The resulting negatively-charged cloud of electrons is called a space charge.These electrons will be drawn to a metal "plate" inside the envelope if the plate (also called the anode) is positively charged relative to the filament (or cathode).
The first letter indicates the year: M 1963 N 1964 O 1965 P 1966 Q 1967 R 1968 S 1989 The Second Letter indicates the month: A January B February C March D April E May F June G July H August I October J September K November L December Check the EIA number.
Fender recommends dating unstamped amps by the EIA number on the transformers.
The format is always EIA 606-Y-XX or 606-YY-XX where Y indicates the year (196Y/197Y or 19YY respectively) and XX the week. Dating by serial number, while less accurate, can be of some use.
In it's simplest form a vacuum tube can be created to operate as a diode: a device that conducts current only in one direction.
A third element called a "control grid" can be added to the design which provides the ability to amplify a signal.