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872A


The John Lawrence Collection
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Sensibly equivalent to:- 4064B - 872 - AH217 - CV642 - ESU872 - VT-42A - CE-872A - RK872A - GL872A
 
See also:
Ediswan Rectifier & Power Triode Advert
 
This exhibit was last updated on 14 February 2013
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Type 872A is a half-wave mercury vapour HT rectifier, and is essentially an upsized type 866/A.
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, ie. more power output per valve. A 'parallel' 6-valve rectifier bank would deliver up to 7.5 ADC at around 3300V (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 wide glass tube envelope is 55 mm in diameter and, excluding the four stub pins is 203 mm tall.
Reference: Datasheet. Type 872A was first introduced in 1938.
 
Absolute Maximum Operating Conditions
Rectifier
Vh
Ah
mAa
5.0
7.5
1250

Thanks to Frank Philipse for supplying the above PDF datasheet