessary, and to keep the desired degree of selectivity, Mr. Scott Taggart has found it necessary to sacrifice slightly the amplification per stage to bring about the desired results in a set using two stages of R.F.

Both the Americans and English have turned to the old tuned plate method as the logical way to get the maximum amplification from a single stage of radio amplification. Using an ordinary transformer of the B.D. type, variation of the size of the primary will affect both amplification and

selectivity, and a little experimenting on these lines will repay the constructor who wants the extra amplification without loss of selectivity. The working of the new valve is quite simple. The extra grid or screen is made of pure gauze, the work being about 1 -16 in square. In the case of the ordinary three-electrode valve, the lines of electric force run from the plate to the grid and is in effect an electrostatic condenser of small size. The screen, which is at a fixed positive potential, intercepts these lines of force and they do not pass out through the shield, so that no effect can be produced on the grid by any changes

in plate potential. In other words, this small capacity between the plate and the grid is no longer present, and no current can be fed back from the plate to the grid, and the valve becomes a truly unidirectional device, making the risk of feed back, and consequent oscillation, a thing of Contingencies of manufacture demand that there is still a very tiny plate grid capacity, but too small to be really harmful. The double grid valve, it must be remembered, is not a new idea; in Germany, particularly, they have been investigated, and the lack of self excitation between grid and anode circuits with suitable valve sockets was demonstrated as far back as January, 1921, and interest was later aroused by the work of Hull and Williams in America.

Other valves of this type will be the Cossor, which takes .1 amp. at two volts, the Robinson valve made by Mullards and used in the new R.I. four-valve set, the UX222. Mullards and Phillips are both bringing cut valves of this type, and samples should be here shortly. Details are not available of all types, but the UX222 +akes 3.3 volts, with circular electrodes, and the Mullard uses only .075 amps. Since writing the above the writer has tried the screened grid valve a stage further, and the results were as follows. Using four valves the set at 9 p.m brought all the following stations in, in such volume that they had to be

r cut down: 2YA, 3YA, 4YA, IYA, 2BL. 2GB, 4QG, 3LO, 2FC, 3AR, 7ZL. At l 11 p.m. about 8.24 Sydney time, to be exact, 2BL. 2FC and 4QG were brought ; in on a 2ft x Ift loop in sufficient i volume to be heard clearly 12ft away ; from the speaker. C The circuit used was a tuned plate i using space round coils. A reaction i coil of the tickler type was added i roughly, and fully justified its addition. . Experimenters can rest assured of an - eventful winter using these new valves, f The writer hopes to give full details i of the experimental set next week, i using screened coils, with construe- =* tional details.