"A voltage-regulator tube (VR tube) is an electronic component used as a shunt regulator to hold a voltage constant at a pre-determined level.
Physically, these devices resemble vacuum tubes, but there are two main differences:
Their glass envelopes are filled with a gas mixture, and
They have a cold cathode; the cathode is not heated with a filament to emit electrons.
Electrically, these devices resemble Zener diodes, with the following major differences:
They rely on gas ionization, rather than Zener breakdown
The unregulated supply voltage must be 15–20% above the nominal output voltage to ensure that the discharge starts" - (en.wikipedia.org 06.03.2022)
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