05-22-2026, 02:25 PM
Whenever a selenium rectifier gets replaced by a diode, there always should be a series resistor used.
If there has been one, it gets increased.
Increase of the resistor, considering the current stays more or less the same as it is defined by the total resistance which is all filaments plus that dropout resistor, will be accompanied by an additional power dissipation (proportional to the increase), so if you simply replace the old resistor with the one of a higher value, increase the power rating by the same factor (say you replace an 80 Ohm 3W resistor with 120 Ohm one, then the power rating of the 120 Ohm will be 4.5W (round to 5W)).
If all this resistor does is being the ballast for the filament, I would then not use the Zener, instead of picking the value such that the voltage across the filaments is right where it should be at 120V.
Of course, Zener is also an additional protection, so it is a judgement call.
If there has been one, it gets increased.
Increase of the resistor, considering the current stays more or less the same as it is defined by the total resistance which is all filaments plus that dropout resistor, will be accompanied by an additional power dissipation (proportional to the increase), so if you simply replace the old resistor with the one of a higher value, increase the power rating by the same factor (say you replace an 80 Ohm 3W resistor with 120 Ohm one, then the power rating of the 120 Ohm will be 4.5W (round to 5W)).
If all this resistor does is being the ballast for the filament, I would then not use the Zener, instead of picking the value such that the voltage across the filaments is right where it should be at 120V.
Of course, Zener is also an additional protection, so it is a judgement call.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
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