I’ve been building some prewar-era laboratory hardware and have been employing voltage regulator (VR) tubes. I found a paper that gives a nice concise explanation, including design rules and information on supply-voltage tolerance. The paper employs graphical aids, but today’s spreadsheets make life easier. Currently, this link works:
VR tubes age and can be tough to tame over time[2, 3]. We now have Zener diodes.
 Raymond C. Miles, “How to Design VR Tube Circuits,” p. 135-137, Electronics Magazine, October 1952.
 Walter R. Jones, “Voltage Regulator Tubes,” Panel on Electron Tubes Research and Development Board, distributed by RCA.
 Sylvania News, “Voltage Regulator Tubes,” Jan-Feb 1943.