1920s. Perhaps from perv- + -eance, or perhaps from pervi- + -ance, after conductance, permittance, etc., with spelling in -eance after words such as permeance". The oldest sources on Google Books, from 1930s or 40s, say that "Perveance is a fundamental tube constant inversely proportional to the impedance".
It does sound suspiciously close to permeance — so close that it makes me wonder whether some physicist or other made a mistake when coining a name for this quantity — but it is impossible to tease out of Google who that physicist was.
(There is, of course, a line from Chaucer with a word spelled as perveance; but this must be a spelling variant of purveyance or something.)
