Nice device, and it uses a 2N3055 transistor!!! I built an amplifier with these transistors 45 years ago, probably the last time I have seen one of them. I remember that they were very popular in Spain in the 1970s.
I’ve got these ones (ST, with the marking on the top) from Reichelt Germany for $0.9 each, probably the last ones, because now they are no longer listed.