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Sensibly equivalent to:
See also:
Ediswan Rectifier & Power Triode Advert
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Type VT-42A is a half-wave mercury vapour HT rectifier. The commercial valve was a selected 872 designated the 872A. 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 mercury is splattered over the inside of the envelope and there is no liquid reservoir.
The anode - supported only at the top with the filament shield at a slight angle.
The filament is completely enclosed.
The wide glass tube envelope is 57 mm in diameter and, excluding the four stub pins is 200 mm tall.
Reference: Data-sheet.


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