When was the last time when serious new instrument was invented ?

The electric guitar of all things. Invented in 1932. It’s based on an acoustic guitar, but it’s a completely different instrument.

Electric guitar is so important, it can do many things, even sing almost like a violin.

Trey Gunn used a Warr Guitar.  It will play bass, guitar and can sound like a synthesizer.  Chapman Stick is another example.  Tony Levin is a master playing that.

Just remembered,

The Harmonium was another newer instrument.

How long ago was the kazoo invented?