ST-70 selenium rectifier vs. diode polarity enigma

TriodeGuy

Active Member
Every schematic of the ST-70 power supply that I've seen to date shows the selenium rectifier as a diode symbol with the red/black power transformer tap being run directly to the cathode/negative side of that diode...

http://diyaudioprojects.com/Schematics/Dynaco-ST70-Tube-Amp-Schematic.htm

...However, the wiring up section step # 1 of the ST-70 assembly manual states that this wire should be run to the positive/ anode side of the rectifier...

"(1) Connect the red-black transformer lead to the lower (+) lug of the selenium rectifier (S)"

...So the two appear to be in disagreement. Which one is correct? Is the lower lug of the selenium rectifier really the anode? What little bit I know about rectification leads me to think that the lead should go to the anode to act as a proper half-wave rectifier. If you install the diode the other way as the schematic shows then I would think the diode would only conduct when voltage swung past the diode's breakdown voltage. I want to turn this diode around but before trying this and potentially blowing something up I really wanted to get some other opinions.

I actually bought a selenium rectifier replacement "kit" off ebay years ago that included a drawing of the ST-70's underside with supposedly the correct installation. This diagram (I still have this) shows it the way the schematic does with the red/black lead running to the cathode. I still see this kit being sold as a NOS item from time to time.

Any thoughts?
 
transformer goes to (-) cathode (side identified with line) . The anode goes to filter network and out to amp
edited fixed
 

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download assembly manual from hifiengine. You have to sign up (free) to get access. This is link but i don't know if it will open for you if you're not a member.
Nevermind it didn't work. Let me check digital docs
 
Its in digital docs.
Red/black transformer lead to cathode,
The Anode feeds the 10k resistor which goes to the bias pots
 
This problem dates back to the old (erroneous) way of teaching electron current flow, and as such had the positive output side of a selenium rectifier marked with a + symbol and red dot. After all, selenium rectifiers were relatively new at the time, and that was the point at which you could first measure a positive DC output in a conventional rectifier circuit of an actual unit, so it made sense -- at the time. Notice also that the manual makes no reference to the terms "anode" or "cathode" as you are doing. Therefore, it (only) becomes confusing today when you start applying the terms and indicators of the 60 year old text, with today's correct understanding of true current flow. Today, with all symbols based on absolute current flow, the old selenium rectifier's + terminal equates to today's cathode/negative/banded terminal on a silicon diode. Therefore, both you and the text are correct for the period of reference: The manual text AND schematic are correct -- the manual being correct for the installation of the component used produced at the time the kit was produced, and the schematic being absolutely correct regardless of time period. Therefore, the bottom line answer to your question is: Follow the schematic -- and now you know why.

I hope this helps!

Dave
 
For those that might need a polarity reminder, if you draw a diode like this ---KI--- it is easy to see which is the cathode (or kathode) because it looks like a letter K. (and the anode looks a bit like a letter A on its side ;))

cathode -----KI----- anode

Simple. :)
 
As I wrote in my restoration article, the wire from the transformer connects to the striped end of the new diode, and the wire going to the rest of the bias supply connects to the non-striped end. In fact, one of the reasons I wrote my article was after a friend of mine got confused while replacing the selenium rectifier in his ST-70; he connected the new silicon diode backwards, and blew up the new electrolytics he'd just installed! Fortunately, no other damage was done to his amp.
-Adam
 
Peter -- The problem with that interpretation, is that there is no such thing as +50 VAC. There can be a positive AC "value", but of course being an AC voltage, then there will also be a corresponding negative AC value as well by definition.

To identify properly connecting a selenium rectifier in the circuit, the old selenium rectifiers had the cathode terminal marked with either a "+", or red dot, or both:

SAM_1810.JPG

At the time, there was no thought of absolute current flow as it relates to identifying the terminals for anode and cathode. The terminal with the + designation was simply a way to show which side of the rectifier produced a positive DC output in a conventional rectifier circuit. Again, the reason it made sense at the time is because pre-SS, current flow was actually taught using the then current "conventional" current flow understanding. When SS devices came on the scene, then device polarity became supremely important, so the conventional understanding quickly became replaced with proper absolute current flow understanding. Since selenium rectifiers also disappeared about the same time, the "problem" disappeared with them -- until today that is, when any of the old equipment is examined by those only knowing of absolute current flow. Then, it leaves you scratching your head!

Dave
 
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Peter -- The problem with that interpretation, is that there is no such thing as +50 VAC. There can be a positive AC "value", but of course being and AC voltage, then there will also be a corresponding negative AC value as well by definition.

To identify properly connecting a selenium rectifier in the circuit, the old selenium rectifiers had the cathode terminal marked with either a "+", or red dot, or both:

View attachment 817765

At the time, there was no thought of absolute current flow as it relates to identifying the terminals for anode and cathode. The terminal with the + designation was simply a way to show which side of the rectifier produced a positive DC output in a conventional rectifier circuit. Again, the reason it made sense at the time is because pre-SS, current flow was actually taught using the then current "conventional" current flow understanding. When SS devices came on the scene, then device polarity became supremely important, so the conventional understanding quickly became replaced with proper absolute current flow understanding. Since selenium rectifiers also disappeared about the same time, the "problem" disappeared with them -- until today that is, when any of the old equipment is examined by those only knowing of absolute current flow. Then, it leaves you scratching your head!

Dave

I found this out the hard way 30 years ago when I thought I was a smart alec kid in engineering school. Someone should make this a sticky.

It can be very confusing for newbies. Thanks Dave.

Great info for all again.
 
ANOTHER PROBLEM that arises quite a bit in this modern day of using PCBs, is that most PCB software I'm aware of uses square holes for positive polarity of capacitors,
but also uses square holes for the negative cathode of diodes !! On my PCBs, the cathode goes in the square hole, but the diode on the silkscreen DOES show the cathode stripe end connecting to the square hole.
 
i seem to remember this confusion many years ago .. looking at the schematic soon sorts things out though so long as + and - are marked ..
 
sometimes It's better not to blindly rely on these (often conflicting and unconventional) conventions but try to understand how a diode basically works and how it is used in the circuit. A diode rectifier (tube, solid-state or selenium) is not a battery and doesn't have a "+" and "-" voltage output, only an anode and cathode who can be connected in many various ways. A diode only conducts when his anode is positive with respect to his cathode and is non conductive the other way, that's all. The polarity of the voltage measured on the anode or cathode can be either positive or negative depending on the way it is connected in the circuit and your measuring reference point. For instance, in the Dynaco bias circuit the diode is used as a half wave rectifier to supply a negative voltage with respect to ground, one side of the transformer winding being referenced to ground and the other side (red-black lead) must be connected to the diode cathode (striped end of a ss diode, or "+" connection of a selenium rectifier) , the anode being the negative rectified supply "output".

Just remember: Cathode = striped end = "+" mark on a selenium rectifier.

Thus, your following excerpt from the assembly manual (if verbatim) is incorrect:


...However, the wiring up section step # 1 of the ST-70 assembly manual states that this wire should be run to the positive/ anode side of the rectifier...

and should read: positive/cathode side.
 
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Do you mind if I ask maybe a not so obvious question ?
Are you building an original st 70 kit with a selenium rectifier now?
 
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