A valve amplifier (UK and Aus.) or tube amplifier (U.S.), is a device for electrically amplifying the power of an electrical signal, typically (but not exclusively) sound or radio frequency signals. Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers during the 1960s and 1970s, and replacement valves are no longer produced in the same large quantities as they were in the past. Specially constructed valves are still in use at high power levels, especially at microwave frequencies; see the Microwave amplifiers section. Valves are high voltage/low current devices in comparison with transistors (and especially MOSFETs) and their transfer characteristics show very flat anode current vs. anode voltage indicating high output impedances. The high working voltage makes them well suited for radio transmitters, for example, and valves remain in use today for very high power radio transmitters, where there is still no other technology available. However, for most applications requiring an appreciable output current, a matching transformer is required. The transformer is a critical component and heavily influences the performance (and cost) of the amplifier. Many power valves have good open-loop linearity, but only modest gain or transconductance. As a result, valve amplifiers usually need only modest levels of feedback. Signal amplifiers using tubes are capable of very high frequency response ranges up to radio frequency. Indeed, many of the Directly Heated Single Ended Triode (DH-SET) audio amplifiers are in fact radio transmitting tubes designed to operate in the megahertz range. In practice, however, tube amplifier designs typically “couple” stages either capacitively, limiting bandwidth at the low end, or inductively with transformers, limiting the bandwidth at high end. Circuit advantages of valves * Good for high power systems * Electrically very robust, they can tolerate overloads for minutes which would destroy bipolar transistor systems in milliseconds. Disadvantages of valves * Heater supplies are required for the cathodes * Dangerously high voltages are required for the anodes * Valved audio equipment is normally heavy because of the weight of transformers * Valves often have a shorter working life than solid state parts because the heaters tend to fail * Valves are fragile and break if hit, since they are usually made of glass. Solid state components don’t have this problem.