To drive the tone stack the signal 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 Fender uses two directly-coupled triodes.

Fender Bassman 5F6-A preamp cathode follower circuit

The 12AX7 triode on the left is configured the same as the 12AY7 bright and normal channel preamps, except that the cathode resistor RK1 is not shared by two triodes and is not bypassed by a large capacitor. The lack of sharing means that in this stage the 820-ohm cathode resistor represents 820 ohms at DC. For the previous stage it effectively acted as 1.64k per triode.

The absence of a cathode bypass 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. The effect is dramatic. We saw, for example, that substituting a 12AY7 in a traditional 12AX7 preamp drops the output impedance from 38k to 20k. Keeping the 12AX7 and configuring it as a cathode follower, on the other hand, drops the output impedance to less than 1k.

An explanation of cathode follower 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 5F6-A tone stack.

DC Operating Points

The parts values for the voltage amplifier are

RL = 100k
RK1 = 820
RK2 = 100k

The plate supply voltage VPP is 325 volts. The load line (red) and grid line (blue) for RL = 100k and RK1 = 820 are plotted here:

Fender Bassman 5F6-A preamp cathode follower DC operating point

The lines intersect at a DC grid bias voltage of minus 1.2 volts. The DC operating point is thus defined by a quiescent grid voltage, plate voltage, and plate current of

VGQ = -1.2 volts
VPQ = 180 volts
IPQ = 1.4mA

This means that the voltage at the plate relative to ground is 1.2 volts more than 180 volts, which is also the voltage of the grid of the cathode follower relative to ground. Because the grid-to-cathode voltage is very small, the voltage across the 100k resistor RK2 is also near 180 volts, which means that the idle plate current through the resistor is approximately

IPQ = 180 / 100k = 1.8mA

The idle plate voltage is approximately

VPQ = 325 - 180 = 145

These results place the operating point for the cathode follower near the green circle.

Voltage Gain and Output Impedance

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 voltage gain of 32dB and an output impedance of 59k. 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 a tenth of a dB but it dramatically reduces the driving stage output impedance from 59k to just 615 ohms, ready to take on even the most demanding tone stacks.

References

1Richard Kuehnel, Circuit Analysis of a Legendary Tube Amplifier: The Fender Bassman 5F6-A, 3rd Ed., (Seattle: Pentode Press, 2009).

1Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Preamps, 2nd Ed., (Seattle: Pentode Press, 2009).