[GreenKeys] Teletype REC11 Rectifier "is NOT suitable for use in the signaling ... circuits of Teletype apparartus"

[Jim Haynes]

If there is an oil-filled capacitor then the REC-11 probably incorporates
a constant voltage transformer.  You might look on the name plate to see 
if the thing was made by North Electric Co. or Power Equipment Co. of
Galion, OH.  (North acquired PECo).  That was Teletype's favorite supplier
for rectifiers, and they were experts in constant voltage transformers.

They work on a principle sometimes called ferro-resonance.  However that
term has a completely different meaning in the electric power industry,
so constant-voltage transformer is a better term.  Constant voltage 
transformers are frequency sensitive, so they are rarely used in military
equipment which might have to be powered from a generator set in the 
field, hence not very good control of frequency.  Also you see speed
governed motors rather than sync motors in military equipment for the
same reason.

The Model 19 as used in the field by the military typically had a power
supply with thyratron tubes to regulate the voltage.  And some later ones
had magnetic amplifier control (saturable reactors)  I once had a
Western Electric power supply for a Model 19 that used a saturable
reactor, and used a couple of VR tubes as the voltage reference.