The above regulator is based on a Philbrick 5910-C regulator circa 1961 published in Electronic Design magazine, page 70, Feb. 4, 2002, in an article by Bob Pease.

Differences are: the reference tube load resistor was increased from 60K to 90K, the pass tube is 6L6 instead of 6AS7, and the lower dual triode is 12AX7 instead of 7025.