[js1138]

So when was the triod invented, and when were the first practical devices marketed?

[Marine_Uncle]

"So when was the triod invented, and when were the first practical devices marketed?"
The triode was invented in 1906. Not sure who was the first to implement it into a system design.
While where at it, the tetrode was invented in 1926, and the pentode in 1930. As far as the "beam power tube" a variant of the pentode, don't ask me, I don't know.