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 signal.
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.
Valve characteristics 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 supply currents. High power solid state transmitters also require a complex combination of transformers 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. Radio amateurs also use valve amplifiers in the 500–1500 watt range mainly for economic reasons.
Audio vs. amplifiers 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 .
An power amplifier is tuned to a single frequency as low as 18 kHz and as high as the range of frequencies, for the purpose of radio transmission or industrial heating. They use a narrow tuned circuit to provide the valve with a suitably high load impedance and feed a load that is typically 50 or 75 Ohms. amplifiers normally operate class C or class AB.
Although the frequency ranges for audio amplifiers and amplifiers overlap, the class of operation, method of output coupling and percent operational bandwidth will differ. Power valves are capable of high frequency response, up to at least 30 MHz. Indeed, many of the Directly Heated Single Ended Triode () audio amplifiers use radio transmitting valves originally designed to operate as amplifiers in the high frequency range.
Circuit advantages of valves
High input impedance Tubes' input impedance is comparable to that of ‑s, higher than in bipolar transistors, which is beneficial in certain signal amplification applications. Tolerant of high voltages Valves are high voltage devices, inherently suitable for higher voltage circuits than most semiconductors. Tubes can be built oversized to improve cooling Valves can be constructed on a scale large enough to dissipate great amounts of heat. Very high-power models are designed to accommodate water- or vapor-cooling. For that reason, valves remained the only viable technology for handling very high power, and especially high power + high voltage use, such as radio and transmitters, long into the age when transistors had displaced valves in almost all other applications. However, today even for high power/voltage, tubes are increasingly becoming obsolete as new transistor technology improves tolerance of high voltages and capacity for high power. Lower investment cost Because of the simplicity of practical tube-based designs, using tubes for applications like amplifiers above the kilowatt power range can greatly lower manufacturing costs. Also, large, high value power valves (steel clad, not glass tubes) can to some extent be remanufactured to extend residual life. Electrically very robust Tubes can tolerate amazingly high overloads, which would destroy bipolar transistor systems in milliseconds (of particular significance in military and other "strategically important" systems). Indefinite shelf life Even 60 year-old tubes can be perfectly functional, and many types are available for purchase as "new-old-stock". Thus, despite known reliability issues (see next section, below), it is still perfectly possible to run most very old vacuum tube equipment. Comparative ease of replacement Being known to be subject to a number of common failure modes, most systems with tubes were designed with sockets so the tubes can be installed as plug-in devices; they are rarely, if ever, soldered into a circuit. A failed tube can simply be unplugged and replaced by a user, while the failure of a soldered-in semiconductor may constitute damage beyond economical repair for a whole product or sub-assembly. The only difficulty is determining which tube has failed.
Disadvantages of valves Cost For most applications, tubes require both greater initial outlay and running expense per amplification stage, requiring more attentive budgeting of the number of stages for a given application compared to semiconductors.
Short operational life In the most common applications, valves have a working life of just a few thousand hours, much shorter than solid state parts. This is due various commonplace modes of failure: Cathode depletion, open- or short-circuits (notably of the heater and grid structures), cathode ‘poisoning’, and breaking the glass shell (the glass “tube” itself). Heater failure most often happens due to the mechanical stress of a cold start. Only in certain limited, always-on professional applications, such as specialized computing and undersea cables, have specially designed valves in carefully designed circuits, and well cooled environments reached operational lives of tens or hundreds of thousands of hours.
Heater supplies are required for the cathodes Beside the investment cost, the share of the power budget that goes into heating the cathode, without contributing to output, can range from few percent points of anode dissipation (in high power applications at full output), to broadly comparable to anode dissipation in small signal applications.
