[GreenKeys] Tube powered loop supply

[Jim Haynes]

As others have said, first there were motor-generator sets, then copper
oxide rectifiers, then selenium.  Someone else mentioned using an 83 tube
and those were popular with Western Union.  The rectifiers using thyratron
tubes and pretty much exclusively for the military market; and the reason
for that is that the power frequency in the military is very uncertain,
since they are using field generator sets.  For the same reason they used
governed motors in military gear.  For use with stable frequency power
sources Teletype liked to use ferroresonant transformer regulated supplies
(same idea as the Sola constant voltage transformers) but they can't be
used where the frequency varies.

You can use any kind of rectifier tube that can handle the current.  This
was pretty common in early vacuum tube TUs if they didn't use a polar
relay; they perhaps used higher voltage to allow for the drop in the
magnet keyer tube.

In my childhood the telco had a customer, a truck line, with some kind
of TTY installation and equipment that involved a 35L6 and a 50Y6 tube.
I never saw the equipment, so I don't know what it was that used these
off-the-shelf tubes instead of some Western Electric model.  But I know
those were the tubes.  It's plausible that the 50Y6 was the rectifier
for the loop and the 35L6 was a loop keyer, especially if it used a 20ma
loop.