A pentode or tetrode vacuum-tube (valve) configured as a common-cathode amplifier (where the output signal appears on the plate) may be operated as:
a pentode or tetrode, in which the screen-grid is connected to a stable DC voltage so there are no signal variations on the screen-grid (i.e. the screen-grid has 0% of the plate's output signal impressed on it)
a triode, in which the screen-grid is connected to the plate (i.e. the screen-grid has 100% of the plate's output signal voltage impressed on it)
a blend of triode and pentode, in which the screen-grid has a percentage (between 0% and 100%) of the plate's output signal impressed on it. This is the basis of the "ultra-linear" circuit, and is usually achieved by incorporating a suitable "tap" on the primary winding of the output transformer that the vacuum-tube (valve) is connected to.
The impression of any portion of the output signal onto the screen-grid can be seen as a form of feedback, which alters the behaviour of the electron stream passing from cathode to anode.
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http://en.wikipedia.org/wiki/Ultra-Linear
