Terms used to describe the intensity of the vacuous space within a vacuum tube’s glass envelope. A hard vacuum denotes a more complete vacuum is created in the tube manufacturing process. Hard vacuum tubes – like the 6L6 and 6550 – have a higher saturation threshold and therefore distort less easily then soft vacuum tubes. Soft vacuum tubes – such as the EL34 or 6CA7 – have a less intense vacuum (sometimes called a gassy vacuum), saturate more easily and therefore break up [distort] more quickly.
