Discussion in 'Tube Audio' started by Mikeorg, Sep 15, 2018 at 12:48 PM.
Need to know how to replace a selenium rectifier with diode rectifier..
Is it a bridge or only a single rectifier? Pics would help...
I just restored this Tube realistic saf 40d put in a new power section and all new caps and resisters, but it still has the selenium rectifier in it and I have never made a diode one. it sounds great but I've heard stories of the rectifier burning up and don't want that to happen.
Again: is it a bridge (4 connections) or a single one (- and +)?
I added photos
Looks like 4 connections from the picture, plus another 2 connection on the rectifier itself...?
There is no schematic to be found on the net... oh well! I don't want to presume only going from the pics but it looks like Leland47 said.
No I've never been able to find any schematics for it only for the earlier Saf 40B...
try this FB page, there is mention of a SAF 40D repair, scroll down a bit, maybe they can help out. https://www.facebook.com/pg/Tube-Amplifier-Repair-Service-and-Forum-1432579480300617/posts/
Looks like a bridge rectifier to me. The schematic for the SAF-40B shows a bridge rectifier here:
Two of the leads going to it will be the AC in, and the other two will be the + and -. There may be markings somewhere on the rectifier telling you which lug is which. I'm not sure if the SAF-40D used a tube rectifier in addition to the selenium bridge, as with the SAF-40B. The bridge in the SAF-40B looks to have been used for relatively low voltage, possibly for the bias voltage of the output tubes (be sure to note the polarity). A silicon bridge rectifier should work fine in its place, possibly with a bit of added resistance to make up for the lowered forward voltage drop of silicon vs. selenium. Good luck!
Thanks for the input!
A modern selenium bridge rectifier (yes, they still make them!) can be seen here. Here are the pictures they provide:
The outer ends of the stack are connected together, and are the negative (-) output, colored black. The very center is the positive (+) output, colored red. In between those are the AC input points, colored yellow in this case. I can't be 100% sure that the rectifier in your SAF-40D is set up the same way, but it's possible. The replacement silicon bridge rectifier will probably look something like this:
The AC input pins may be labeled with the sine wave (~) symbol. Also note the notched corner near the + lead. Mount the new silicon bridge on a terminal lug strip like this one. Use the extra lugs to mount a dropping resistor (100 ohms or so, perhaps a watt or so) to compensate for the reduced voltage drop of the new silicon bridge. Again, good luck!
I really don't think that you have anything to worry about. I still use a 60 year old amp with its original selenium and its still going strong. If mine burns up, I will just replace it with modern diodes. Its not going to destroy my amp or transformers as everything I own is fused.
If you want to worry about something, worry about how long the tubes will last or how long will the resistors last before going out of spec or if your power transformer will smoke tomorrow. My point is that there is no way of telling when or if it will burn up in our lifetime. If you replace it, the new one could burn up. Not realistically and it is rare, but I have seen it happen.
If voltages check out, I just wouldn't give it much thought. A short circuit grounding could smoke it, such as a failed filter cap, but the normal failure mode is merely very slowly increased series resistance with lowered output voltage.
Boy do they stink when they go.
First hand experience here, I prefer lilacs.
I've never had one spontaneously self-destruct.
Smelled way worst than rotten cabbage. And filled the entire room with that terrible smelling smoke. My father was working on a Motorola business band transceiver I believe. ie 1973 ish.
Sulphurous Farts, awful.
Selenium is known for producing some horrible-smelling compounds. Behold the wonders that are Carbon Diselenide and Selenophenol. When you hear of a selenium compound described as being akin to "six skunks wrapped in rubber innertubes and the whole thing is set ablaze," you can tell that just holding your nose probably isn't gonna be enough if one of these rectifiers goes poof..........
PS: Check out the rest of the "Things I Won't Work With" section of that blog for more tales of 'interesting' chemicals.
But its the main rectifier fellas'. I say get it out of there. It will only degrade more and more, lowering your high voltage. Do as Adam Ant said to do, go with a new silicon bridge rectifier.
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