 High-power RF power tubes are directly heated; the heater voltage must be much smaller than the signal voltage on the grid and is therefore in the 5...25 V range, drawing up to hundreds of amperes from a suitable heater transformer.In some valve part number series, the voltage class of the heater is given in the part number, and a similar valve might be available with several different heater voltage ratings.The system assigned numbers with the base form "1A21", and is therefore also referred to as the "1A21 system".