Rocksonics Professional Audio has released Triode, a simple plug-in (1 control) that generates the asymmetric transfer characteristic of a Class A Triode vacuum tube pre-amp. The effect adds both even and odd harmonic distortion which creates the characteristic tube "warmth" that is NOT generated with filters.
"Triode" can be daisy-chained with the Rocksonics "Saturator" plug-in (previously released) to emulate a Class A pre-amp followed by a push-pull output stage.
Triode is free and is available for Windows as a VST Effect plug-in.
