Valve amplifier
- For valve amplifiers in audio use see Valve audio amplifiers. This page is about the circuit design and applications of valve amplifiers.
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.
Characteristics of linear valve amplifiers
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.
Classes of operation
Valve amplifier circuits, like other types of amplifier circuits, are classified as A, B, AB and C. Each class defines what proportion of the input signal cycle flows through the amplifying devices. See the Amplifier article for more details.
History
The first application of valves was in the regeneration of long distance telephony signals, and the triode (called the 'Audion' by its inventor) was developed specifically for this purpose.
Later, valve amplification was applied to the 'wireless' market that began in the early thirties. In due course amplifiers for music and later television were also built exclusively using valves until the 1950's. During this period power levels were usually very low - indeed radio's often used headphones only, with no loudspeaker at all. The overwhelmingly dominant circuit topology during this period was the single ended triode gain stage, operating in class A, which gave very good distortion performance despite extremely simple circuitry with very few components : important at a time when components were hand made and extremely expensive.
After the invention of the transistor, the dramatic reduction in size cost and power consumption very quickly allowed transistors to displace valves in undemanding applications, however the limitations of early, especially germanium, transistors kept valves as the active device of choice for demanding and higher power applications, and during the 1960's, the increasing spread of Gramophone players, and the beginnings of "HiFi" able to drive loudspeakers to significant volume levels, combines with the spread of B&W TV represented a golden age in Valve development and also in the development of Valve amplifier circuits
Notably the introduction of Negative Feedback (by Black) which reduced distortion levels (and also gave other benefits such as reduced output impedance) revolutionised power amplifier design, the introduction of the Williamson amplifier, operating a push pull circuit in class AB1 being a milestone in the development process. Since the williamson the majority of power amplifiers intended to drive loudspeakers (Radio, TV as well as gramophones etc) have been minor variations of the Williamson topology,
From the 70's the silicon transistor became increasingly pervasive and valve production was sharply ramped down, with the notable exception of Cathode Ray Tubes, and a dramatically rationalised range of valves for amplifier applications, low power tubes being mostly dual triodes, power tubes mostly being Tetrode/Pentodes, in both cases with indirect heating
It has come to light that the Soviets retained valves to a much greater extent than the west during the cold war, for the majority of thier communications and military amplification requirements, one commonly cited reason being that tubes withstnad EMP moch better that (especially early) transistors, although this is perhaps an oversimplistic and incomplete explaination
Power triodes remain in use in extreme voltage/power applications, mainly very high power radio transmitters
Audio amplifiers
There are many categories of audio amplifier. Historically telephony was a driving application. However today the main application for valves is audio amplifiers for HiFi and performance use. These two related fields in fact have quite defferent demands and this is refelected in different design details
Hi-Fi amplification
As the name "HiFi" (Originally "high Fidelity") suggests, Audio amplifiers for hi-fi audio amplification demand extremely low levels of distortion and colouration. Frequency response is expected to be essentially flat accross the audio band (20-20,000Hz) to within a fraction of a dB, with extremely low distortion levels.
Although consumer products are today almost exclusively transistor based, the extreme upper end of the market saw a dramatic resurgence in demand for tube based amplifiers in the late 1990's, and today tube products hold a significant market share in the niche of amplifiers costing >> $1000
Preamplifiers are invariably SE triode circuits. Premaplifiers historically contained equalisation, tone and filter circuits (different kinds of circuits designed to give a controntrolled non linear frequency response). However the decline of the vinyl record, the inclusion of equalisation inside modern tape and cassette decks, and especially the introduction of digital audio devices that all have a "flat" frequency response at line level, needing neither amplifiaction (prior to the power amp) correction or euqlaisation continues to undermine the need for preamps as a mainstream product.
The majority of power amps are Class AB1 push pull circuits (usually using Tetrode/pentodes) however it is worth noting that a small nich of low power (~ 5 watt class) Single ended triode amplifiers continue to fascinate a egment of this market and very small numbers of hand builts triode power amplifiers continue to be marketted at amazing prices (>> $10,000)
It is worth noting that the simplicity of valve ampifiers, especially Single Ended deigns, combined with the ability to select components that are not is production any more and are not available in commercial volumes, makes valve amplifiers easy and very attractive for hobbiests, often extremists, to construct : there are many hobbiest constructed amplifiers, many of them unique, albeit based closely on mostly of mainstream designs.
Instrument and vocal amplification
Valve amplifiers for guitars (especially, vocals and other applications to a lesser degree) are usually designed to very different specifications from those of hi-fi amplifiers. There is usually a greatly reduced demand for fidelity and the valves are pushed much harder to maximise output power at the expense of increased distortion levels. Output stages are usually Push Pull class AB1 (often using the El34 output tube.)
Small signal circuits are often deliberately designed to have very high gain, driving the signal far outside the linear range of the tube circuit, to deliberately generate large amounts of harmonic distortion. the distortion and overdrive characteristics of valves are quite different from transistors (not least the amount of voltage headroom available in a typical circuit) and this results in a distinctive sound
Narrow band (tuned) amplifiers
Most high power radio frequency amplifiers are of valve construction.
Anode circuits
Because valves are designed to operate with much higher resistive loads than solid state devices, the most common anode circuit is a tuned LC circuit where the anodes are connected at a voltage node. This circuit is often known as the anode tank circuit.
Active (or tuned grid) amplifier
An example of this used at VHF/UHF include the 4CX250B, an example of a twin tetrode is the QQV06/40A. The tetrode has a screen grid which is between the anode and the first grid. The purpose of the screen grid is to increase the stability of the circuit by reducing the capacitance between the first grid and the anode. In RF pentodes the additional grid is another screen grid which improves the isolation between the first grid and the anode. In audio equipment the third grid in a pentode is used to cure the tetrode kink.
