- Triode – simplest type – three active elements, cathode, grid, anode
- Cathode is heated by “heater” (filament), emits electrons which are
attracted to the anode (or plate [U.S.]),
connected to high tension (HT) supply of hundreds or thousands of volts
- A small change in Grid voltage controls Anode current giving gain
- Variant of this, beam tetrode (four elements), common in HF power amplifiers
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