Richardson's handbook of projection (1930)

1362 HANDBOOK OF PROJECTION FOR always check the filament current, adjusting it to 270 mils if it does not already stand at that value. When the plate current under test, with the possible exception of a shorted input transformer in the 42A amplifier, is all right, then the grid circuit of the tube under test probably is also all right. When the tube is taking its proper plate and filament current, there can be certain troubles only in the grid circuits, in case of VI, namely an open in the secondary circuit of the input transformer; an open in the potentiometer, which is the gain control across the secondary; a break in the wiring between the secondary and the potentiometer, or a ground on the high side of either of these. Incidentally, a ground on the low side of either of these might cause trouble, but probably would not. A ground here would short the 100,000 ohm resistance Rl and the worst results to be expected from this would be uneven amplification at different frequencies. This is very unlikely, however, as the various circuits in this amplifier are very well filtered. A grounded grid lead or a ground at the grid terminal of the socket, has the same effect as a ground on the high side of the secondary or the potentiometer. Obviously a ground at any of these points will result in no sound, except the ground between the transformer and Rl. With an open potentiometer the sound usually comes through all right, unless the break happen to be above the point where the wiper arm happens to be, in which case you can shove the gain up and thus again having sound. The potentiometer should, however, be replaced or else repaired as soon as possible, because while jumping across the break will cause the potentiometer to function again,