Testing Grid Leaks

Radio Record, Volume III, Issue 32, 21 February 1930, Page 28

Testing Grid Leaks

‘A Source of Interference LTHOUGH the average grid leak now sold is a thoroughly reliable little accessory, especially when purchased from one of the well-known makers with a reputation to maintain, there are occasionally cases where a noisy grid leak causes all kinds of trouble, frequently attributed to other sources, such as the "B" battery or accumulator. It is very easy ‘to rig up an arrangement which will test whether your grid leak is noisy. All that is necessary is to connect a grid leak in series with the primary of an audiofrequency transformer and a couple of dry cells. The secondary of ‘the transformer should be connected to a valve in the usual way, and preferably this valve should have another audio-fre-quency transformer in its output circuit (giving a two-stage amplifier following the grid leak). For example, a three-megohm grid Jeak in geries with the primary of a transformer and a couple of dry célls, i£ correctly graded and of good quality, should pass a perfectly steady curgent of approximately one micro-am-pere, ‘ If now we listen at the output end of the amplifier with a pair of tele- phones we should hear nothing whatever save when the current is started and stopped. If, however, the leak is faulty and "noisy" (which means constantly changing its value), the current passing through the primary of the transformer will vary with the variations of resistance in the grid leak, and we shall get noises and erackles. ; The position of the grid leak in a2 receiver is such that it never has to carry more than a very small current, as the voltage applied to the grid of a detector valve is probably on the average considerably less than one volt,-so it is not ‘fair to test grid leaks, as some people do, with a "B" battery of 100 volts or so, for a grid leak which may be perfectly satisfactory for all ordinary receiving purposes is often ruined by such a test. Anode resistances and ¥Yesistance-capacity-coupling units are, ef course, tested with much higher voltages.