A Hint for Using Damaged Valves
One sometimes finds that valves with thoriated filaments show a falling off of emission after they have been in use for some time, or the drop may happen even though little work has been done if the filament is much overheated. This is caused by the fact that the layer of thoria on the surface of the tungsten is driven off. The valve therefore becomes temporarily a 'bright emitter', and will not work with less than 4 or 5 Volts unless some means can be found of bringing a further supply of thoria to the surface. This can be done, if one is careful, by 'flashing the valve'. Connect one of the filament prongs to the positive terminal of a high-tension battery giving about 50 Volts. Attach a wire to the other, and with it just flick the negative terminal, making contact for the briefest possible instant. The valve should now be tried again to see whether an improvement has been made. If not, the process should be repeated until an increase is observed. There is, of course, always the risk of burning out the valve by making a contact accidentally for a fraction of a second too long. However, as a valve whose emission has fallen off is not of very much use to anyone, we may regard this as a desperate case which justifies a desperate remedy.
Before you undertake flashing make quite sure that the emission has fallen off. I was using a DEV the other day in whose plate circuit the milliammeter showed no readable current. Investigation, however, proved that the filament in this case was not at fault. The little metal caps of these valves are liable to become in time rather dull, as are the clips into which they fit. When this happens in the case of the plate contacts the emission may fall off to almost nothing. This is actually what had occurred with the valve mentioned. A rub with a piece of fine emery cloth put matters right at once. Four-pin valves may suffer in the same way, and their prongs should be brightened every now and then, especially if they are left in the ordinary way for long periods without being removed from their sockets.
Operating for a while at bright emitter temperature would cause thoria to diffuse to the surface and prolonged running at the high temperature would cause the thoria to evaporate. Thus flashing at 4 to 5 Volts should work as well.(ed)