[Penthode]

I have been considering replacement of the Selenium rectifiers. They are connected in a full wave volage doubler arrangement. I plan to replace them with a pair of 1N5208 diodes which are rated at 3A with a peak inverse voltage of 800volts.

First I thought I should test the old Federal units to determine their current state and performance. I calculated the typical full B supply current from the schematic diagram as indicated in the photos below. The full load current appears to be 600mA and I used my adjustable low voltage bench supply to see what the forward volatage drop of the selenium rectifier is at 600mA.

I was surprised to find the drop only to be 10 volts. If I was to substitute silicon diodes and ignoring the junction drop of about 0.6 volts, the series resistance to effectively account for the diode substitution works out to be 15 ohms. Power dissipation would be 10 x 0.6 = 6 watts so I should get away with a 10 watt resistor.

I am curious as to the failure mode of selenium rectifiers. I understand the forward resistance rises with age. But 15 ohms at 600mA does not seem bad and both rectifioers have almost identical forward resistance. I suspect that the rectifiers may reach a point where the dissipated heat leads to a thermal runaway situation. Will need to do more research on this.

Not: In my forward voltage drop test, I arbitrarily used 10 ohms in series as a current limit as I brought up the voltage supply. The voltage aimed for across the series load resistance was 6 volts which woulkd man 600mA is flowing.