Video demos of the Solo v5.3 guitar amplifier and a book called
Guitar Amplifier Design: Tubes and Semiconductors Play Together.
The book video comes from a presentation at Gerald Weber's Tube Amplifier Webinar 6/9/2020.
The book describes the design and build of the Solo v4.3 amplifier.
The v5.3 amp is the next amp in the Solo series. It is far more flexible and capable, but still based on the same design concepts and principle.