This waiting period can be an annoyance, and it seems bizarre at first to people whohaven’t dealt with tubes before.Cold cathodeIn a gas-filled voltage-regulator tube, the cathode might not have a filament to heat it.Such a device is called a cold-cathode tube. The schematic symbol for a cold-cathodetube is shown at C in Fig. 29-1. The solid dot indicates that the tube is gas filled, ratherthan completely evacuated. Various different chemical elements are used in gas-filledtubes; mercury vapor is probably most common. In this type of tube, the “warmup” periodis the time needed for the elemental mercury to vaporize, usually a couple of minutes.PlateThe plate, or anode, of a tube is a cylinder concentric with the cathode and filament(Fig. 29-2). The plate is connected to the positive dc supply voltage. Tubes typically op-erate at about 50 V to more than 3 kVdc.The triode54129-2Construction of diode tube.For clarity, electrodes areshown transparent. Actuallythey are opaque metal.Because the plate readily attracts electrons but is not a good emitter of them, andbecause the exact opposite is true of the cathode, a diode tube works well as a rectifierfor ac. Diode tubes can also work as envelope detectors for AM, although they are nolonger used for that purpose.The triodeIn a diode tube, the flow of electrons from cathode to plate depends mainly on the dcpower supply voltage. The greater this voltage, the greater the current through thetube.The flow of current can be controlled by means of an electrode between the cath-ode and the plate. This electrode, called the control grid, is a wire mesh or screen thatlets electrons physically pass through. But the control grid (also called simply the grid)interferes with the electrons if it is provided with a voltage that is negative with respectto ground. The greater this negative grid bias, the more the grid impedes the flow ofelectrons through the tube.