Help concerning the 1n4007 Diode in the Fender Super Reverb

[coreysan]

Can someone teach me something here? I'm building a Super Reverb kit for the first time. The 1n4007 diode's forward voltage goes from anode to cathode, and the manufacturer labels the anode with a plus symbol ("+") and the cathode with a minus ("-").

In technical literature, the diode symbol looks like this:  ---------- >| ------------
                                                                                              +    -

But when you look at all Fender schematics, the plus sign is positioned at the cathode,
not the anode, like this: --------------- >| ---------------
                                                       -    +
                                                                                                                                         

Why? I'm guessing the people who draw schematics use the "+" and "-" to indicate direction of current flow, *not* polarity.

[sluckey]

In technical literature, the diode symbol looks like this:  ---------- >| ------------
                                                                                              +    -

Anode is > symbol and cathode is | symbol. They are saying that in order for current to flow thru the diode, the anode must be positive and the cathode must be negative. This is the "on" state or forward biased condition. If you reverse the polarities, the diode will be reverse biased and will not pass current (off state).

But when you look at all Fender schematics, the plus sign is positioned at the cathode,
not the anode, like this: --------------- >| ---------------
                                                       -    +

Confusing ain't it. Fender is not the only mfgr to do this. What they mean is that if you input an AC voltage on the anode you can expect a positive DC voltage on the cathode. Or if you input an AC voltage on the cathode, you can expect a negative voltage on the anode.

In the early days before there were any silicon or even germanium diodes, there were selenium diodes. Selenium rectifiers were marked with a + or - symbol. These symbols carried forward to the early days of silicon. These symbols soon gave way to the more familiar (and more intuitive) cathode stripe or actual diode schematic symbols that we see today.

[HotBluePlates]

But when you look at all Fender schematics, the plus sign is positioned at the cathode,
not the anode, like this: --------------- >| ---------------
                                                      -    +
Why? I'm guessing the people who draw schematics use the "+" and "-" to indicate direction of current flow, *not* polarity.

The end of a power transformer winding presents an a.c. voltage to the rectifier diode. When you look at a Super Reverb, there is a grounded center-tap to that winding, so the end of the winding looks positive (with respect to ground) one half-cycle, then negative (with respect to ground) the next half-cycle.

When the winding end is negative with respect to ground, the diode is reverse-biased and blocks any current.

Sluckey explained "forward-biasing" of the diode. When the winding end is positive with respect to ground, the diode could be forward-biased. If the filter cap already has some voltage stored, then the diode will conduct when the winding-end is more-positive than the filter cap voltage. This is the same as saying "when the diode anode is more positive than the diode cathode" because the things attached to the ends of the diode hold the voltages that either forward-bias or reverse-bias the diode.

But diodes are most-often used as rectifiers. You get used to assuming there is a positive (rectified) d.c. voltage at the | end of the diode (when that end is the output of the rectifier), just as you get used to seeing a negative d.c. voltage at the > end of the diode (when that > end is the output of the rectifier)

[coreysan]

You guys - thank YOU!!!! Sluckey and HotBluePlates - may you both get that new Ferrari for Christmas!

That really cleared it up for me. I called the supplier for my kit, and they said they just ignore the
symbols, and I suppose that's fair, but I just really wanted to know.

Again, I can't thank you enough.

Corey