Obtaining Accurate Grid Bias

Radio Record, Volume III, Issue 27, 17 January 1930, Page 30

Obtaining Accurate Grid Bias

Biassing under Operating Conditions

[t goes without saying that to get the very best out of any amplifying valve exactly the right amount of negative bias must be applied to its grid. Tha "right amount" is that which under operating conditions, and when no signal is being received, brings the working point exactly in the middle (or very slightly lower than this) of the straight portion of the grid-volts-plate-current characteristic which lies to the left of the zero line. By far the best way of determining the exact amount of grid bias required is to take a curve of the valve when it is in position in the receiving set. with the normal resistance of its plate circuit in: action. We thus obtain what we may call a semi-operating curve which enables grid bias to be determined exactty To take such a curve is a very simple business, the only aid required being a milliammeter, a voltmeter and a piece of squared paper. Draw a horizontal line near the bottom of your paper and a vertical line close to the right-hand edge. Mark off the horizontal line into divisions of fifteen small. squares each corresponding to 1.5 volts, Mark off the vertical line into divisions of one whole square apiece, each indicating one milliampere. Next connect the positive terminal of the voltmeter to the ‘highest tapping of the grid battery and apply a lead

‘attached to the negative terminal to each of the other sockets in turn. The I.M,F. of a dry cell is usually taken as 1.5 volts, though it may be a good deal more or a good deal less. Make a note of each of the readings- that you obtain. Adjusting the ‘Bias. WVE will suppose that these are 1.6, 3.0, 4.7, 6.1, 7.6, 9.0, 10.4; 11.9, 13.5, 15.1, 16.6, 18.2, 19.6, 21.1, and 22.6, Plot each in along the bottom line. Since the first reading is 1.6 a dot is made at the sixteenth small square, 8.0 corresponds to the thirtieth square, 4.7 to the forty-seventh, 6.1 to the sixty-first, and so on. We are now ready to-take the curve. Switch off all valves but that to be dealt with, and set the gerid .of this at. zero potential. This, by the way, .does not mean _ simply pulling ont the grid-battery wander plug, for when this is done the grid will set itself as a rule at something rather different. The negative wander plug must

EE 4 actually be removed ‘from ‘the. battery and applied firmly to. the low-tensioén negative busbar at some. point. Note the milliammeter reading. and- plot it in by means. of a dot-on the vertical line on the right of the: paper, Now place the plug in the first socket above zero, Which means that the grid is given a bias of 1,6 volts negative. . Note the milliammeter scurrent and make a corresponding: dot opposite the number of milliamperes indicated’ on the right-hand: vertical line and sixteen small: squares to. the left of it.; Other readings:are taken .and. plotted: in the same way until’ the negative bias ‘is such that the milliammeter records the passage of no current, ; The Complete Curve, Now lay the edge of a ruler’ along your row of' dots. You: find, let us say, that ‘so far as 18.2 voltS negative all lie pretty well on a straight ‘line. Rule in the line. . ‘From the 19.6 volt reading down: to the: place at which. current becomes nil, the. line ‘is obviously. curved. .° Draw in: the curve. Now does the straight line-really. end at, 18:2 volts or should it. be continued: to. perhaps 18.5 or even 19. volts?::. It. is’ impossible to determine by further experiments with the grid battery, since nothing. closér than 14-volt steps can be obtained: from it:.. From the general shape of the curve, though,:one may infer ‘that the straight portion really runs to about 19 volts, and that: the bend. begins at this point approximately. The ideal biasing voltage then is one half of 19, or 9.5 volts. Sincé we cannot obtain this with the dry-cell. grid battery used, without any other: aid, the choice lies between 9 and 10.4 volts. To apply 10.4 is clearly , to . overdo things ; any "peak": grid swing will result in bottom bending, should rob ourselves of this. } volt of grid. bias, for we can obtain it in. quite a simple way. Instead of eolnecting the positive of the grid battery to low-tension negative, wire a potentiometer across the "A" battery leads and connect the positive of the grid battery to. its slider. This provides at once a means of making minute adjustments of the grid biasing potential, and it is well worth while to add a potentiometer to a set relying upon a small power’ valve for its output, for it enables the very utmost to be got out of the ‘valve. Final Adjustments. API UST the slider of the potentiometer so that its potential is about 1}-volts positive to the negative busbar. This.means that if the "A" supply ‘is 2 volts it will be almast-over -to the positive end of its travel; with a 4volt supply it will be: rather less than half-way, and with a 6-volt accumulator something less than one-third. ‘Now place the grid battery . wander-plug, in the socket found by "the yolnebir to give a reading’ of 10,4 volts. . The actual negative grid bias is now 10.4 volts, less’ the 14 volts due’ to: the potentiometer, or: approximately’ 9 volts negative. . By moving: the slider gradually towards ..the.: negative: -end.

= — = — the bias can be adjusted to anything between 9 and 10.4’ volts negative. "Wire. the’ milliammeter ‘into the plate circuit of the output valve, tune in the loéal station and-adjust ‘the potentiometer until the needle of the instrument ‘is as‘ nearly as possible stationary when a signal is’ coming in at normal ' loud-speaker strength. Detune, ‘if necessary, until the needle . remains : absolutely: still, and you’ then know first of all the’ exact grid bias required by the output valve’ and, secondly, the. greatest signal strength that the ‘set can produce without ‘overloade ing: due"to:distortion,.... 4. ... ok