GRID-LEAK VALUES

Sun (Auckland), Volume III, Issue 705, 3 July 1929, Page 14

GRID-LEAK VALUES

EFFECT ON QUALITY EXPERIMENTS NECESSARY Because a .0025-mfd. condenser and a 2-megohm grid leak are generally specified in connection with valves used as detectors, it has become almost a religious belief that these values are correct and beyond reproach. Anyone who has done a bit of experimentation soon arrives at the conclusion that the grid leak may run anywhere from i to 8 megohms, depending on the valve, the circuit and the signal strength, while the grid condenser may give better results when .001-mft capacity than the dogmatic ,003-mfd. However, the condenser is not as critical in its contribution toward better radio results as the grid leak, hence we shall confine ourselves to the latter. With the usual three-electrode valve there are two methods of obtaining rectification, namely: (1) By means of a bias battery, and (2) by means of a grid leak, which is the more popular method. The grid leak method is more sensitive than gridbias detection. However, marked distortion may take place with this method on powerful signals unless the resistance value of the grid leak is materially reduced. Thus for distant reception, or the detection of weak signals, grid leak values as high as 5 megohms may be employed, with surprising increase in loud-skeaper response, while on local and powerful signals the grid leak value may be reduced to 2,1, or even h megohm, with ample loud-speaker volume accompanied by far less distortion. With the introduction of the grid-leak clarostat, representing a resistance range of from .1 to 10 megohms, with precise adjustment and permanent resistance of any value. It becomes possible to place the grid leak function on the same precision basis as capacity or inductance in the radio circuit. Noise, heretofore the main fault of variable grid leaks, has been entirely eliminated. Conditions Vary The indirectly-heated valves require a critical grid leak for best detection. The value Is not known without trial, as valves and other conditions vary widely. Certain A-C valve manufacturers recommend trying several values between 1 megohm and 9 megohms.

With a variable grid leak, however. It becomes possible to determine the correct value. The variable grid leak also acts as a satisfactory volume control and eliminates the need for a separate device for this purpose. In short-wave reception the grid leak is quite critical in value. Experimenters sometimes try as many as fifty grid leaks before the satisfactory value has been found. The best value is usually between 4 and 8 megohms. Aside from the detector, the grid leak has an important function to serve in resistance and impedance coupling. While the plate coupling resistance usually is not critical, the grid resister is of considerable importance. By means of precise resistance of grid leak it becomes possible to employ larger coupling condensers, therefore passing lower frequencies for the desired bass notes, while effectively preventing blocking or choking by lowering the grid-leak resistance. Resistance Coupling It is well to note In passing that resistance coupling is capable of responding to very low frequencies, with the result that there is sometimes set up a low-frequency oscillation, due to coupling between circuits, known as “motor-boating.” The simplest way to offset motorboating when it does occur is to use a lower resistance of grid leak for one or more stages, together, with smaller blocking or coupling condensers. Often the lowering of the grid-leak resistance in the first or second stage will serve the purpose. In this respect a variable grid leak, substituted for the first or second fixed grid leaks, will serve as a simple cure. TIPS FOR LISTENERS If you are using a soldering paste which spluters a little, do not let this get on to the panel, but stick down a piece of paper over the terminal shanks or soldering tags, and tear it away when the operation is completed. * * * If you use a fairly large solderingiron, remember that It is not necessary to tin all the sides of this, and it is much easier to keep one side thoroughly clean. W * If you are trying short-wave reception for the first time, remember that a swinging aerial may give trouble on short-wave work, although no fading effect was noticed when receiving ordinary broadcasting. s * # Variations in the volume of sound do not affect the ear so much as might be expected, and most people would be unable to detect a difference of even up to 25 per cent, on certain notes. * * s When trying to drive a screw into a place where it cannot be held with the fingers, get a slip of paper or stiff cardboard, pass the screw through the end of it and hold it in position with one hand while you drive the screw with the other. * * * The liner, S.S. Olympic, works on a i wave-length of 20 metres, under the call-sign GLSQ. * * * When overhauling your aerial remember that a large shell type insulator may be employed instead of a pulley at the mast head. Provided it is of ample proportions, there is then little danger of the wire supporting the aerial fouling or breaking.