X-ray tube, CRT.
The CRT is at least a triode (pentode or more depending on whether you count the focus electrodes etc.)
That would be a project. To get a CRT to function as an amplifier.
You'd have to de-focus the beam
a lot to get any significant anode dissipation, otherwise you'd generate an internal cloud of Aluminium and phosphor vapor! There are Beam Triodes though, which have a basic electron gun assembly, firing into a cup shaped anode. They were used as high voltage shunt regulators for things like regulating the EHT supply to CRTs in high end monitors to prevent image blooming.