[GreenKeys] Original M19 power supply

[Jim Haynes]

Teletype preferred to use constant voltage transformers in power supplies.
But CVTs are quite frequency sensitive, so where the frequency was not
well controlled (like military generator sets) they used thyratron
rectifiers to regulate the voltage.  I had a Western Electric M19 power
supply that used a magnetic amplifier for regulation, and a couple of
VR tubes for the voltage standard.  An interesting circuit - two VR
tubes in parallel, with spring-loaded switches to disconnect either of
them momentarily.  When you put two VR tubes in parallel only one will
fire and the other will stay dark.  I guess as the tube ages the firing
voltage increases and then the other one will fire and the first one
will stay dark.  So they increase the time between visits to replace a
VR tube.

jhhaynes at earthlink dot net