SPL TwinTube
This is a tube-emulation plug-in from SPL and is a software implementation of one of SPLs well-known, but now discontinued, hardware processors.
It is designed to let you add, and control, harmonic distortion and/or tube saturation effects in controllable amounts and some pre-determined points. Harmonics can be applied from one of four configurable center frequencies, with a controllable degree of intensity (up to +15 dB). And saturation effects can be controlled in terms of their level/intensity (up to +6 dB).
There’s an overload indicator so you can easily tell when your settings are causing distortion/clipping that is not intentional.
Like a lot of professional audio plug-ins, TwinTube adopts a skeuomorphic UX - the appearance and controls are represented as they’d look on a physical device:
If you’re not already audibly-familiar with how saturation or harmonics affect sound, it takes a bit of time experimenting to get a feel for. But the results become clearly audible, as you push the intensity up.
You can apply very subtle effects here, to the level it might take protracted listening to “get it”, and you can also introduce gross changes that result in significant and brutal distortions.
It’s something fun to play with if you’re interested in having some “tube flavor” without actually going down the tube-amplifier route.
It’s available via “Plugin Alliance” and has a 14-day free trial. Normally it runs $199, but periodically goes on sale for as low as $29.99.