17Y9 tubes are a type of vacuum tube used in audio amplifiers and vintage radios. They are a type of pentode vacuum tube, and were amongst the first tubes to be developed in the early 1940s. They are characterized by their large base, often around 1.5 inches (35mm) in size, and tall cylindrical shape, typically around 8 to 10 inches (200-250 mm) in length. 17Y9 tubes are known for their large warm sound and ability to produce extreme highs and lows. They are usually found in push-pull configurations and can be used in guitar amps and Hi-Fi amplifiers.