What is Gain?

Gain is the multiplication factor of a tube; it is the ratio of the output to the input. It can be voltage gain, current gain, or power gain. Usually tubes are characterized as having voltage gain, stated as a dimensionless number. A 12AX7 for instance typically has a gain of 92. But they can be as high as 110 (EH 12AX7-WXT) or as low as 85 (early Mullard). This number does not change very much over the life of a given sample tube and is somewhat harder to use a differentiation value.