The fifth stage is almost identical to the fourth and creates a voltage gain of 43 (33dB). Both stages end with cathode followers that provide low-impedance outputs to drive relatively heavy loads. The fourth stage drives an optional effects loop and the fifth stage drives a Fender Bassman 5F6-A tone stack with modified parts values.
The voltage amplifier's DC load line has one end at the DC plate supply voltage of 380 volts and the other end at a plate current of
(380) / (220k + 2.2k) = 1.7mA
If the grid voltage is minus 1 volt then the plate current passing through the 1.8k cathode resistor is 1 / 2.2k = 0.45mA. For grid voltages of minus 1.5 and minus 2 volts, the plate current is 0.68mA and 0.91mA, respectively. These points create a grid line that puts the DC operating point at minus -1.75 volts.
Because the cathode follower has a very high input impedance, the AC load line is nearly identical. Based on the resulting DC operating point, a 1.75 volt grid voltage swing drives the tube into saturation. It takes about 2.75 volts to achieve cutoff.
If the grid voltage rises from its DC value of minus 1.75 to a value of zero, then the plate current rises from 0.8mA to 1.43mA. The current through the 2.2k cathode resistor thus increases by 1.43mA - 0.8mA = 0.63mA. This raises the cathode voltage by (0.63mA)(2.2k) = 1.4 volts. It therefore takes an input signal amplitude, measured grid-to-ground, of 1.4 + 1.75 = 3.15 volts to overdrive this stage.
Using 34 for the gain of the first stage, 60 for the gain of the second stage, 0.68 for the gain of the second stage output circuit, 1.8 for the gain of the third stage, and 1.5 for the gain of the fourth stage, the signal amplitude at the amplifier input jack required to drive the fifth stage into overdrive (volume set to maximum) is
3.15 / 34 / 60 / 0.68 / 1.8 / 1.5 = 840 microvolts
This is at least 40 percent higher than for the fourth stage, so the fifth stage remains within its headroom limits even under severe overdrive conditions. The fourth stage, which is notably upstream of the optional effects loop, produces a clipped output that is within the headroom limits of the fifth stage.
The voltage amplifier has a gain of 44, about half the gain that could be achieved with a large cathode bypass capacitor. The cathode follower has almost unity gain. Thus total stage gain is about 44, or 33dB, which more than compensates for the attenuation of the tone stack that follows.
Having examined each stage individually, let's look at the Soldano Super Lead Overdrive channel from a systems perspective and summarize some lessons learned.
1Richard Kuehnel, Circuit Analysis of a Legendary Tube Amplifier: The Fender Bassman 5F6-A, 3rd Ed., (Seattle: Pentode Press, 2009).
2Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Preamps, 2nd Ed., (Seattle: Pentode Press, 2009).
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