Documentation and circuit diagrams for Steinhart Audio equipment have long been notoriously difficult to track down, turning service and restoration work into a significant challenge. To help fellow enthusiasts and technicians keep these amplifiers running, I’m sharing the reference diagrams I’ve collected over the years for the Steinhart Bolero Jnr. (USA Export).

While I haven't worked hands-on with a physical Bolero Jnr., I’ve relied on these schematics extensively when bench-testing and restoring the standard Bolero monoblocks. The topologies share substantial architectural similarity. The main difference lies in the output stage: the Bolero Jnr. runs a 4-valve arrangement, whereas the full-scale Bolero scales up to an 8-valve push-pull output section. The front-end phase splitting, driver architecture, and general rail distribution align closely enough to make these drawings an invaluable service reference for either version.

The factory documentation clarifies the output section is built around 6Pi45c horizontal sweep valves rather than standard audio tetrodes. This directly shapes the power supply topology, which pairs large filter capacitance with a low-voltage plate rail designed not to exceed 280V. Because sweep pentodes deliver high peak current at the expense of steep screen grid sensitivity and thermal drift, the circuit takes a hybrid approach - using discrete high-voltage transistors to stabilise the driver supply and individual bipolar buffering.

The tuning procedure below reflects those valve characteristics. Setup requires zeroing the master and individual bias trimmers, setting an initial baseline, and running a burn-in period to let the tube elements settle mechanically and thermally before making final adjustments. Only after that extended run are the individual trimmers set without signal to achieve the final operating target across the cathode test points.

Below is the (suspected original) bias calibration sheet that I found some years ago. While the original author and origins are unknown, I’m archiving and sharing it here as a practical bench reference for anyone setting up or servicing these amplifiers.

I’ll soon be adding a capacitor replacement schedule for the Bolero chassis—aimed at experienced technicians planning a bench overhaul rather than a full turn-key restoration guide. 

Disclaimer: These reference diagrams and instructions were collected from various online sources and fellow hobbyists over many years and are shared here in good faith, strictly for educational, archival, and non-commercial restoration purposes. All original copyrights and intellectual property remain with their respective owners.

Valve amplifiers operate at potentially lethal DC rail voltages. If you undertake any testing, service, or modifications, you do so entirely at your own risk.