Rocksonics have released Triode, a freeware Class-A vacuum tube preamp emulation in VST plugin format for Windows.
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.
I must also recommend Saturator, another simple but very useful freebie by Rocksonics. Both Triode and Saturator are available for download on this page.
Links
Download Triode: click here
Rocksonics Website: click here