APPARATUS TESTED

Sun (Auckland), Volume II, Issue 517, 21 November 1928, Page 14

APPARATUS TESTED

THE MULLARD WONDER VALVE There are two main characteristics of a valve, the ability to magnify signals, called the amplification factor, and the ability to handle loud signals without distortion, called the mutual conductance. The usual practice is to use a valve with a fairly high magnification as the first tube in an audio frequency amplifier, and a power tube with a high mutual conductance in the last socket. Owing to the methods of construction employed hitherto, a valve with ability to handle loud signals has had a low magnification, which explains why many radio enthusiasts who have substituted a power tube for one of the standard type have been disappointed to find that the volume was decreased. Thus most of the power tubes sold to-day have only a magnification of 3, as against 8-9 of the ordinary tube. ‘The New Mullard tubes, PM24 and 22, are of special construction, somewhat of the type of the screened grid valve, and employ three grids; their magnification is from 60-80, while their mutual conductance is 1300, which is a value greater than that of many power tubes on the market to-day. On test, it was found that only one stage of audio frequency amplification was required with one of the new tubes, the volume being quite equal to that of a two-stage amplifier; when the valve was used in the last socket of a twostage amplifier, the volume was enormous, signals which were only at weak speaker strength when the ordinary tubes were used taxed the capacity of the speaker with the PM24 in use. Although sold as a power tube, the writer found that both the PM22 and the PM24 acted well as radio frequency amplifiers in any neutralised circuit, giving a great increase in amplification and easily neutralised. A three-valve Roberts with the PM24 in the first socket gave better signal strength than sc^L ra * ve six-valve receivers. .The PM24 is a four-volt tube taking -Id amperes, while the PM22 requires two volts and .3 amperes. The total B battery consumption of either tube is about 10 milliamperes—a very low figure for tubes of such power. Being easy to adapt to such circuits as the Roberts, Browning Drake, etc., the writer is of opinion that these super tubes will be of greater utility than the screened plate type which require special circuits