B409 / RE134 / L413 / U415 / L424
We are shortly after the dawn of the mass diffusion of Radio and all its technologies. Among these, of course, there was the beating heart of the amplification circuits, that is the VALVES, in particular those of power amplifiers. The TRIODE was the second tube structure after the Diode. In particular, their technology has had a very strong boost from military research. Almost all the Triodes of that era were direct heating = DHT.