My first tech job was as a mainframe computer operator. Lucky for me, the computers were DEC PDP-10s. These timesharing systems supported remote access for customers throughout the United States and the UK. But more importantly (at least for me) was that DEC-10s were also used in the best computer science research labs, e.g., BBN, CMU, MIT, Stanford, etc... ARPANET, the computer network that eventually became the Internet was full of DEC PDP-10 systems. Checkout an early map of it:
So why do I think I was lucky?
Because... as a programmer, I worked on the operating system and systems software, reading lots and lots of source code, modifying it, extending it. I was working with software written by some of the best CS minds in the world. I learned a lot about the roots of the architectures we see today. Timesharing of the 1960s, 1970s, and early 1980s is a mirror of cloud computing we see prevalent today. I attended conferences ripe with brilliant programmers. What a gift to be able to talk with others, pick their brains and learn so much...
And a bonus... the PDP-10 machine-level instruction set was such a pleasure to work with.