Why does Fender label diodes in reverse?

[Randall]

Replaced the selenium rectifier with a robust diode in a Harvard 5F10. When I put in the diode according to the schematic, I was confused by the + at the cathode, and - at the anode on the schematic. So I looked at the layout, and it shows the selenium rectifier as + coming off 6.8K feeder resistor, and - to the 56K.

So I installed the diode with + going to 6.8K, just like the selenium rectifier. Didn't work. I put the rectifier back in according to the layout, didn't work. I swapped it around, and it works. So, the layout is incorrect, this I am sure. But why does Fender draw a rectifier in many of it's schematics with it pointing in the correct direction, but labeled the opposite of it's ends?

[Enzo]

You make it sound like a policy. If a diode is drawn backwards, it is just a drawing error.

I am looking at the 5F10 drawing:
http://www.webphix.com/schematic%20h...5f10_schem.pdf

The selenium looks right to me. The bias supply is negative, so the anode should be towards the circuit and negative end of filter caps. The cathode towards AC. The layout seems to agree with the schematic. Is your schematic different from this one.

WHatever the schematic might say, we should all know which way a diode is installed to make a negative supply.

DIodes have cathodes and anodes. I find it only confuses people when we use + and - signs.


Fender also appeared to specify a 25v filter cap with 27v across it. You might replace it with a 35v or 50v version.

[Randall]

But why is there a plus drawn on the cathode and a minus drawn on the anode? That is my question.

[Enzo]

Imagine you have a transformer, voltage doesn;t matter, let us say 12vAC. A winding. Ground one side and we have a 12vAC source. Now connect a diode in series with it, connect the AC to the anode of the diode. WHat voltage will appear at the cathode with respect to ground. In other words, if we put a cap there, what would it charge up to? I'd expect a positive voltage. And the opposite, I send an AC voltage through a diode, feeding it into the cathode, I expect a negative voltage at its anode.

Does the bias supply look backwards in this AB763?
http://www.webphix.com/schematic%20h...b763_schem.pdf

Anode is the arrow and cathode is the line.

[Steve Conner]

I'm looking at a 57 amp, 2000V SCR and diode module here. There is a little laser engraved schematic on the side. The cathode of the SCR is labelled with a + and the anode of the diode is labelled with a -.

The reason is presumably as Enzo said. When you use the thing in a rectifier circuit, the positive DC rail comes out of the cathodes and the negative rail comes out of the anodes. This is always the case, I can't think of any exceptions.

Maybe Fender put it on the schematic because at the time they were using selenium rectifier modules that were labelled that way. It is common on bridge rectifiers, but I've never seen it on a single diode.

[Enzo]

The selenium stacks looked like a heating radiator on a pipe, square fins. Unless you had a seriously trained eye, the only way to detect anode and cathode was for it to be printed on the thing. I used to mainly see them as stacks, so dual diode or full bridge. But even single diodes were made and printed the same way.

[Randall]

All I know is if I wired the leg of the selenium stack labeled with + to the 6.8K resistor as in the layout drawing, it didn't work. But in reverse it did work. So I'm still confused.

[g1]

I take it that when you said "didn't work", you meant that it was making a positive voltage rather than proper negative voltage. Careful with that, reverse voltage will blow that cap pretty quick.
Perhaps the original rectifier Fender used was labelled "backwards" (like Enzo and Steve spoke of) and maybe it was replaced with one that is labelled - on the cathode and + on the anode ?

[Jazz P Bass]

This statement strikes me as odd: From Post #1: "So I installed the diode with + going to 6.8K, just like the selenium rectifier. Didn't work."
So the selenium AND the silicon diode have to be reversed to get both to work.
This may be a case of blindly following a layout but not using a meter & a bit of knowledge to verify the circuit.
Can it be that the 6.8K & the 56K resistors are in reversed positions?

[olddawg]

There are those on this board much wiser than me, but I'm not sure I would have looked at the layout or the schematic at all. I would have looked at the polarity of the filter caps and biased the diodes appropriately.

[Jazz P Bass]

Bingo on that thought.
Or at least check which end is towards ground.

[Enzo]

Here, look for yourselves:

http://www.webphix.com/schematic%20h...5f10_schem.pdf

[Jazz P Bass]

My thoughts were that the amp resistors are not as in the layout.
56K to transformer, 6.8K to ground.

[g1]

Here's a shot of the original type (stock) selenium rect.:
The upper leg (+) gets it's AC feed from the 6800ohm resistor. The lower leg is where the negative voltage will be measured. It connects to the neg. terminal of the bias cap. and the 56K resistor. The positive end of the bias cap goes to ground.

[g1]

And another example of Fender schematic showing +/- on diodes to indicate voltage rather than cathode or anode: