The second stage has a 12AX7 triode driven via a large grid-stopper.
The DC load line has one end at the DC plate supply voltage of 350 volts and the other at a plate current of (350) / (100k + 1.8k) = 3.4mA.
If the grid voltage is minus 1.5 volts then the plate current passing through the 1.8k cathode resistor is 1.5 / 1.8k = 0.83mA. For grid voltages of minus 2 and minus 2.5 volts, the plate current is 1.1mA and 1.4mA, respectively. The resulting grid line puts the DC operating point at minus 2 volts.
The effective plate load is 94k at high frequencies where the cathode is shorted and the plate is loaded by the 1.47M output circuit. In that case a drop in plate voltage from the DC operating value of 240 volts to zero volts creates an increase in current of 240 / 94k = 2.55mA. The current thus rises from 1.1mA (the DC value) to 3.65mA, which is the upper left end of the blue AC load line show here.
Notice that the AC load line is much closer to the DC load line in this stage compared to the first stage. This is because the latter has a 220k plate resistor that creates a greater output impedance. It also drives a 500k load, which demands more current than the 1.47M load in this stage.
A grid voltage swing from zero to minus 4 volts causes a plate voltage swing from 115 volts to 332 volts. The gain over this range is therefore (332-115) / 4 = 54.
Compared to the first stage the DC operating point is closer to the middle of the AC load line. A 4 volt grid voltage swing thus drives the tube into saturation and cutoff nearly simultaneously. Distortion is therefore characterized by increasing second harmonic distortion as the signal amplitude approaches 2 volts, then rapidly increasing third harmonic distortion as the signal level increases further. Ultimately a sinewave input signal becomes a square wave at maximum overdrive.
At high frequencies the first stage has a gain of about 74 at its plate. With the volume control at maximum the first stage output circuit has a gain of about 0.5, creating a total first stage gain of (0.5)(67) = 34. Thus the amplitude at the input jack needed to drive the second stage into overdrive is 2 / 34 = 59 millivolts. This is certainly achievable, even with low gain pickups. Stay tuned, however, because Soldano's legendary preamp gain doesn't stop here.
Small signal voltage gain increases from 35 at 82Hz to about 55 at 400Hz due to the relatively small, 1uF cathode bypass capacitor. Maximum treble gain is 60. The 0.02uF coupling capacitor is effectively shorted at all guitar frequencies. The voltage divider formed by the 470k and 1M resistors at the output attenuates the signal by a factor of 0.68 (3.3dB).
The 470k resistor is in series with the first stage output impedance, which is well under 100k. From the perspective of the second stage, the effective first stage output impedance increases to about 500k, many times higher than in a typical 12AX7 voltage amplifier.
The high output impedance causes Miller capacitance in the second triode to have more of an effect, enough to significantly attenuate high frequency guitar signals. The grid stopper attenuation for a 4 kilohertz signal, for example, is 4dB to 6dB depending on the volume control setting. This response offsets the substantial treble boost created by the first stage.
In overdrive the high output impedance becomes a distinct advantage. It ensures that the first stage cannot drive the second stage grid positive by any appreciable amount and is not bogged down by any grid current that flows. It always sees a load of at least 470k independent of the state of the second triode. This greatly reduces bias excursion that can lead to blocking distortion.
Large grid stoppers are often used in high-gain, master volume control preamps, but are absent in classic amps that rely on power amp distortion. In the Fender Bassman 5F6-A, for example, the power stage is overdriven long before the preamp stages. Preamp bias excursion is therefore not an issue. Bright boost to compensate for large grid stoppers is also unnecessary in a more traditional design. Let's move on to the third stage.
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).
Copyright © 2005-2015 Amp Books LLC All Rights Reserved