Neutralization is a term used in valved electronics for negative feedback which is used to make the system more stable. Negative feedback counteracts the positive feedback in valve circuits.
It is possible by the correct choice of the ratio of the turns in the inductive coupling to obtain a step up in drive voltage, allowing a very high gain. However, the high gain increases possible instability and with this type of amplifier, good layout is vital.
In common with all three basic designs shown here, the anode of the valve is connected to a resonant LC circuit which has another inductive coupling which allows the RF signal to be passed to the output.
Operation
The anode current is controlled by the electrical potential (voltage) of the first grid. A DC bias is applied to the valve to ensure that the part of the transfer equation which is most suitable to the required application is used. The input signal is able to perturb (change) the potential of the grid, this in turn will change the anode current (also known as the plate current).
In the RF designs shown on this page, a tuned circuit is between the anode and the high voltage supply. This tuned circuit is brought to resonance, and in a class A design can be thought of as a resistance, since a resistive load is coupled to the tuned circuit. In audio amplifiers the resistive load is a loudspeaker coupled via a transformer to the amplifier.
As the current flowing through the anode connection is controlled by the grid, then the current flowing through the load is also controlled by the grid.
One of the disadvantages of a tuned grid compared to other RF designs is that neutralization is required.
Passive grid amplifier
An example of a passive grid used at VHF/UHF frequencies include the 4CX250B; an example of a twin tetrode would be the QQV06/40A. The tetrode has a screen grid which is between the anode and the first grid, the purpose of which is to increase the stability of the circuit by reducing the capacitance between the first grid and the anode. The combination of the effects of the screen grid and the damping resistor often allow the use of this design without neutralization.
The signals come into the circuit through a capacitor, then are applied to the valve's first grid. The value of the grid resistor determines the gain of the amplifier stage. The higher the resistor the greater the gain, the lower the damping effect and the greater the risk of instability. With this type of stage good layout is less vital.
Passive grid design is ideal for audio equipment, because audio equipment must be more broadband, i.e., handle a wider relative range of frequencies, than RF equipment. For example, a RF device might be required to operate over the range 144 to 146 MHz (1.4% of an octave), while an audio amp might be required to operate over the range 20 Hz to 20 kHz, a range of three orders of magnitude.
Advantages
- Stable, no neutralizing required normally
- Constant load on the exciting stage
Disadvantages
- Low gain, more input power is required
- Less gain than tuned grid
- Less filtering than tuned grid (more broadband), hence the amplification of out of band spurious signals, such as harmonics, from an exciter is greater
Grounded grid amplifier
This design uses a triode, the grid current drawn in this system is larger than that required for the other two basic designs. Because of this, valves such as the 4CX250B are not suitable for this circuit. This circuit design has been used at 1296 MHz using disk seal triode valves such as the 2C39A.
The grid is kept at ground, the drive is applied to the cathode through a capacitor. The heater supply must be isolated with great care from the cathodes as unlike the other designs the cathode is not connected to RF ground. The cathodes are also at a DC potential more negative than the grounded grid, and the DC supply for the valve is likely to be more complex than the supply required for the other two designs.
Advantages
- Stable, no neutralizing required normally
- Some of the power from exciting stage appears in the output
Disadvantages
- Very low gain, much more input power is required
- The heater must be isolated with greater care from the valve with chokes
Neutralization
The capacitance which exists between the anode and the first grid provides some positive feedback within the valve. For the higher gain designs the positive feedback must be counteracted. Additional screen grids in RF valves reduce the unwanted capacitance between the anode and the first grid.
High voltage amplifiers
Valves are high voltage devices. Tubes remain robust and reliable even if faced with occasional short term overload. For very high power at high voltage (such as large TV transmitters), tubes remain the only viable technology
Valve amplifiers in TV sets
Prior to the invention of the transistor, all active circuitry in TV sets used valves. Many of the 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.
Oscilloscope amplifiers
Prior to the advent of the transistor, all amplification in oscilloscopes was performed by valves. They were usually used as dual triodes or tetrodes in differential pairs in the same envelope and there may be 3 or 4 sets of amplification per display channel. In later oscilloscopes, the distributed amplifier was employed to amplify very high frequency vertical signals before application to the display tube. This type of amplifier used a series of tubes connected at equal distances along transmission lines.
Vibration table amplifiers
Electronic circuits and systems for aerospace and military applications must operate under extreme conditions. Vibration tables, sometimes called shaker tables, are used to test the systems under simulated vibration conditions. A vibration table is usually driven by a large motor similar to a moving-coil loudspeaker, and the motor is powered by high-power valve amplifiers in the multiple-kilowatt range. The amplifier can be set to generate either randomly changing vibration frequencies, or specific vibration frequencies can be generated to detect resonances.
Microwave amplifiers
According to Symons, while semiconductor amplifiers have largely displaced valve amplifiers for low power applications, valve amplifiers are much more cost effective in high power applications such as "radar, countermeasures equipment, or communications equipment" (p. 56). Many microwave amplifiers are specially designed valves, such as the klystron, gyrotron, traveling wave tube, and crossed-field amplifier, and these microwave valves provide much greater single-device power output at microwave frequencies than solid-state devices (p. 59). [1]
Footnotes
- ^ Robert S. Symons (1998). "Tubes: Still vital after all these years". IEEE Spectrum. 35 (4): 52–63.
See also
References
- Radio communication handbook (5th Ed), Radio Society of Great Britain, 1976, ISBN 0-900612-28-2
External links
- The Audio Circuit - An almost complete list of manufacturers, DIY kits, materials and parts and how do they work sections on valve amplifiers
- Conversion calculator : distortion factor to distortion attenuation and THD