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RE: replacing a silicon rectifier with a diode

Posted by Lee of Omaha on September 12, 2012 at 09:35:28:
I assume you mean replacing with a tube rectifier such as a 5U4. First, you're going to need a supply for the heater. You can use a filament transformer or a DC supply. A DC supply will need to float, so you're better off using a filament transformer. Chunky rectifiers such as the 5U4 draw 3 amps at 5 volts. You will get a greater forward voltage drop across the tube than a silicon rectifier.

There is a nearly constant voltage drop of 1 volt or a little less across the silicon diode, but the voltage drop across the tube increases with current. So your power supply with a tube will have lower voltage and greater impedance than with a solid state device. You will also have a bit of built-in time delay in your high voltage supply as the cathode needs to heat before it starts emitting. A directly-heated rectifier such as the 5U4 will have only a 1-2 second delay, but an indirectly heated tube will have a delay of 8-12 seconds.

If your circuit was designed for solid state, I can't see a good reason to redesign it, with one exception. By paralleling a tube with silicon you avoid the 0.6 volt or so turn on delay every half-cycle. The silicon does the "heavy lifting" while the tube is just there to get things started. You will still need the filament supply. The tube, since it does so little, should last a long time. As much as I like the idea, I haven't done it, and suspect it will be little "bang for the buck."

Good luck.