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I want to replace this selenium diode on an old tube amplifier with a silicon equivalent. Normally, this diode would have a lower value resistor in series with it such as a 15 ohm. Instead, this one has a 15K ohm resistor, only having about 20mA of current going through it. Is it safe to say, in this case that I can simply do a swap of the selenium for a 1N4007 and not worry about adding more resistance?

From what I've read because the voltage drop of a selenium is higher about 7-10V, and the 1N4007 is only 0.7V, you would need to add a small resistor such as 100 ohm in series with it to achieve the same voltage drop and get the same voltage in the circuit as before.

Thanks

Schematic diagram showing selenium diode

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    \$\begingroup\$ Is that a single selenium disk, or is it a stack? Each disk has a forward voltage of about 1V. \$\endgroup\$
    – JRE
    Aug 17, 2020 at 16:32
  • \$\begingroup\$ It has about 10 disks I believe, I'll have to check \$\endgroup\$
    – Jon
    Aug 17, 2020 at 16:41

2 Answers 2

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Looks like you can just swap it out and trim the bias to be the same as it was before. The bias has more than a 10:1 range.

The voltage drop of a selenium rectifier depends on how many plates it has.

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  • \$\begingroup\$ Thanks, I think about 10 plates but I'll check. Any way to measure the voltage drop across it without powering up the amp? I want to finish replacing caps etc before I do that. \$\endgroup\$
    – Jon
    Aug 17, 2020 at 16:41
  • \$\begingroup\$ You could take it out and put about 5-7mA or so through it with a series resistor and a bench power supply. eg. a 1K resistor and crank the supply up slowly until the voltage across the resistor is 6V or so, then measure the voltage across the rectifier. \$\endgroup\$ Aug 17, 2020 at 16:45
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One way to achieve a voltage drop without using a high wattage series resistor if replacing a Selenium unit for higher b+ : use a 12 KV microwave oven rectifier. They are silicon, stacked units in a very compact case. Good for 1 amp average and some pretty high surge ratings. The one I recently used provided about a 12 volt drop, very similar to the characteristics of the original Selenium unit.

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  • \$\begingroup\$ You could also just use a regular diode in series with a zener diode. \$\endgroup\$
    – Hearth
    Jan 14, 2023 at 15:38

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