Equipment designers should normally take care to ensure that valves are operated at a heater voltage which is, on an average, almost exactly equal to the rated figure. Fluctuations of heater voltage due to mains voltage changes will occur, but provided they they are not greater than ±10%, the life and performance of the valve should not be seriously affected. The recommended British practice is, generally, that heater voltage fluctuations should not exceed ±7%. These remarks apply to indirectly heated valves.
High Heater Voltage
If the heater voltage is too high, the oxide coating will tend to evaporate from the cathode, resulting in some loss of emission. Some of the oxide coating will almost certainly move across to the grid where it will emit electrons and lead to reverse grid current. Heater failure or heater-cathode leakage may also be caused by an excessive heater voltage.
Low Heater Voltage
Valves operated at low heater voltages will have a much reduced emission, but unless the valve is operated for long periods under conditions such that the anode current is large enough for the smaller space charge to be exhausted, no permanent damage will occur. At low heater voltages oscillators may stop functioning. Output valves and rectifiers which operate at nearly their maximum cathode ratings are the valves most likely to be damaged by the use of a low heater voltage. Resistance-capacitance coupled stages will often operate satisfactorily at low heater voltages if their anode voltage and current are comparatively low.
lf valves must be operated at heater voltages which differ from their correct value by more than 10%, the valve ratings (anode voltage, anode dissipation, etc.) should be reduced.
Valves For Car Radios
Valves used in car radio receivers have to withstand large changes in heater voltage (up to 30% or even 35%) because a battery having a nominal 12 Volts will supply 14 Volts or more whilst it is being charged by the car dynamo, but when the battery is being discharged, the voltage may be less than 12 Volts. Valves expressly designed for such purposes (e.g. the Loctal series) have heaters which will give a satisfactory performance under such conditions. Their nominal heater voltage is that of a battery on charge, but their normal operating voltage is 6.3 or 12.6.