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TUBE ULTRAFEX T1954
and anode, a current flow is not possible because the unheated anode emits more or less no electrons. This
design was used, for example, as a rectifier in the power supplies of amplifiers. The magnitude and velocity of
the flow of electrons depend on the cathodeî
s temperature, the material it consists of, and the magnitude of the
anode voltage. When the electrons hit the anode they produce heat that is dissipated by using large anode
plates.
Fig. 4.3: Triode
The triode has an additional metal grid between anode and cathode. By applying a negative voltage, this grid
can be used to control the internal resistance of the tube, and hence the anode current. When the grid bias
voltage (voltage between cathode and grid) becomes negative, the current flowing to the anode is reduced
because the negatively charged grid repels the arriving electrons. As a consequence, there are less electrons
to reach the anode. When the bias voltage shifts towards zero, the flow of electrons accelerates. When it finally
becomes zero or even positive, the grid current begins to flow which considerably reduces the current flowing
to the anode and can possibly destroy the tube. Triodes are most commonly used in pre-amps, often in pairs
arranged in one tube (twin triode).