6.2 Design of a vacuum tube amplifier input and phase splitter stages

In previous section, we noted that the maximum possible peak amplitude of the input signal to the power stage is 12V. With such a signal, the amplifier delivers its maximum power. Accordingly, when designing the input and phase splitter stages we should guarantee that they are actually able to provide the power stage with this voltage, in correspondence of the input signal received by the input stage. Many audio sources provide a signal with a maximum amplitude peak around 1V, and some CD players and DAC can even reach 2.5V or 3V.

To have maximum output power with a 1V peak amplitude input signal, we need a combination of input and splitter stage with a gain at least of 12 (or 21.58 dB).

12AX7 vacuum tubes are good candidates for this job. In fact, in Section 3.4, we said that 12AX7 vacuum tubes can provide a gain greater than 60 (around 35.56 dB). This is much more than what we need. However, this also gives a lot of space to design the negative feedback circuit of the amplifier, which, on one hand reduces harmonic distortion, on the other hand reduces the gain of the amplifier.

We will use a directly coupled concertina configuration, as discussed in Section 4.3.1. In this configuration there is no coupling capacitor between input and splitter stage, and the anode of the input stage also provides the grid bias voltage to the concertina vacuum tube.

The complete schema and the values of the various components is given in Figure 50. Let us see how these values were chosen.

6.2.1    Concertina phase splitter

The high-tension voltage of the concertina is set to 290V, the cathode resistor R^s_k and anode resistor R^s_a have both a resistance of 100K Ohm. This gives a total load of 200K Ohm and produces the DC loadline depicted by the red line in Figure 51. Under AC operation, the two grid leak resistors of the power stage are in parallel with the anode and cathode resistors of the concertina vacuum tube. The AC loads seen respectively at the anode and the cathode becomes 100K∙200K/(100K+200K)=67K Ohm, for a total of 134K Ohm. Suppose we set the operating point of the concertina at 94.5V and 0.98 mA, as depicted by the red spot. This, along with the AC load, gives the AC loadlinedepicted by the green line in the figure. The concertina will operate along this loadline.

The figure shows the anode characteristic graph of a 12Ax7 vacuum tube. Red line is the DC loadline of the concertina phase splitter. Green line is its AC loadline. Pink line is the loadline of the input stage. Operating point, as well as operating intervals are reported.

We already mentioned that the maximum possible peak amplitude of the input signal to the power stage is 12V. Anode and cathode of the concertina phase splitter should be able to move from their quiescent voltage of such quantity. The voltage indicated in the anode characteristic graph, in Figure 51, represents the voltage measured between the anode and the cathode of the vacuum tube. In a concertina, when anode voltage increases, cathode voltage decreases of the same quantity. This means that when the anode voltage increases of 12V, the cathode decreases of exactly the same amount. Therefore, the maximum voltage increase measured between anode and cathode, with respect to the quiescent voltage, is 24V. The orange and cyan spots, in Figure 51, highlights the maximum and minimum anode to cathode voltage (+24V/-24V with respect to the quiescent voltage), which correspond to the 12V output signals seen at the anode and the cathode. This interval represents the operating range of the concertina phase splitter, when providing the power stage with the needed signal for maximum output power. We can see that the operating range is in a rather linear area and has a reasonable distance from the vacuum tube saturation, reached when V_g=0.

In Section 4.2.2, we already mentioned that the voltage gain of the concertina phase splitter is A=0.98. Expressed in dB we have A_db=20∙log(0.98)=-0.18 dB.

6.2.2    Input stage for directly coupled concertina

The high-tension voltage of the input stage is set to 280V. With an anode resistor Rⁱ_a of 220K Ohm we obtain the loadline depicted by the violet line in Figure 51.

In a directly coupled concertina, the anode of the input stage vacuum tube provides the grid voltage bias to the concertina. The concertina operating point depicted by the red spot in Figure 51, corresponds to a grid to cathode voltage of approximatively V_g =-0.6V. The concertina quiescent current is 0.98 mA, therefore, the cathode voltage, elevated by the cathode resistor, is 100K Ohm∙0.98 mA= 98V. To provide the concertina vacuum tube with the correct grid to cathode bias voltage of -0.6V, the input stage should be configured so that the anode quiescent voltage is very close to 98V-0.6V=97.4V.

According to the input stage loadline, with a grid bias voltage V_g=-0.7V we obtain an anode to cathode voltage of 96.7V and a current of approximately 0.8 mA. This can be obtained using the self-bias technique, described in Section 3.6.2, with a cathode resistor Rⁱ_k of 0.7V/0.8mA= 870 Ohm.

With this configuration, the anode to ground quiescent voltage of the input stage is 96.7V+0.7V=97.4V, which is what needed to provide the bias voltage to the grid of the concertina phase splitter and to use directly coupling between the two stages.

In order to eliminate the local feedback effect and the gain loss, introduced by the cathode resistor, we bypass it with a bypass capacitor Cⁱ_k,of 150μF, as discussed in Section 3.6.3 and Example 5.

The grid of the 12AX7 vacuum tubes should be connected to the grid stopper resistor Rⁱ_g, to produce low pass filters for very high frequencies, exploiting the Miller effect, as discussed in section 4.1.1 and other places in this book. Values around Rⁱ_g=47K Ohm are generally used for 12AX7 vacuum tubes.

Input signal arrives to the input stage grid through the volume potentiometer R_v. This potentiometer also acts as a grid leak for the input stage and determines the input impedance of the amplifier. We chose a logarithmic potentiometer of 100K Ohm for this purpose. The input coupling capacitor C_c isolates the amplifier from possible DC voltage arriving with the input signal. It basically forms a high-pass filter with the potentiometer. A capacitance of 0.22μF, with a 100K Ohm resistance provides a 7Hz cut-off frequency.

The concertina phase splitter has a gain near to unity, as discussed in Section 4.2.2. Therefore, also the peak amplitude of the output signal of the input stage should be around 12V, as required by the power stage. The green and brown spots, in Figure 51, show the operating range of the input stage, when the anode voltage signal (the output signal of this stage) swings from -12 to +12V, with respect to the quiescent condition. We can see that the operating range is again in a linear area and far from the vacuum tube saturation (V_g=0).

The voltage gain of the input stage can be computed as discussed in Section 3.4 as

.

Expressed in dB, we have A_db=20∙log(74.6)=37.45 dB.

As we said before, this is much higher than what required and gives us space for conveniently setting the global negative feedback circuit, as discussed in next section.