Valve amplifier
From Free net encyclopedia
- This page is about the circuit design and applications of valve amplifiers. For valve amplifiers in audio use see Valve audio amplifiers.
A valve amplifier or tube amplifier (UK & Aus., and U.S.), is a device for electrically amplifying the power of an electrical signal, typically sound or radio frequency signals. Tube amps were largely replaced by solid state amplifiers during the 1960s and 1970s, and replacement vacuum
Tubes are no longer produced in the same large quantities as they were in the past.
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Characteristics of linear valve amplifiers
Valves are extremely high voltage / low current devices in comparision with transistors (and especially MOSFETs). This makes them well suited to, for example, radio transmitters and valves remain in use today for very high power radio transmitters, where there is still not other technology available. However, for most applications requiring current drive, this characteristic requires the use of some kind of matching transformer. The transformer is a critical component and heavily influences the performance (and cost) of the amplifer.
Many power valves have good open-loop linearity, but only modest gain. As a result, valve amplifiers mostly have only modest levels of feedback. Signal amplifiers using tubes are capable of very high frequency response ranges - up to RF. Indeed many of the DH-SET 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 or using transformers and these devices do limit the bandwidth at both the high and low end.
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
- In audio equipment, valves are credited with having a 'warm' sound when compared with solid state equipment.
Disadvantages of valves
- Heater supplies are required for the cathodes
- High voltages (mortal threat) 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
Classes of operation
Amplifier circuits are classified as A, B, AB and C. Each class defines what proportion of the input signal cycle actually flows through the amplifying device:
- Class A 100% of the signal cycle is used (conduction angle a = 360° or 2π)
- Class AB more than 50% but less than 100% is used. (181° to 359°, π < a < 2π)
- Class AB1 is a valve amp in class AB where the grid is more negatively biased than it is in class A.
- Class AB2 is a valve amp in class AB where the grid is often more negatively biased than in AB1, and the input signal is often larger. When the drive is high enough to make the grid positive, the grid current will increase. It is possible depending on the level of the signal input for the amplifier to move from class AB1 to AB2.
- Class B 50% of the input signal is used (a = 180° or π)
- Class C less than 50% is used (0° to 179°, a < π)
Audio amplifiers
Template:Main Many high end audio companies (EG: Musical Fidelity, still produce tube power and preamps for reasons involving sound quality. Properly designed modern tube amps are said to provide a warmer, richer sound than their solid state counterparts. Prices usually range from $1,000.00 up to several thousand dollars for top models. One of the most expensive amplifier the Wavac SH-883, is a tubed monoblock costing $350,000.00 a pair.
Instrument and vocal amplification
The basic valve amplifier design has remained largely unchanged since the 1950's. Valve amplifiers for guitars have also maintained a fairly large following; they are often described as being warmer and more organic sounding than their solid state counterparts.
Recently hybrid models, using both tubes and integrated circuits, have been created in an attempt to take the best from both worlds. There has also been a recent flurry of amp modellers that can quite accurately capture the "tube amp" sound.
A vacuum tube has a life of about 2-3 years.
Narrow band (tuned) amplifiers
Most high power transmitter amplifiers are of valve construction because of the high power required.
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.
Grid circuits
Active (or tuned grid)
Here is . An example of this used at VHF/UHF 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 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 feed back which is used to make the system more stable, this feed back counteracts the positive feedback which exists within a valve. an inductive coupling, they enter a resonant LC circuit. It is possible by the correct choice of the ratio of the turns to obtain a step up in drive voltage which enables this design to have a very high gain. The very high gain is a two edged sword, as it increases the risk of instability. With this type of stage 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 output to be taken away
How does it work
For a fixed anode voltage the anode current of a triode can be described by the following equation
Ianode = {K1.(Egrid-N1)} + {K2.(Egrid2-N2)} + {K3.(Egrid3-N3)} etc
For a tetrode the equation will be:
Ianode = {K1grid1.(Egrid1-N1grid1)} + {K2grid1.(Egrid12-N2grid1)} + {K3grid1.(Egrid13-N3grid1)} etc + {K1grid2.(Egrid2-N1grid2)} + {K2grid2.(Egrid22-N2grid2)} + {K3grid2.(Egrid23-N3grid2)} ... etc
Note that the K constants for the second grid are smaller than those of the first grid because the second grid is further away from the cathode.
As the second grid (screen grid) in a tetrode is maintained at a constant potential the equation for the tetrode can be reduced back to that of the triode as long as the screen grid is kept at the same potential.
In short 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. Another term for the anode in a valve is the plate so hence on many designs the anode current is named the plate current.
In the RF designs shown on this page between the anode and the high voltage supply (known by convention as B+) is a tuned circuit. This tuned circuit is been brought to resonance, and in a class A design can be thought of as a resistance. This is because a resistive load is coupled to the tuned circuit. In audio amplifiers the resistive load (loudspeaker is coupled via a transformer to the amplifier. In short the load formed by the loudspeaker driven via the transformer can be thought of as a resistor wired between the valves anode and B+.
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.
- Disadvantages of tuned grid compared the other RF designs.
- Neutralization is required
Passive grid
Here is . An example of this used at VHF/UHF 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 the screen grid 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, they are then applied to the valve's first grid directly. 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.
This passive grid design is ideal for audio equipment, this is because audio equipment must be more broadband than RF equipment. 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 (three orders of magnitude of difference).
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 spurii (such as harmonics) from an exciter is greater
Grounded grid
This design uses a triode, the grid current drawn in this system is larger than that required for the other two basic designs. Hence 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, 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 a clear need exists for the design to counteract this.
The main reason for adding screen grids to RF valves is to reduce the unwanted capacitance between the anode and the first grid.
High voltage amplifiers
Valves are more suited to high voltage output amplifiers as valves usually have higher voltage ratings than transistors. The problem with valve hv amplifiers is that they cannot normally supply much current (only a few mA) unless an output transformer is used.
Vibration table amplifiers
In the aerospace and military electronics industries, engineers often test electronic circuit cards both individually and in their equipment boxes whilst mounted on a flat vibrating table. The table is driven from a large motor similar to a moving-coil loudspeaker. The frequency of vibration can be random, or specific frequencies can be used to detect and permit the assessment of resonances. Usually high power (many kW) valve amplifiers are used to drive these tables.
Video amplifiers
See also
External links
- Conversion: distortion factor to distortion attenuation and THD
- [1] GEOFEX
- McIntosh, Renown engineer's lab constructing tube amplifiers
References
Radiocommunication handbook (5th Ed), RSGB, 1976.de:Röhrenverstärker