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In Chapter 1 It Was Shown How to Design Triode Gain Stages, Which Are the Main Building Blocks of Guitar Preamplifiers

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In Chapter 1 It Was Shown How to Design Triode Gain Stages, Which Are the Main Building Blocks of Guitar Preamplifiers In chapter 1 it was shown how to design triode gain stages, which are the main building blocks of guitar preamplifiers. Most readers will know that small-signal pentodes as opposed to the power pentodes found in power-output stages, also appear in some amplifiers and this chapter will discuss how they are used. Interestingly, it seems pentodes were once more popular with manufacturers than they are now. New pentodes are usually more expensive than the popular double-triodes and usually contain only one tube per envelope which may account for their lack of use in commercial amplifiers. However, the independent builder is not bound by such constraints. The most popular current-production pentode is the EF86/6261 but there are also many others available as NOS, which can be put to good use by the inventive designer. Useful pentodes include the EF91, EF93/6BA6, EF94/6AU6, 6AM6, 6BR7, 6SJ7 and 5879. There are also many triode-pentode tubes such as the ECF80, ECF82/6U8A, 6GH8, 7199 and 6BL8. The design of such stages is slightly more involved than for triodes but not difficult, provided we understand how pentodes work. In addition to the cathode, control grid and plate found in the triode, the pentode contains two more electrodes: the screen grid and the suppressor grid. The screen grid is normally held at a high voltage and accelerates electrons toward the plate at greater velocity than could be achieved with the control-grid alone. The electrons hit the plate with such force that other electrons may dislodge from its surface and these are known as secondary electrons. At low plate voltages it is possible for more electrons to be knocked out of the plate than received by it and without the suppressor grid the secondary electrons would be collected by the nearest positive electrode, the screen grid, meaning total plate current would appear to be flowing backwards. This would result in the tube having negative resistance over a part of its operating characteristics, rendering it prone to oscillation and making it useless for audio amplification. To prevent this, the suppressor grid is interposed between the screen grid and the plate and is made considerably more negative than the plate. Secondary electrons are prevented from reaching the screen grid since they are repelled by the suppressor grid, back to the plate, as illustrated in fig. 3.1. The suppressor grid is made more negative than the plate by connecting it directly to the cathode or to ground. In many pentodes it is already internally connected to the cathode. If there is any impedance in the cathode such as an unbypassed cathode bias resistor then, connecting the suppressor to ground will dramatically increase the 2nd and 3rd harmonic distortion. This is detrimental for hi-fi designs but might be useful in a guitar amp. Apart from this, the suppressor grid is regarded as benign and plays no other part in the design of the stage. If in doubt, connect it directly to the cathode. Some pentodes, including the EF86, have an internal screen around the whole tube to shield against external electromagnetic (EM) interference. This should be connected to ground or to the cathode, whichever is most convenient. The screen grid (g2): The screen grid is the most important electrode in the pentode and has the greatest influence over its operation and we must understand it’s function before proceeding. Many designers do not fully understand the screen grid and this inevitably leads to unreliable, unpredictable or noisy circuits. This may be because there is no solid state equivalent to the pentode, so its operation may seem elusive to those with only solid-state experience. The screen grid gets it name from the fact that it shields or screens the control grid and cathode from the plate. In a triode the only thing which attracts electrons towards the plate is the plate's EM field. In a pentode the screen grid is able to attract or impede electrons, depending on the strength of its EM field. In a triode, capacitances between control grid and plate appear to be multiplied due to the large voltage swings at the plate, the Miller effect but in a pentode the screen grid is usually held at a constant voltage so, the control grid does not see the voltage swing at the plate, greatly reducing the Miller effect and allowing a pentode to have a very wide bandwidth. Furthermore, changes in plate voltage will not greatly affect plate current, since the screen-grid's fixed voltage continues to accelerate electrons towards the plate regardless of changes in plate voltage. This makes the tube more efficient, since it effectively magnifies the controlling element of the control grid, allowing the plate to swing much lower than is possible with a triode. The effect is obvious when looking at the static plate- characteristics graph. The grid curves are nearly horizontal and not dissimilar to a transistor's characteristics, indicating that for a given bias voltage, plate current will remain substantially constant even though the plate voltage may vary intensely. This indicates that the pentode has a very high plate resistance ra so, the pentode is sometimes referred to as a constant-current device. The grids in a multi-grid tube are arranged concentrically. The control-grid wire is nearest the cathode and is wound with close spacing so that its electromagnetic (EM) field is dense, providing excellent control over the electron flow. The screen grid is wound more coarsely but its voltage is usually high. Most electrons pass between its wires and are further accelerated by its strong EM field, towards the plate. Some electrons do strike the screen grid and form the screen current, Ig2, though it is much less than the plate current. The suppressor grid is wound even more coarsely and is so negative compared to the plate and screen grid that it collects no electrons. The total current flowing through the cathode is the sum of the plate and screen currents and this may need to be taken in account when choosing a cathode resistor. The characteristics of a transistor are fixed when the device is manufactured and the circuit designer has no control over this. Similarly, the characteristics of a triode are assumed to be fixed. In theory we could alter them somewhat by changing the heater voltage, though it is not recommended since it may adversely affect the tube's working life and is not a very useful or predictable method of control. The characteristics of pentodes, on the other hand, are controlled by the screen voltage. By manipulating the screen voltage we can morph the characteristics from those of high current, low input sensitivity and high gain, to those of low current, high input sensitivity and lower gain or we can turn the pentode into a triode. This is best understood by looking at the change in the plate characteristics as the screen voltage is varied from a high value to a low value. The oscillograms in fig. 3.2 show the measured characteristics of a Mullard EF86, probably the most popular small-signal, audio pentode ever produced. For ease of comparison, the plate current and plate voltage scales and the grid-curve divisions have been kept the same in each graph. The top-left graph shows the static plate characteristics when the screen voltage is fixed at l30V which is relatively high for an EF86. The higher the screen voltage, the stronger its EM field will be and the more powerfully it will attract electrons, so the greatest plate current is able to flow in this situation. The input sensitivity of the pentode is lowest in this situation since the control-grid voltage must be varied by a large amount in order to override the constant attraction of the screen voltage, so the pentode would be able to handle the largest input signals before clipping. Note that the grid curves are virtually horizontal, indicating an extremely high ra and µ, much too high to calculate from the graph. The grid curves are the most widely spaced when the screen voltage is high, indicating that the transconductance and the possible gain is greatest in this situation. It is also clear that because the grid curves are virtually parallel above Va = 50V, the gm is constant for a given bias voltage. The value of gm varies with bias voltage, being greater at lower bias voltages, which make the pentode more non-linear for larger input signals. The top-right graph shows how lowering the screen voltage to 100V squashes the grid curves down, since the strength of the screen's EM field is reduced, for a given bias voltage the plate current is reduced. The transconductance has reduced somewhat, so the possible gain is less, while the input sensitivity has increased. Lowering the screen voltage to 70V compresses the curves down even further, continuing the trend. When the screen voltage is very low then its EM field is minimal and it will have a weak influence on electrons. In the bottom-right graph the screen voltage is reduced to 30V and the transconductance is very low. However, the input sensitivity is very high, since small changes in control-grid voltage now have a greater effect on plate current. Reducing the screen voltage to zero would almost completely cut-off all plate current. Fig. 3.1: Simplified operation of a pentode. The screen-grid accelerates electrons towards the plate. The suppressor-grid repels secondary electrons back to the plate. Fig. 3.2: Lowering the screen voltage squashes the grid-curves down, reducing gm and increasing ra.
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