It is still possible to access datasheets for most valves which provide very useful insight to valve parameters and also sets of curves to help set-up & correctly bias the valve.

The datasheets provide the design-intent specifications for the valve but the actual valve that you are using may differ in several different ways,

• All valves has a natural level of variation, no matter how carefully they have been constructed, they all are slightly different, and some manufacturers are better than others.

• Valves do wear out, and there are various mechanisms that can cause a valve to deteriorate, including how long, and how hard, the valve has been used.  Often when valves are advertised, some data is provided to indicate how worn the valve is, ranging from unsubstantiated statements like "New Old Stock - NOS", to a single test point which is normally stated as a percentage of a new valve's performance, or a measurement of the valve's mutual conductance at a stated plate current (often the anode and grid test voltages are not included.)

• You might be wanting to use the valve in a way, or configuration, that was not originally intended by the manufacturer, and therefore the data that you would like is not actually included in the original datasheet.  Example: Using pentodes and tetrodes as triodes in audio applications.

Valve characterisation is a technique to actually test and measure the valve's performance, across its operating range, so that you have solid data on which to base your design choices and use-cases on.  The test data can be displayed as sets of curves, or traces, and it is also possible to fit the data to a mathematical valve model, so that you can include the valve in circuit simulations.