Figure 16 depicts the basic schema of Single Ended vacuum tube amplifier built around a tetrode (or a pentode, here the suppressor terminal is not shown) in ultra-linear configuration (see Section 2.2.4) and an output transformer.
The load of the vacuum tube is an output transformer. It is used to couple the vacuum tube and the speaker. In fact, the required load impedance of a power vacuum tube is generally much higher than that provided by commercial speakers. The output transformer transforms the speaker impedance into the load impedance needed by the vacuum tube.
As an example, Figure 17 depicts the average anode characteristics of the EL34 power vacuum tube. Red lines represent various loadlines corresponding to various loads applied to the anode of the vacuum tube. The maximum power that can be dissipated by the anode of an EL34 is 25W, corresponding to the dashed line marked as “Wa=25W”. All loadlines lay below this dashed line. In the figure, each loadline is labelled with the corresponding load impedance.
We can see that the various possible load impedances vary from 1.1K Ohm to 5.4K Ohm. These values are all far from reasonable values of impedance of commercial speakers, where impedance generally ranges between 4 Ohm and 16 Ohm. Accordingly, the main purpose of the output transformer is to adapt the impedance of the speakers to the impedance required by the power vacuum tubes, as discussed more in details in Section 4.1.2.
The DC voltage V+ is applied to the anode of the power vacuum tube through the centre tap of the output transformer primary. The output transformer also eliminates the DC applied to the anode, so that speakers just see the AC corresponding to the amplified signal.
The screen terminal of the power vacuum tube is connected to a tap from the primary of the output transformer, corresponding to the wanted percentage of the anode output signal, to obtain the ultra-linear configuration (see Section 2.2.4). Many power amplifier designs use a percentage around 43% of the signal to be given to the screen.
The resistor Rscreen, called the screen stopper, which connects the screen to the transformer screen tap, is mainly used to limit the current from the screen, and to avoid parasitic oscillations of the circuit. Screen current becomes dangerous when the power vacuum tube is overdriven. Overdrive should be avoided in Hi-Fi amplifiers, even if it is generally used in guitar amplifiers. Typically, values around 1K Ohm 2W are used. Smaller values or no resistance at all, is sometimes used in Hi-Fi amplifiers.
The resistor Rg, called the grid stopper, is used to block high frequency parasitic oscillations and reduce radio interference. The internal components of a vacuum tube produce some parasitic capacitances, generally referred as the Miller effect. The grid stopper forms a low-pass filter with these capacitances. The value of the grid stopper mainly depends on the capacitances. For example, values around 4.7K Ohm are generally used for EL34 power vacuum tubes.
Rl is the grid leak resistor. The vacuum tube receives the grid bias voltage through this resistor. Rl offers a high impedance path to ground to the AC signal coming from the previous stage and, as we will see in section 4.2.1, contributes to determine the loadline of the previous stage. Finally, Rl has also the important function of discharging the positive charge that might accumulate on the grid because of the gas ions forming in the vacuum tubes. This effect is very dangerous since it can cause thermal runaway and destroy the vacuum tube. When positive charges accumulate on the grid, the grid becomes less negative and the vacuum tube conducts more. More current in the vacuum tube produces more gas ions, which accumulate on the grid, and more current that will damage the vacuum tube itself. The correct value for the grid leak is a matter of compromise. Large values are preferable to provide previous stage output signal with a high impedance to ground. Small values are preferable to better help the grid to discharge the accumulated positive gas ions. Datasheets generally provide maximum allowed values for the grid leak, in terms of maximum impedance from the grid to the cathode. For instance, Philips EL34 datasheet specifies maximum 0.5M Ohm in class B, or 0.7M Ohm in class A or AB.
When fixed biasing is used, as in our example, a high value decoupling capacitor Cd is also generally connected, from the –Vg side of Rl, to ground. This capacitor, discussed in Section 3.6.1, prevents the input signal from reaching the grid of other vacuum tubes, by providing these residual signals with a very low impedance path to ground.
The capacitor Cc is the coupling capacitor. It blocks DC in the signal arriving from previous stage. AC continues to the grid through the grid resistor, and to ground through the grid leak resistor. The coupling capacitor Cc and the grid leak Rl form together a high-pass filter. The value of the capacitor has to be chosen according to the desired low cut-off frequency. Note that no current goes in the vacuum tube, since the grid has a very high impedance and receives just a voltage signal.
|Example 6: Inter-stage coupling capacitor of the power stage
Suppose the grid leak is 200K Ohm and we want a cut-off frequency at 7 Hz. We can use the low-pass filter equation to obtain the capacitance of the coupling capacitor: