The Significance of Grid Voltage

Radio Record, Volume II, Issue 13, 12 October 1928, Page 30

The Significance of Grid Voltage

1 continuing his lectures from 2YA, Saturday, September 29, Mr. W. M. Dawson dealt with the important question of grid voltage. This is constantly referred to through the columns of the "Record," and readers should acquaint themselves with these terms.

"rea KH have noted (said Mr. it Dawson, in continuing his: lectures from 2YA) that the sole purpose of the filament is to emit a vane stream of electrons which are minute particles of negative electricity. The plate or anode is postively charged by connecting it to the positive terminal of the B battery (through the ’phones or speaker) in order that it would attract the elec. trons emitted by the filament. The more positive, or the greater the charge on the plate the more electrons flow across from filament to plate. The grid is placed between the filament in order to most effectively control the grid. The voltage of this grid battery is varied in successive steps and a plate current reading taken for each and shown on the graph. This control is eitected by changing the charge on the grid: When the grid is made positive the negative electrons are hurried along in their passage to the plate and more get across. When the grid is made negative, numbers of the electrons are repelled back to the -_

filament (as like charges repel) and fail to reach the plate at all. Remember then that the more positive the grid is made on the plate, the greater the electron flow. This is often spoken of as the plate current or plate "draw." The more negative, that is the less positive the grid or the plate, the smaller the electron or plate current flow. It can be shown in graphical form just how the plate current varies in sympathy with the changes in plate voltage and grid voltage. Mr. Dawson then directed listeners to construct a graph which shows at a glance what would need a_e great amount of time and concentration to obtain from a set of figures. If a milliameter is connected in the circuit diagram published in the "Record" of Friday, September 28, and a grid bias (C) battery placed between the terminals marked G.V., the negative being towards the grid, the effect of varying this battery can be readily noticed. The curve appears on this page.

To commence (continued Mr. Dawson), I will disconnect the grid battery altogether and join the terminals so that the grid is directly connected to the negative end of the filament and therefore at zero potential with respect to it. I make the plate battery 1590 volts. Looking at the milliameter I find that it shows a current of 16 milliamperes, represented on the vertical line of the graph. As there is no voltage on the grid this vertical line. from 0 to 16 represents the curve. The lecturer went on to describe that as grid voltage is applied it is noticed that the reading on the milliameter deereases until when 16 volts are applied the meter fails to register. When these are graphically represented the result is a curve, as shown in the diagram. It is noticeable that while the top of the eurve is almost straight, the bottom is gently cuprved. A SIMILAR graph can be plotted when 100 volts are applied to the plate, it being noticed that when there is no grid bias the meter reads 8 and

fades out when 12 volts are applied. Any number of curves can be thus constructed, but two will suffice to explain the principal characteristics of the valve. HB first one of these is the amplitication factor. In practically every valve set all the valves but one are there for the purpose of amplifying or magnifying the impulses impressed on them. This amplified impulse is passed on to the next valve to be further amplified. Upon the amount of amplification given by each valve, and the number of amplifying valves used, depends the total amplification of the set. and given a certain input signal the louder the results will be. The amplification factor of a valve is an important one, other factors remaining unchanged, the higher the amplification factor of the valve the louder the signals. The fact that an increase in plate voltage causes an increase in plate current can be checked. The 100-volt curve crosses the zero grid volts line at 8 milliamperes, while the 150-volt curve crosses at 16 milliamperes. Thus an increase of plate volts of 50 has caused an increase in plate current of 8 milliamperes. Grid voltage and. plate voltage therefore both control] plate current and the amplification factor is a measure of the relative effectiveness of the grid and the plate in this control of plate current. R. DAWSON will be heard next on October 18, when he will explain the important terms amplification fac-

aoe t tor, internal resistance, and slope. A full understanding of these is essential to everyone who wants the best from his set and few are more able than Mr. Dawson to explain their significance. The graph is the one Mr. Dawso directed listeners to prepare during h lecture on Saturday, September 29. It will be referred to again during his next lecture on October 13.