Some guy at a audio show in NYC told me it help to reduce second-order, "harmonic" distortion, why you want to remove this harmonic which is quite musical.
krellmk,
I enjoy attending audio shows almost as much as I enjoy live music. The former because I can never predict what I will hear from
man or machine, and the latter because, well, for reasons that I will never understand completely, music just makes my mind go happy. :0)
In this video, with no music playing (thereby negating the musically-stimulated 2nd harmonic theory) you can hear the actual
mechanical resonant frequency of a piece of metal inside a vacuum tube. The source of the noise must be at least one of the 4 items in a triode: heater > cathode > control grid > plate. If I had to make an educated guess, the physical implementation of a control grid is such that it would be the 'flimsiest' & therefore most microphonic of the 4 major parts involved?
In order to hear for yourself what I'm trying to describe, please open the video below & skip directly to ~8:40, and listen carefully to the ~30 second demo/description:
NOTE: Bonus points if you go back to the ~8:10 mark & listen to the constant hiss created by the 1st amp.
Anyway, hope this helps explain what we're up against when using tubes to amplify music signals. (Well, at least with cheap tubes...no doubt the genuinely well-engineered tubes don't suffer from these microphonic sins of amusical addition. :0)
Cheers --
3D