The preamp stage takes the INPUT signal and amplifies it into a larger or louder OUTPUT signal.
Most often the preamp in a tube amp is a very linear Class-A, or self-biased stage.
Linear means what you get out is accurately related to the input.
Class-A means the stage is always conducting current, even on the negative input peaks.
Self-biased means that the components and voltages are selected so that by the virtue of the tube conducting current, it creates its own voltage drop to bias itself.
To do this, the Grid has to be at Ground. If you measure the Grid with a voltmeter, it will show 0 (zero) volts.
The Cathode will raise off of Ground and, for a 12AX7 tube, settle down at 1.5 Volts. (1.5 Volts just happens to be the turn-off voltage for the 12AX7.) Changing the Plate voltage will make a small difference in the Cathode voltage.
The input resistance of the vacuum tube is very high. It is so sensitive that you cannot leave it unconnected.
The tube is sensitive to static charges in the air, and to electrostatic waves around it - mostly just hum and buzzing.