Design Considerations
The pinout of VR tubes was designed so that power could be forced to flow "through" the VR tubes to the load. That is, the load current would flow in one pin of the VR tube and out through a second pin. In this way, the circuit could be arranged so that unplugging the VR tube would disconnect the load. (Otherwise, unplugging the VR tube would have allowed the voltage to become unregulated, possibly rising as high as the source voltage and damaging the downstream load.)
Because the glow discharge is a "statistical" process, a certain amount of electrical noise is introduced into the regulated voltage as the level of ionization varies. In most cases, this could be easily filtered out by placing a small capacitor in parallel with the VR tube or using an RC decoupling network downstream of the VR tube. Too large a capacitance (>0.1μF for an 0D3, for instance), however, and the circuit will form a relaxation oscillator, definitely ruining the voltage regulation and possibly causing the tube to fail catastrophically.
VR tubes could be operated in series for greater voltage ranges. They could not be operated in parallel: because of manufacturing variations, the current would not be shared equally among several tubes in parallel. (Note the equivalent behavior with series and parallel connected Zener diodes.)
Presently, VR tubes have been almost-entirely supplanted by solid state regulators based on Zener diodes and avalanche breakdown diodes.
Read more about this topic: Voltage-regulator Tube
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