The Soldano Super Lead Overdrive channel is renown for gain. In fact, input sensitivity for preamp overdrive, as we will demonstrate, is less than 1 millivolt with the volume control at maximum. This is more than an order of magnitude greater than classic amps like the Fender Bassman and its offspring.

Soldano's master volume control is placed between a modded Fender Bassman Tone Stack and a 5F6-A phase inverter and power amp. Crank the potentiometer to maximum and the preamp drives the power amp into classic Class AB distortion. Turn it way down and the preamp creates its own second-to-third harmonic transition and sustain that can last well beyond the band's next smoke break. Somewhere between minimum and maximum, however, is a realm of pure magic that is unique to Soldano's carefully crafted design. Let's take a closer look.

DC Operating Conditions

The first stage has a gamma network at the input and a bright boost circuit at the output.

soldano super lead overdrive first stage preamp schematic

The DC load line has one end at the DC plate supply voltage of 360 volts and the other at a plate current of (360) / (220k + 1.8k) = 1.6mA.

soldano SLO first stage DC load line

If the grid voltage is minus 1 volt then the plate current passing through the 1.8k cathode resistor is 1 / 1.8k = 0.56mA. For grid voltages of minus 1.5 and minus 2 volts, the plate current is 0.83mA and 1.1mA, respectively. These points create a grid line that puts the DC operating point at minus -1.5 volts.

soldano SLO first stage DC operating point

Headroom and Distortion

At high frequencies when the 470k resistor is shorted by the capacitor across it, the AC load becomes 220k in parallel with 500k for an effective impedance of 154k. In that case a drop in plate voltage from 175 volts (the DC value) to zero volts creates an increase in current of 175 / 154k = 1.1mA. The current thus rises from 0.83mA (the DC value) to 1.93mA, which forms the upper left end of the blue AC load line show here. (At lower frequencies the AC load line is closer to the DC load line.)

soldano SLO first stage headroom

An input signal with an amplitude of 1.5 volts drives the plate voltage between 68 and 268 volts (the green lines). Large signal voltage gain is therefore (268-68) / 3 = 67.

The gain at maximum swing is slightly less than the gain at low signal levels because of compression as the grid voltage approaches minus 3 volts. The grid lines are closer together so the plate voltage swings less. This compression creates second-harmonic distortion that increases as the signal amplitude approaches 1.5 volts. Beyond 1.5 volts the triode goes into saturation on positive swings of the input signal. Above 2.5 volts the grid swings to below minus 4 volts, forcing the tube into cutoff as well.

Distortion is thus characterized by steadily increasing second harmonic distortion as the signal amplitude approaches 1.5 volts, rapidly increasing second harmonic distortion as the amplitude increases from 1.5 to 2.5 volts, and severe third harmonic distortion at higher levels. Ultimately an input sinewave creates an output square wave when the input signal is high enough.

With a high-gain pickup, guitar volume set to maximum, and amp volume set low enough, this stage could conceivably be driven into overdrive while keeping subsequent stages clean. In typical operation, however, the triode operates well below its swing limits, creating substantial voltage gain and a modest amount of second harmonic distortion that is enhanced by the bright boost of the output circuit.

The Input Circuit

The overdrive channel's first stage uses a classic gamma network for the input circuit.

Soldano SLO overdrive channel first stage gamma network

Assuming a voltage gain of 74 and a grid-to-plate capacitance of 1.7 picofarads this means the -3dB high-frequency cutoff is at 18kHz. There is unity gain for guitar frequencies so the input headroom at the jack is the same as at the grid: 1.5 volts. Radio frequencies are effectively suppressed.

Bright Boost

Small signal voltage gain increases from 51 at 82Hz to about 74 at 400Hz due to the relatively small, 1uF cathode bypass capacitor. At the top of this range the output impedance is 49k, equal to the plate load resistor in parallel with the plate resistance of a 12AX7 triode. With a load ranging from 500k to 970k, depending on frequency, this creates a loss of 1dB or less across the 0.02uF coupling capacitor.

The 0.002uF capacitor in the output circuit provides bright boost at higher frequencies by shorting the 470k resistor across it. With the volume control at maximum the 12AX7 output is attenuated by almost 6dB at 82Hz but down by only 1dB at 1kHz. (This assumes the bright boost circuit is driven by a constant 49k output impedance, which is a reasonable approximation for treble frequencies in which the cathode is fully bypassed.)

Soldano's first stage certainly brightens up the signal. Let's turn our attention to the next stage.

References

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).

3Technical Correspondence with Paul Reed.

4Soldano SLO Overdrive Preamp Schematic