[GreenKeys] Vacuum tubes in TTY power supply

[Jim Haynes]

On Sun, 16 Jun 2019, Nick wrote:

> I have seen a picture about a year ago (or was it a YouTube video, can't
> remember) of several blue illuminated vacuum tubes inside of a teletype
> power supply / rectifier. 

I don't know the type number offhand, nor where to go look it up, but
they are thyratrons.  Teletype liked voltage regulated power supplies.
For use on 60 Hz commercial power they used a lot of constant voltage
transformers in power supplies.  But these transformers are quite
frequency sensitive.  So for situations where the power frequency was
not very reliable they had to use speed governed motors and thyratron
regulated power supplies.  The military version of Model 19 is a very
typical example.

Later there were some power supplies with magnetic amplifier regulation
to get away from the tubes.  I have one of these that is the power supply
for the military BD-100 field switchboard; and I once had a Model 19
power supply by Western Electric that used a couple of VR tubes as a
voltage reference.  It was interesting in that two VR tubes were wired
in parallel.  With that connection whichever tube regulates to a slightly
lower voltage will take all the current and leave the other one dark.
So I guess they couldn't get acceptable life with one VR tube but could
with two of them.  There were toggle switches so you could test the
VR tubes by turning one of the pair off and seeing that the other one
regulated correctly.