Large circuit temperature swings in on/off cycles Massive stray heat from cathode heaters in common low power tubes means that adjoining circuits experience changes in temperature that can exceed . This requires heat resistant components. In applications this also means that all frequency-determining components may have to heat to thermal equilibrium before frequency stability is reached. While at broadcast (medium wave) receivers and in loosely tuned sets this was not a problem, in typical radio receivers and transmitters with free-running oscillators at frequencies this thermal stabilization required about one hour. On the other hand, miniature ultra-low power direct-heated valves do not produce much heat in absolute terms, cause more modest temperature swings, and allow equipment that contains few of them to stabilize sooner.
No "instant on" from a cold start Valve cathodes need to heat to a glow to start conducting. In indirect-heating cathodes this could take up to 20 seconds. Apart from temperature-related instability, this meant that valves would not work instantly when powered. This led to development of always-on preheating systems for vacuum tube appliances that shortened the wait and may have reduced valve failures from thermal shock, but at the price of a continuous power drain, and an increased fire hazard. On the other hand, very small, ultra low power direct-heated valves turn on in tenths of a second from a cold start.
Dangerously high voltages Anodes of tubes may require dangerously high voltages to function as intended. In general, tubes themselves will not be troubled by high voltage, but high voltages will demand extra precautions in circuit layout and design, to avoid “flashover”.
Wrong impedance for convenient use High impedance output (high voltage/low current) is typically not suitable for directly driving many real world loads, notably various forms of electric motor
Valves only have one polarity Compared to transistors, valves have the disadvantage of having a single polarity, whereas for most uses, transistors are available as pairs with complementary polarities (e.g., / ), making possible many circuit configurations that cannot be realized with valves.
Distortion The most efficient valve-based RF amplifiers operate class C. If used with no tuned circuit in the output, this would distort the input signal, producing harmonics. However, class C amplifiers normally use a high output network which removes the harmonics, leaving an undistorted sine wave identical to the input waveform. Class C is suitable only for amplifying signals with a constant amplitude, such as , , and some (Morse code) signals. Where the amplitude of the input signal to the amplifier varies as with single-sideband modulation, amplitude modulation, video and complex digital signals, the amplifier must operate class A or AB, to preserve the envelope of the driving signal in an undistorted form. Such amplifiers are referred to as linear amplifiers.
It is also common to modify the gain of an amplifier operating class C so as to produce amplitude modulation. If done in a linear manner, this modulated amplifier is capable of low distortion. The output signal can be viewed as a product of the input signal and the modulating signal.
The development of broadcasting improved fidelity by using a greater bandwidth which was available in the range, and where atmospheric noise was absent. also has an inherent ability to reject noise, which is mostly amplitude modulated. Valve technology suffers high-frequency limitations due to cathode-anode transit time. However, tetrodes are successfully used into the range and triodes into the low GHz range. Modern broadcast transmitters use both valve and solid state devices, with valves tending to be more used at the highest power levels. transmitters operate class C with very low distortion.
Today's digital radio that carries coded data over various phase modulations (such as , , etc.) and also the increasing demand for spectrum have forced a dramatic change in the way radio is used, e.g. the cellular radio concept. Today's cellular radio and digital broadcast standards are extremely demanding in terms of the spectral envelope and out of band emissions that are acceptable (in the case of for example, −70 dB or better just a few hundred kilohertz from center frequency). Digital transmitters must therefore operate in the linear modes, with much attention given to achieving low distortion.
Applications
Historic transmitters and receivers (High voltage/high power) Valve stages were used to amplify the received radio frequency signals, the intermediate frequencies, the video signal and the audio signals at the various points in the receiver. Historically (pre WWII) "transmitting tubes" were among the most powerful tubes available, were usually direct heated by thoriated filaments that glowed like light bulbs. Some tubes were built to be very rugged, capable of being driven so hard that the anode would itself glow cherry red, the anodes being machined from solid material (rather than fabricated from thin sheet) to be able to withstand this without distorting when heated. Notable tubes of this type are the 845 and 211. Later beam power tubes such as the 807 and (direct heated) 813 were also used in large numbers in (especially military) radio transmitters.
Bandwidth of valve vs solid state amplifiers