Eliminating Grid-Bias Batteries

Radio Record, Volume III, Issue 18, 15 November 1929, Page 29

Eliminating Grid-Bias Batteries

Introduction of Resistance IN order to maintain perfect reproduction, it is advisable to renew average set-owner, after installing this average set-dwner, after installing the battery, usually forgets all about it, and later wonders why the quality of his reception is falling off. Constant purity of reception, however, can be obtained by dispensing with the gridbias battery, and securing the necesSary . potential from the "B" battery supply. Although the following method’ is quite applicable for use with dry batteri or accumulators, it will be found most satisfactory for use’ with a mains eliminator, either A.C. or D.C. Roughly, the scheme works this way. A resistance is connected between the A battery supply and the negative terminal of the B battery supply. For a safeguard, and also for preventing motorboating, it should be shunted with a large-capacity fixed condenser. The presence of the resistance in the circuit results in a voltage drop, and this can be utilised to provide gridbias. The bias wander plug is carried to some » point on this ‘resistance, de

— pending on the grid voltage required. The one’ disadvantage is that the "B" battery voltage is reduced: by ‘ the amount of the grid voltage thus tapped off, but tsually this is of no importance when using mains. eliminators. Two or three valves can be supplied with grid-bias current in this way, but each tapping must be shunted by an earthing condenser of 2 microfarads or so The value.of the resistance depends on the B battery current, and should be calcdlated accurately when the value of grid-bias voltage required is known: A resistance which can be -varied from 500 to 2000 ohms is generally quite suitable.