English: A so-called "lighthouse tube", a high frequency triode vacuum tube used as an amplifier or oscillator at UHF frequencies up to 2 GHz. The purpose of the unusual design is to reduce the inductance of the leads which connect to the electrodes, which limits the frequency of ordinary triodes. Lead inductance can cause the conductors in the tube to act like resonant circuits at high frequencies, causing instability and parasitic oscillations. The grid terminal is the flat metal ring around the center of the tube while the plate terminal is the button at the left.