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Sensibly equivalent to:
4064B 872 AH217 CE-872A CV642 ESU872 GL872A RK872A VT-42A
See also:
Ediswan Rectifier & Power Triode Advert
Extras ▼


Type 872A is a half-wave mercury vapour HT rectifier. The 872A was a selected 872, the 872A could replace the 872 but not vice-versa.
Additional to the ratings given, the peak inverse anode voltage was 10 kV, and the peak cathode current was 5 Amps. A pair of these valves in a bi-phase circuit with an inductor input filter would deliver up to 7.5 kW (2.5 Amps DC @ 3000V), rather more than the normal load rating of many single-phase supplies. This, and the relatively high (5A) peak current rating imply that these valves are designed for use on 3-phase supplies.
In such cases it is normal to use a set of 6 rectifiers to give '6-pulse' rectification because this provides a low level of HT ripple (about 5% RMS, easily smoothed) and a high power factor (around 90%).
For the same PIV, 6-pulse rectification provides rather more output voltage than bi-phase rectification, i.e.. more power output per valve. A 'parallel' 6-valve rectifier bank would deliver up to 7.5 ADC at around 3,300V (25 kW) and the same valves used in the (much commoner) '6-diode bridge' arrangement would provide half the current (3.75A) at twice the voltage (6600V), about right for a conventional 15 kW AM transmitter. Not bad for 37.5 W heating power and 15V forward voltage drop per valve!
Of course, mercury rectifiers are notoriously delicate. To obtain long, trouble-free life it is essential to ensure very strict quality control during manufacture, to mount the valves upright in a temperature-controlled airflow, and to observe strict rules about pre-heating, RF screening, fault protection and overload protection. These are definitely professional valves!
The ribbon filament is entirely enclosed. There is some mercury on the envelope plus a small pool of bright liquid mercury.
The anode is fixed only at the top of the envelope and is dished and ribbed for mechanical rigidity.
The wide glass tube envelope is 57 mm in diameter and, excluding the four stub pins is 202 mm tall.
Reference: Data-sheet. Type 872A was first introduced in 1938.


Absolute Maximum Operating Conditions
Thanks to Frank Philipse for supplying the above PDF datasheet.
Updated May 21, 2021.
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