Mercury rectifier question

[Unrealcrafter2]

Hi y'all! In working on a 15kw vttc which is essentially a class E. Normally these use a silicon diode on the input but I wanna go full old fashioned and use a mercury rectifier. I have two 1.25a 10kv 872a that I can use. While I fully understand the danger I don't know the most about electrical characteristics. Mostly I wanna know is how well to they handle overloaded RMS current and if I can put two in parallel? I know they don't like high peak but I think for short periods and reduced lifespan I can over current. If I put two in parrales i am concerned just one will fire and drop the voltage to where the other will not

 

[JonSnell Electronic]

To employ them in parallel you will need balast resistors. They don't behave well at high current. I would use your silicon diode as normal and use a dropper/balast resistor placing the rectifier valve across the resistor if single phase. The resistor will drop enough voltage for the current to give the desired blue U/V glow without causing too much of an overload.
Here is an extract from the valve data sheet if you think it is still suitable.

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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.​

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