Nowadays constant voltage is easily obtained by Zener diode or three terminal voltage regulator but in the vacuum tube era, voltage regulator tube was widely utilized. It can generate high (more than 75V) constant voltage and it was quite useful for oscillator stabilization or some high precision amplifier. (Actually I have no idea how to generate constant 150V DC, even now ;-) Several kinds of voltage regulator tube was fabricated and this time I got 75V version, named VR75. The following video shows how it works. It is quite simple but useful tube!