Valve RF amplifier

A valve RF amplifier (UK and Aus.) or tube amplifier (U.S.), is a device for electrically amplifying the power of an electrical radio frequency .

Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers during the 1960s and 1970s, initially for receivers and low power stages of transmitters, transmitter output stages switching to transistors somewhat later. Specially constructed valves are still in use for very high power transmitters, although rarely in new designs.

Valves are high voltage/low current devices in comparison with transistors. Tetrode and pentode valves have very flat anode current vs. anode voltage indicating high anode output impedances. Triodes show a stronger relationship between anode voltage and anode current.

The high working voltage makes them well suited for radio transmitters and valves remain in use today for very high power short wave radio transmitters, where solid state techniques would require many devices in parallel, and very high DC supply currents. High power solid state transmitters also require complex combining and tuning networks, whereas a valve-based transmitter would use a single relatively simple tuned network. Thus while solid state high power short wave transmitters are technically possible, economic considerations still favor valves above 3 MHz and 10,000 watts. Amateurs also use valve amplifiers in the 500-1500 watt range mainly for economic reasons.

Valve audio amplifiers typically amplify the entire audio range between 20 Hz and 20 kHz or higher. They use an iron core transformer to provide a suitable high impedance load to the valve(s) while driving a speaker, which is typically 8 Ohms. Audio amplifiers normally use a single valve in class A, or a pair in class B or class AB.

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Wikipedia