Soldano SP-77 Series II Purple Tube Pre-Amp Made in USA

There were two versions made: A japanese-made one that had a silver faceplate, and a USA-made one with the legendary purple-anodized faceplate. The USA models are more desirable, but based on my research there was really no differences in the circuits, and it was probably more due to aesthetic and patriotic reasons. I doubt most guitar players really care about the difference, but the USA models I’ve seen online have a silkscreened and elegantly laid out PCB, where the Japanese ones had a simpler unlabelled PCB. The identification of circuit components is definitely a bonus when modifying a circuit – but more on this later.
For me – I just wanted the USA version for that gorgeous purple faceplate.

Introduction

Build

The SP77 fits in a two-space rack. The case is steel, with an anodized aluminum faceplate mounted to the front of it. There has been incredible care put into the fit and finish, more like a guitar than your typical utilitarian, box-of-electronics that makes up the majority of the audio equipment world. Careful selection of hardware is evident. The PCBs on the USA-made SP77 are the traditional green with solder masks  and screened labels for the components. The Japanese models are more “homemade”, with no labels. The components are laid out for easy assembly and modification. There is considerable room in the case for heat dissipation, and the PCBs are not crowded. All jacks and potentiometers are PCB mounted.

The Circuit

Warning: The following is a technical deep dive for electronics geeks.
The preamp is comprised of eight triodes (four 12AX7 tubes), with the two channels sharing three of the triodes.

Guitar input is fed into a 12AX7 buffer, which acts as the first gain stage for both channels.
Channel one starts with a switchable 120pF capacitor as high-cut filter, labelled as “Bright”, followed by a gain control and three more 12AX7 gain stages, totalling four successive tube gain stages.
Channel two does not not have a bright switch and uses five gain stages. The fourth stage is actually the third stage from channel one.
Each channel has it’s own three band passive EQ circuit, derived from the standard Marshall tone stack and then merged into a single 12AX7 phase inverter stage.

Stage 1: A 12AX7 triode feeds into a buffer giving a bright boost. This stage is similar to the SLO, but the brightness filtering is slightly different with an additional RC filter.
Stage 2. Grid Stopper circuit provides the first stage of overdrive. (same as SLO)
Stage 3. Asymmetric distortion creating second and third order harmonics
Stage 4: Voltage amplifier DC coupled to a cathode follower. This adds another layer of distortion that causes the waveform to become more symmetrical. This stage also drives the tone stack.
Stage 5. Final stage serves to recover from losses after the tone stack, and a Phase Inverter.

Switching is made possible through the use of  four Vactrol (optical) switches, eliminating switch noise.
Power is supplied by a custom AC transformer with three different DC voltage feeds, supplied via two 30µF/450V and two 10µF/350V capacitors.

That Soldano Tone

Michael Soldano designed this thing for one purpose. Tone. You just cannot make this preamp sound bad or mediocre. The clean channel is amazing. The lead channel,  is one of the greatest sounding high-gain tones ever made.