what exactly does this cap do?

[12AX7]

I thought a cap from plate to cathode was used to attenuate highs. But i tried this and if anything it almost seemed to add them. The net result to my ears was a slightly thinner sound, tho not in a bad way. And a more sustainy clearer tone that seemed a bit more complex. Isn't this the opposite of what that cap is supposed to do, or if not what exactly IS it supposed to do. Used a 500pf by the way and tried it on the first then second stages of a cascaded preamp section.

[HotBluePlates]

A cap from plate to cathode *is* supposed to attenuate highs. But 500pF seems overly-large.

[12AX7]

I'll give 250pf a  try then. But i honestly don't hear less high end, but more. Very odd, but also very good. This has really done something to the tone to get it closer to that "sound in my head". (No, not the voices )

EDIT:Doh...it WAS a 250 PF ! Don't hae a smaller one, but thats ok. It just sounds so good i'm going to leave it.