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A Travelling Wave Tube  (TWT, thus nickname twit) was for many years the main method  of microwave power amplification. I once investigated a failed BBC repeater and it was the valve based multiplier used as a local oscillator that had failed.

They are based on an electron gun with a wire helix around the beam. The helix amplifies the signal, with input at end near cathode and output near the anode. There is a second kind I've never encountered and wasn't mentioned in my BBC course. Though known since the 1930s the commercial exploitation is since the 1950s.

The anode is usually a spike in an earthed heatsink, thus the EHT is negative at the cathode. The door had an interlock switch to disable EHT.

They were used on terrestrial video links (FM originally) and telephone trunk terrestrial links (probably initially analogue multiplex and later PCM, but I'm not sure). They were also used on TV Satellites, both links and direct to home broadcasts. Radar more commonly used magnetrons (still used in microwave ovens), but Wikipedia also claims TWTs used for Radar.

They are still in use due to power, gain and wide bandwidth, though it must be dwindling.

See also Wikipedia Traveling-wave_tube.