In vacuum tube technology, HT or high tension describes the main power supply to the circuit, which produces the current between anode and cathode. It is also known as the plate supply or voltage, B battery supply, or simply labeled ->B on circuit diagrams, from the days of battery powered circuitry.
EHT or extra high tension similarly describes the accelerating voltage for a cathode ray tube such as used in a television, computer monitor or cathode ray oscilloscope.
The HT supply circuit most commonly provides a positive voltage of some hundreds of volts to the anode (or plate) and in later circuits also to the screen grid, with the cathode running at near ground or chassis potential. The main exception to this was in the case of tubes with water-cooled anodes, once used for radio transmission, electric furnaces and similar applications. These tubes used a negative HT supply to the cathode, so that the anode could run near ground DC potential (but typically at many hundreds of volts of RF).
Other power supplies to vacuum tube circuits include:
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In vacuum tube television sets, the EHT supply was typically generated by rectifying the output from the scanning circuitry rather than directly. Specialized efficiency diodes, also known as damper diodes or energy recovery diodes were produced for this application.