Allied Radio catalog 1966 (copyright 1965), p. 370. (Uses a Nuvistor. Probably introduced earlier than this.)
QST, Aug 1976, p. 131.
QST, Jun 1979, p. 155. (The PLF-2 is a solid state version of the PLF.)
QST, Feb 1980, p. 112.
QST, Apr 1981, p. 150.
QST, Jan 1982, p, 115.
