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.
That thingy is a current limiter. These typically have iron filaments that operate in a hydrogen atmosphere. The operating temp is a dull red heat. It's basically a primitive CCS, often used to keep Nernst tubes from going into thermal runaway.