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Vintage dual diode part
Vintage dual diode part
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Old 11th November 2019, 09:31 PM   #1
msubully82 is offline msubully82
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Default Vintage dual diode part

I have been studying vintage phono preamps. I am looking at the Sansui AU-717 phono stage and I see parts that are drawn as two series diodes in a single package. According to the parts list, these are SS-3 and SS-3R. Are these essentially two series diodes in a single package, or something different? I couldn't find a datasheet.

Thanks,
Don
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Old 12th November 2019, 02:36 AM   #2
sgrossklass is offline sgrossklass  Germany
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Little mixup there, SS-3 and SS-3R are 3-pin dual rectifier diodes (bridge rectifier halves) in the power supply. The ones in the phono stage are VD1212s. These were called "varistor diodes" but I guess were more or less exactly what you suspected (or perhaps a pair of transdiodes, transistors with B-C tied together), and people commonly replace them by a pair of 1N4148s as VD series parts have a history of less than stellar reliability (which is not what you want when they are responsible for power amp biasing).
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Old 12th November 2019, 04:43 AM   #3
PRR is offline PRR  United States
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The phono stage parts are VD1212 and surely just two jellybean diodes in one. (OK, could be some odd couple junctions, even dud Darlingtons packed in 2-leg pots, but same thing.)
Attached Images
File Type: gif AU-717-phono-42.gif (62.7 KB, 101 views)

Last edited by PRR; 12th November 2019 at 04:48 AM.
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Old 12th November 2019, 05:59 AM   #4
msubully82 is offline msubully82
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Thanks. I can't believe I missed that.
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Old 14th November 2019, 03:28 PM   #5
msubully82 is offline msubully82
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I used a pair of series 1N4148 diodes in place of the VD1212 diodes to simulate the circuit. The simulation shows the DC bias for the input diff pair matches measurements given on the schematic. So these parts are indeed just two series diodes.

The schematic implies the output stage is biased at ~3.6mA. I had to tweak the current source to get the output biased close to that. I guess that is to be expected due to how dependent the biasing is on transistor VBE.

I would have attached a screen shot of the simulation, but I don't see how to add an attachment (Insert Image button wants a URL??)
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Old 18th November 2019, 09:28 PM   #6
sgrossklass is offline sgrossklass  Germany
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Quote:
Originally Posted by msubully82 View Post
I would have attached a screen shot of the simulation, but I don't see how to add an attachment (Insert Image button wants a URL??)
Use the "normal" reply function, not the quick reply field. Then you'll get the "Manage attachments" button and misc. other things.

Once the attachment shows up in the main reply form, you can also copy its URL and feed "insert Image" this way.

If you only attach an image, it will show up as a thumbnail that can be enlarged by clicking on it. If you use "Insert Image", you can place the image anywhere in the text; it'll then disappear from the thumbnails.
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Old 20th November 2019, 08:50 PM   #7
PRR is offline PRR  United States
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Quote:
Originally Posted by sgrossklass View Post
Use the "normal" reply function, not the quick reply field. Then you'll get the "Manage attachments" button....
Click the image to open in full size.

Or-- "Quick Reply" to get started, except then click "Go Advanced" to gain access to more formatting frills and the file-upload function.
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