Based on our analysis of the SLO overdrive channel, let's see what conclusions we can draw concerning Soldano's strategy for high-gain preamp design.
1. Make the Master Volume Control Frequency Selective
The Soldano Super Lead Overdrive's tone stack and overdrive control are in series.
The controls work together to attenuate the preamp output, thereby making the distortion dynamics highly frequency selective. If the tone stack were located further upstream and separated from the overdrive control by one or two stages of voltage gain, the results would be quite different.
2. Place the Master Volume Control in Front of the Phase Inverter
Soldano's tone stack and overdrive control are placed between the last preamp stage and the phase inverter, so the phase inverter always has enough headroom to drive the power amp to full power and beyond. Except in extreme cases, overdrive is limited to the preamp or the power amp, not the phase inverter.
3. Place Preamp Clipping in Front of the Effects Loop
Soldano's effects loop follows the fourth stage, which has the overdrive channel's lowest input sensitivity. This ensures that signal clipping is upstream of the loop. Effects thus operate on the distorted signal, not the input to the distortion-producing stage. In the case of reverb, for example, the preamp creates reverberated distortion, not distorted reverberation.
4. Limit the Signal Passband
Three of Soldano's voltage amplifiers use relatively small cathode bypass capacitors. Partial bypassing creates negative feedback for low frequencies, thereby creating bass attenuation.
Unlike the Fender Bassman and other classic amps created before the era of extreme overdrive, Soldano's third stage includes a plate resistor bypass capacitor with a -3dB cutoff of only 2.2kHz. Any treble from the guitar pickups that manages to run the gauntlet of the upstream grid stopper suffers a substantial cut. The nature of overdrive, however, is harmonic distortion, and this amp creates bucket loads of its own high-frequency content. Soldano's design introduces harmonic distortion generated predominantly by the guitar's fundamental frequencies and low-order harmonics. High frequency content is created by overdriving the amplifier.
Solo guitar voice usually has a range of only 2 or 3 octaves. Limiting the passband reduces intermodulation distortion and a plethora of extraneous and undesirable harmonic artifacts that can pollute the mix.
5. Keep Input Impedances High
In traditional designs the power amp is the first stage in the signal chain to break into extreme distortion. Except under unusual circumstances it is the only stage to succumb to bias excursion, which contributes substantially to the distortion dynamics of Class AB. Without proper foresight, however, bias excursion can lead to highly undesirable effects like blocking distortion.
By incorporating a 4-knob master volume control, Soldano creates the opportunity to overdrive the preamp while allowing the power amp to run relatively clean. This shifts the potential for bias excursion to the overdriven preamp stage. To prevent blocking distortion, Soldano uses grid stoppers to ensure that the overdriven stage's input impedance remains high even when its grid tries to exceed zero volts. Only the first stage and the effects return, which are unlikely to be overdriven, have an input impedance of less than 220k.
From the perspective of the grid to which the grid stopper is attached, the resistor adds substantially to the output impedance of the upstream stage. The high output impedance is incapable of providing the current necessary to drive the grid positive by any appreciable amount. This makes the driven tube's transition from near linear operation to severe distortion relatively sharp.
Large grid stoppers also have a substantial impact on treble response, but in the context of treble-creating overdrive there is plenty of brightness in the final result.
6. Overdrive a Directly Coupled Cathode Follower
The first triode to break into overdrive is the directly coupled cathode follower in the fourth stage. This has a substantial effect on clipping of the output waveform.
Overdriving the cathode follower in the positive direction causes a drop in the plate-to-cathode voltage, making electrons more attractive to the grid. The resulting increase in grid current causes more current to flow through the driving triode's plate load resistor, thus limiting the decreasing voltage across it.
As the driving triode approaches cutoff the cathode follower's input impedance drops from an extremely high value, representing a very light load, to a very low value that bogs down the driving circuit. This sharpens the clipping of the signal and makes it a more symmetrical match to clipping in the opposite direction. As Rutt observes, there is no equivalent to this phenomenon in transistor circuits.6
7. Provide Adequate Low-Frequency Decoupling
Soldano's signal chain has an incredible 7 triodes upstream of the phase inverter. Nevertheless, no more than 2 inverting voltage amplifiers are driven by the same plate supply. This ensures adequate decoupling to prevent low-frequency feedback that can lead to motorboating. To keep voltages up, Soldano uses a parallel power supply filter for the first two stages.
8. Prevent Parasitic Oscillation
Wires and components have inherent inductance and capacitance that can lead to parasitic oscillation. This highly undesirable affect, which often occurs at radio frequencies, is caused by feedback from output circuits to input circuits. High-gain preamps are particularly susceptible.
Soldano's generous use of grid stopper resistors helps attenuate the outbreak of parasitic oscillation. The SLO's plate bypass capacitor in the third stage also chokes off high frequency signals, effectively isolating upstream stages from downstream stages for the span of frequencies over which oscillation can occur. It is worth emphasizing, however, that grid stoppers, plate bypass capacitors, and other proactive measures are effective only in the context of good circuit layout. Parts and wire placement are crucial in a high-gain preamp.
9. Keep the Signal-to-Noise Ratio in Check
High gain works for both signals and noise. At the input jack, Soldano uses a classic gamma network with a 68k grid stopper and 1M grid resistor. This allows all guitar signals to pass to the first tube. Attenuation and frequency shaping take place only after the signal is strong enough to compete with the noise generated by downstream circuits.
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).
3Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Power Amps, (Seattle: Pentode Press, 2008).
4Technical Correspondence with Paul Reed.
6T.E. Rutt, "Vacuum Tube Triode Nonlinearity as Part of The Electric Guitar Sound," 76th Convention of the Audio Engineering Society, New York, 1984.
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