Name That Diode . . .

[Joe Kramer]

Hi Friends,

I've been tweaking with a triangle wave LFO and using various diodes to approximate a sine wave.  A formula of two pairs of 914s and one pair of 1N270s in series seemed to work well enough, but I decided to try one last experiment.  There were some unidentified house-numbered diodes in my parts bin, sitting for more than fifteen years because I didn't know what they were.  To my surprise, a single back-to-back pair of these mystery diodes yielded an almost perfect sine wave!   Apparently the "knee" is very soft, because the waveform doesn't have the usual flattened contours of other diodes I've tried, and there is only a slight "point" at the peak of the curve. 

So far, the only thing certain about these is that the forward voltage is about 1500-1600 mV, but LEDs with the same fV made a waveform nowhere near as rounded, so there's obviously more going on than just the fV.  They are made of black glass, about the size of a 1N34A, and numbered "702-120".  I got them from an electronics factory I worked at when they were throwing out excess inventory.   I have about fifteen of them.  Seems like they would make excellent clippers in a distortion config, but I haven't tried this yet.

Any help in identifying these would be greatly appreciated!  I can post pix of the diodes and a scope shot of the sine wave if that would help. . . .

Regards,
Joe

[stm]

Based on the unusually high conduction threshold I can only think of constant current diodes.

[Joe Kramer]

Thanks STM!  You may have hit the nail on the head.

Interesting info here:

http://www.centralsemi.com/cld/index.aspx

http://margo.student.utwente.nl/el/componen/diodefaq.html#IV.5%20-%20Cu


And a possible part number here:

http://www.datasheetcatalog.com/datasheets_pdf/1/N/5/3/1N5313.shtml

Apparently using an FET with the gate shorted to the source is the very same thing.  I seem to recall that topic coming up around here before.  I'll have to give that a try on my LFO to see if it works the same.  Seems like this sort of diode would also be useful in FWR circuits. . . .

Thanks again for your help!

Joe

EDIT UPDATE:

Just tried a couple of FETs (2SK30 and 2N5457) as above, but no dice.  They didn't seem to even pass the signal, or maybe the FV was so low that it was almost flat.  Either way, the search may still be on. . . .

[ambulancevoice]

these black ( ) diodes sound very useful and interesting
i wouldnt waste them if i were you

i like the sound of the "noise diode"
what would be classified as an avalanche zener diode?

[Joe Kramer]

these black ( ) diodes sound very useful and interesting
i wouldnt waste them if i were you

I hear you.  At the moment, I have a couple of LFO projects that I intend to use them in.  Being able to get a decent sine wave from a triangle with only two diodes is like a dream come true.  I wish I could find out what these things are in case they sound good for distortion too. . . .

i like the sound of the "noise diode"
what would be classified as an avalanche zener diode?

Do you have a part number I could look into?

[ambulancevoice]

go to that site http://margo.student.utwente.nl/el/componen/diodefaq.html#IV.5%20-%20Cu
scroll down to noise diodes
and search some of those sources

[stm]

Just tried a couple of FETs (2SK30 and 2N5457) as above, but no dice.  They didn't seem to even pass the signal, or maybe the FV was so low that it was almost flat.  Either way, the search may still be on. . .

I think a J201 would be closer to what you described since it has a limiting current (Idss parameter) around 0.6-1.1 mA, rather than aprox 3.5mA of the 2N5457.  I don't know for sure about the 2SK30, since I haven't used them and the scarce datasheet information available has a huge spread.

I tend to think that a silicon diode + 2 to 4 germanium diodes could work in a similar fashion, just as it is used in the SSS in the Layouts Gallery here: http://aronnelson.com/gallery/STMs-Circuit-Ideas/Speaker_Saturation_Simulator

[Paul Perry (Frostwave)]

I suspect that although it appears that the diodes have the magic property of converting trainagle to sine all by themselves, that there is a series resistance (obvious or not) built into the triangle source. So the circuit is really a voltage divider, back to back diodes on the bottom & an unknown resistance on top.
But, if it works, it works!

[Joe Kramer]

Hey STM: I'll try to get some J201s and give those a try.  I've seen that Tape Saturator schemo that your SSS is based on and wondered how it sounded.  Looks like a good, simple circuit.  The problem I run into is trying to cram ten extra diodes into an already limited perf space.  At this point, the two-diode solution is a very lucky find.  Thanks. . . .

Hey Paul:  You're right on about the series resistance.  I hadn't quite put that together.  Right now I'm running 47K in series with the diodes.  I haven't tried varying the series resistance yet with other diode combos, so I'll give that a try and see if results are similar.  I did find out that the load resistance also makes a big difference.  With a voltage-divider type output control of 100K, the wave shape started to warp back into a triangle.  But with a 1meg, all was round again.  Thanks. . . .

If it turns out I can't duplicate the smooth sine shape with any other combo of diodes/series resistance, I'll post a scope shot for further contemplation. . . .