Cathode follower tone stacks...

[Pierre]

Now if I understand well, a cathode follower tone stack is basically a sweepable value for one of the bypass capacitors usually tied to ground with the bias resistor, right...? This bit lost me, but on a lot of schematics the tone stack is indeed attached to the cathode.

Also for the PI... If I understand right, it consists in using a triode backwards relative to the other to get negative voltage, or am I totally off?