Valve amplifier
From Wikipedia, the free encyclopedia
- 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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] Classes of operation
- Further information: Amplifier
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.
[edit] Audio amplifiers
The first application of valves as audio amplifiers 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, the primary use of the valve in audio amplification was for the 'wireless' market that began in the early thirties. When television emerged, the valve technology in use meant that the audio stages of the TV were also composed of valve amplifiers. Single ended amplifiers were invented prior to World War II and early valve audio amplifiers were mostly Class A types as efficiency and output power were not important.
As more people became interested in hi-fi, the need for more powerful amplifiers resulted, and the push-pull Class B amplifier design became popular due to its higher efficiency. The basic push-pull valve audio amplifier design has remained largely unchanged since the 1950's, following the introduction of the Williamson amplifier and many high end audio companies still produce tube power and preamps because of their perceived sound quality.
[edit] Hi-Fi amplification
Audio amplifiers for sound reproduction in high fidelity (hi-fi) applications, especially in the high-end of the market, are different from amplifiers used by guitarists and other instrumentalists. High end hi-fi components are designed to reproduce sound with great clarity/accuracy, whereas 'instrument' amplifiers, especially those used by guitarists, are deliberately intended to add a distinctive colouration to the sound. Typically harmonics, AKA harmonic distortion, and frequency boosts/cuts.
[edit] Instrument and vocal amplification
Valve amplifiers for guitars are sometimes built to a different specifications from those of hi-fi amplifiers. The valve circuits are driven past the linear amplification range, producing large amounts of harmonic distortion and creating the classic tube sound. What looks bad on a meter in fact sounds more pleasing to our ears.
[edit] Narrow band (tuned) amplifiers
Most high power radio frequency amplifiers are of valve construction.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] Advantages
- Stable, no neutralizing required normally
- Constant load on the exciting stage
[edit] 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
[edit] 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.
[edit] Advantages
- Stable, no neutralizing required normally
- Some of the power from exciting stage appears in the output
[edit] Disadvantages
- Very low gain, much more input power is required
- The heater must be isolated with greater care from the valve with chokes
[edit] 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.
[edit] 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
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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). <ref>Robert S. Symons (1998). "Tubes: Still vital after all these years". IEEE Spectrum 35 (4): 52–63. </ref>
[edit] Footnotes
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[edit] See also
[edit] References
- Radio communication handbook (5th Ed), Radio Society of Great Britain, 1976, ISBN 0-900612-28-2
[edit] 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 THDde:Röhrenverstärker
