The CGS65 is a tube based Voltage Controlled Amplifier/timbral gate. While this module basically operates as a VCA, it does add a degree of distortion to the signal. How much distortion depends on how hard it is driven. Add feedback and it begins to oscillate, synchronizing to the incoming signal to some extent. All this while running on a standard synthesizer power supply, with no extra heater supply required.
Component Kit Discontinued - Contains 1SH24B Tube
CGS65 Datasheet and Build Guide