All Mullard valves designed for operation in television series heater chains now possess equalised heating characteristics.
The current flowing in the heaters of valves comprising a series heater chain depends on the total resistance of the chain, but the voltage developed across one heater depends almost directly on the resistance of that heater. The resistance of the heaters depends on their temperature, and is greater when the valves are hot than when they are cold. When a receiver is switched on, if one valves warms up more rapidly than the rest, the effect on the current will not be appreciable, but the increase in the voltage developed across the heater of that valve can exceed the amount above the nominal heater voltage (50%) which can be tolerated during the warm-up period. This can shorten the life of the valve considerably, and to prevent it, heater chains have normally been designed to incorporate a thermistor. The resistance of the thermistor is high when it is cold, but falls as heat is generated in the thermistor by the heater current. The thermistor thus reduces the rate of increase of the heater current, and prevents unequal rises in the temperature of the heaters from producing an excessive voltage across any heater.
The rate at which the temperature of a valve heater rises depends very much on the mounting of the heater in the valve. Any point of contact between the insulated heater and the surrounding cathode sets up a small drainage of heat away from the heater and thus retards the rate at which the temperature rises. Since the space inside the cathode is small, it is difficult with mass-production methods to ensure consistent positioning of the heater within the cathode, so that the number and extent of these contacts, and therefore the rate at which the heater temperatures rises, can vary considerably from one valve to another. Now, however, Mullard have developed accurate and carefully controlled methods of manufacture which produce equalised rates of temperature rise in all their television valves. Extensive tests with a large number of their valves in typical heater chains have shown that these equalised heating properties ensure that, without added protection, the voltage developed across any heater will not exceed the permitted 50% above nominal during the warm-up period. Use of Mullard television valves thus enables heater chains to be designed without a thermistor, and this, in addition to the economy, leads to shorter warm-up times and a faster appearance of the picture on the screen.