To drive the tone stack the bright channel needs an extra gain stage to overcome tone stack attenuation. It also requires a low output impedance to drive the stack's relatively low input impedance. To satisfy these requirements Vox uses two directly-coupled triodes.
The triode on the left is configured the same as the bright and normal channel preamps, except that the cathode resistor RK1 is not shared by two triodes and often is not bypassed by a large capacitor. The lack of sharing shifts the DC operating point for the same parts values and the absence the capacitor lowers gain and increases output impedance. The effect on gain is significant. The increased output impedance is irrelevant in this case, because the output is directly coupled to the grid of the next triode, which has a very large input impedance.
The second triode operates as a cathode follower, which is widely used to drive tone stacks because of its low output impedance. An explanation of its operation is described in the text and figures accompanying our Cathode Follower Output Impedance Calculator, so here we will simply use the calculator to verify that the output impedance is low enough to drive the AC30 tone stack.
The parts values for the voltage amplifier are
RL = 100k
RK1 = 1.5k
RK2 = 56k
The plate supply voltage VPP is 320 volts minus a drop across a 10k power supply filter resistor. The amount of the drop depends on the idle plate currents of the two tubes. To compute the currents, on the other hand, we need to know the value of the plate supply voltage. Let's assume that the first triode draws 1mA and the second draws 3mA, for a total of 4mA. This gives us a voltage drop of 40 volts, making the plate supply voltage VPP = 280. The load line (red) and grid lines (blue) for RL = 100k and RK1 = 1.5k are plotted here:
The lines intersect at a DC grid bias voltage of minus 1.45 volts. The DC operating point is thus defined by a quiescent grid voltage, plate voltage, and plate current of
VGQ = -1.45 volts
VPQ = 180 volts
IPQ = 1mA
This means that the voltage at the plate relative to ground is 180 volts plus a volt or two across RK1. Because the magnitude of the grid-to-cathode voltage for the cathode follower's triode is very small, the voltage across its 56k resistor RK2 is also approximately 180 volts, which means that the idle plate current through the resistor is about
IPQ = 180 / 56k = 3.2mA
The idle plate voltage (always measured relative to the cathode) is the plate supply voltage minus the cathode-to-ground voltage:
VPQ = 320 - 180 = 140
These results appear to place the operating point slightly above the zero-grid-voltage curve, which is difficult to achieve when driven by a high output impedance. We expect the DC operating point for the cathode follower to be very close to the zero grid voltage curve because of the relatively low 56k cathode resistor value. Fenders and Marshalls, for example, use 100k in this position. By design, however, the cathode voltage tends to "follow" the grid voltage because of generous quantities of negative feedback. Under these circumstances pinning down the exact DC operating point analytically is hardly necessary.
The sum of the two plate currents is 4.2mA and we estimated 4mA at the beginning of our analysis, so our estimate of VPP = 280 appears close enough.
As mentioned, the voltage gain in the voltage amplification stage is lower and the output impedance is higher because of the lack of a cathode bypass capacitor. Our Preamp Output Impedance Calculator shows that the voltage amplifier has a gain of 30dB and an output impedance of 68k. This compares to 36dB and 38k when RK is fully bypassed. The unloaded gain represents the voltage gain that is achieved if the preamp is disconnected from its load, in this case the cathode follower. Since a cathode follower draws very little current, the loaded gain is approximately the same and the output impedance is insignificant.
The cathode follower provides no gain. In fact it introduces a slight amount of attenuation. It has a much lower output impedance than a voltage amplifier, however, making it ideal for driving a current-demanding tone stack. Our Cathode Follower Calculator shows that the cathode follower attenuates the signal by only 0.2dB but that it dramatically reduces the driving stage output impedance from 68k to just 612 ohms, ready to take on even the most demanding tone stacks.
1Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Preamps, 2nd Ed., (Seattle: Pentode Press, 2009).
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