selenium diodes for clipping?

freq67 · Post by **freq67** » 10 Apr 2015, 22:42

hey all what do you guys think about using selenium diodes for a clipping circuit in an overdrive pedal?

teemuk · Post by **teemuk** » 11 Apr 2015, 08:56

You can try it for the sake of exotism but generally selenium diodes were abandoned because they have a finite lifetime and "aging" degrades them. They start to run hotter and hotter and in worst case catch in fire releasing a cloud of toxic smoke. Nice.
Not to mention that they have painfully low reverse voltage and rather high forward voltage ratings because "plates" are stacked to increase reverse voltage, which simultanously pumps up forward voltage. The forward voltage (which varies tremendously amongst different selenium diodes) can range from 5 to 10 volts. This will introduce a problem if you planned to use traditional 9V battery power, in fact if you planned to use low supply voltages in general and extract "high-gain" tones.

All this of course begs a question; why I can't simply use generic silicon diodes for the job? Well you can: Just reference them to higher voltages to increase clipping threshold, and add some cleverly planned series and parallel resistance if needed. You can even stack up "ladders" of resistors and diodes and construct some rather complex forward voltage characteristics. Effectively you can "model" selenium forward conduction characteristics with modern components. So basically there is no need for exotic devices except for the sake of exotism.

Go ahead if you feel like you need a real "Selenium Distortion". Do realize that it's a somewhat unsensible project if you only want that same "Tone" sans all other drawbacks associated with selenium diodes in general.

teemuk · Post by **teemuk** » 11 Apr 2015, 09:18

Note how the selenium characteristic curve is less "steep". This is due to its higher series resistance. Putting series resistance to ordinary silicon diode will change its characteristics towards similar direction. The "knee" from reverse bias to "effective" forward conduction is also "softer" in selenium. This characteristic can be replicated with parallel resistance to diode. Experiment with values.

Though a similar characteristic curve can be faked you may not be able to "match" forward voltages per se. This, however, is just a matter of signal amplitudes and their ratios to diode forward voltage. If "faked" forward voltage ends up being higher just drive the diodes with equivalently "hotter" signal.

Do note that for selenium the portrayed are single "cell" characteristics. Since one cell may have a reverse voltage of mere 50 volts a typical diode may stack up cells, which naturally lifts the characteristic curve "upwards" in voltage.

deltafred · Post by **deltafred** » 11 Apr 2015, 16:49

They are basically crap (IMO, YMMV etc.). I don't know if they ever made selenium signal diodes, if they did I have never seen them.

The best thing you can do with them is strip them down for parts. They have some nice washers in there, a length of insulated (paxolin) tube and a piece of threaded rod down the middle.

freq67 · Post by **freq67** » 12 Apr 2015, 16:50

The reason I asked was because I had some lying around in a molded packaging as opposed to the stacked plate version which do indeed suck. I've replaced my share on old tube amps. Here's a pic of the diodes. One was twoin series the other was single.

freq67 · Post by **freq67** » 12 Apr 2015, 16:57

: Molded selenium diodes

deltafred · Post by **deltafred** » 12 Apr 2015, 17:03

freq67 wrote:
image.jpg

First time I have seen those, give them a try and see what they are like.

DrNomis · Post by **DrNomis** » 14 Apr 2015, 04:07

I seem to remember reading somewhere that when Selenium Diodes go faulty and blow up they really stink, and the smell hangs around for a very long time.....

Personally, I've never heard of Selenium Diodes being used as clippers but can't see why they wouldn't work provided that they are small signal types, best thing to do is first test them with a multimeter to verify that they work, and just try them in a breadboarded circuit.....