1. 3CY5 tubes are thermionic devices in a VS (vertical symbol) configuration. They have an octal base and feature a large, 8-pin filament.
2. These tubes are dual-function, with two sections that can be used for amplification or switching. The "front end" of the tube consists of a Pre-Amplifier so that the input used for amplification can be pre-filtered before being presented to the main amplifier stage.
3. The "back end" of the tube can be used for a driver section or an isolated switch. Its function is to provide a high impedance output signal without introducing noise into the circuit. 3CY5 tubes are typically found in guitar amplifiers and other vintage audio equipment.