WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Yup what Colt said, the word is phototube. The cathode is coated with cesium which has a lot "work potential", basically electrons are more prone to being bumped out by light rays. Add 300V and you get a few micro to miliamperes plate current depending on intensity. I think response drops off around 10-20kHz typically.