In 1900 Peter Cooper Hewitt observed that if an arc is struck in mercury vapour at low pressure, current will flow in one direction only. In 1902 he patented the mercury vapour valve rectifier, its sole purpose - to convert AC electricity to DC. Used throughout the 20th century into the 1970s by which time they'd become redundant, superceded by solid state semiconductors.
Previously, this rectification process had been mechanical, at large scale, and much more expensive.
Early rotary converter. Credit: @Matt Blaze
By the 1920s these valves were commonplace. Produced in vast quantities they're still readily available today and in good supply. Capable of supplying huge current, a relatively low voltage drop between cathode and anode also ensures extremely good voltage regulation. Perhaps overkill for an audio amplifier, but what a foundation to start on!
816 mercury vapour valve rectifiers
The glass bulb holds a pool of liquid mercury at the bottom acting as the cathode. This is heated, producing mercury vapour through which rectification is achieved via the anode. The blue-green light produced is rich in UV-C, a high energy germicidal radiation that is blocked by the glass envelope of the valve. At significantly higher voltages X-ray radiation is produced, thankfully impossible in this application however.
866A mercury vapour valve rectifiers
They do require some extra care beyond regular valves. The mercury will become distributed around the inside of the valve if shaken or moved, e.g. due to transportation. To allow the mercury to condense back to the bottom of the bulb, the valve must be preheated via the filament before the high anode voltage is applied. This preheating time varies with the type of valve but is typically 30 minutes for the larger 866a. Subsequent starts then require up to 30 seconds to begin the vapourisation of the mercury.
866A RCA globes
Davison Audio's power supplies use a three position power switch to facilitate preheating. The full wave bridge rectifier arrangement incorporates damping valves with a built in soft start such that a 15 second preheating time is recommended before powering on.
Preheat power switch
Mercury vapour valves are extremely durable with a long lifespan, particularly when used gently for audio amplification. A favourite at Davison Audio, the hypnotic glow adds to the warmth of the amplifier valves and makes every listening session an experience.
816 mercury vapour valve rectifiers