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Radio Record, Volume I, Issue 28, 27 January 1928, Page 13

NOTES

DOUBLE-GRID VALVES. Philips Laboratories turn ont fom patterns of double-grid receiving valves. The Al41 requires a filament voltage not exceeding 1.3 volts, the A241 maxiinum 2 volts, A341 maximuin 38.3 volts, and A441 a maximum of 4 volts on the filament. They all work on the low filament current of 0.06 amp. They can be used as amplifiers or detectors, the latter only requiring a plate voltage of 2 to 4 volts. A filament rheostat of not less than 12 ohms is required. These valves .were on sale in Wellington over twelve months ago, but do not appear, so far, to have received proper recognition. A shipment has now arrived. THE GRAVITY DANIEL CELL. Several correspondents constructing this cell have written stating that when the weak sulphuric acid solution has been put in there has been violent action on the zinc. Whilst this qetion should not be unduly violent, it must be there, for the object of the sulphuric acid to ‘start’ the cell is to act upon the zinc, so that the acid solution is turned to zinc sulphate, the required solution for the working of the cell. The zinc sulphate forms a layer of its own above the solution of copper. sulphate lower down, and the object of keeping the zinc a shallow or flattish shape is to ensure it mot reaching down to the copper sulphate. solution. Normal voltage should not be expected until the ‘starting’? is complete, and the cells have settled down. Another way of starting the cells is to place ready-made zine sulphate solution in them, instead of the weak acid. A HANDY RESISTANCE UNIT. An extremely conipact and handy re-sistance-coupling unit was handed to "Megohm" for testing purposes. This unit, the product of the Philips Laboratories, is in keeping with their many other excellent and well-known products. The chief thing looked for from an audio amplitier is tone, combined with reasonable volume. Tone cannot be measured with a meter, so the unit was simply dropped into the first audio stage of a Browning-Drake, and its excellence was apparent at once. The writer is at present engaged in finding out the very best way of improving the first audio in this set, and this unit can certainlv be recommended to replace with advantage any transformer of doubtful pedigree. The unit is provided with four terminals, just like a transformer, and = simply connects to the same four wires. The retail price is 25s. , A high-amplification detector and increased plate voltage is necessary for best results.