HT (vacuum tube)

In vacuum tube technology, HT or high tension describes the main power supply to the circuit, which produces the current between anode and cathode.

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).

In very early vacuum tube television sets, the EHT was derived directly from a high voltage winding on the mains transformer using a half wave rectifier.

Although this provided a greater degree of safety, the reason for the change was that the mains transformer had been eliminated from sets produced from the 1940s onwards.