[AMRadio] Too Much HV

[JAMES HANLON]

According to my ARRL Handbook, 1952 edition, page 210, "The voltage drop through a rectifier of this type (Mercury Vapor) is practically constant at approximately 15 volts regardless of the load current."  Silicon diodes have a voltage drop of approximately 0.7 volts each, but as several silicon diodes are put in series to make up the 10KV peak inverse voltage rating of an 866A, the voltage drop across such a replacement silicon stack is more likely to be on the order of 7 volts.  In any case, the difference in voltage drop between an 866A and a replacement silicon diode unit is negligible.  
 
If you are seeing something like 1000 volts more our of your HV power supply than it "should" be delivering, something other than the diodes has gone wrong.  If the supply has a choke-input filter, check the choke.  It should measure several hundred ohms DC resistance.  If it has shorted out, that would convert the supply to a capacitor-input circuit and the voltage output would soar.  Neglecting voltage drop across the choke due to its dc resistance and the load current and the rectifier voltage drop, the voltage output of a choke-input filter provided the choke is at least the "critical value" is 0.9 times the rms voltage of the power transformer.  The voltage output of a condenser output filter at no load is 1.414 times the rms voltage of the power transformer.  Going with these two figures, if a power supply was designed to give 1500 volts output with a choke-input filter, the power transformer would have to supply 1500/0.9 = 1667 volts rms.  If a condenser-input filter were used with a 1667 volt rms transformer, the voltage would increase to 1667 x 1.414 = 2357 volts.  If these voltages look familiar, check out your choke!  
 
Jim, W8KGI