Diode polarity confusion

[kanderson70]

This is really a rudimentary question, but I'm a little confused on something and hope you guys can help. I'm building a blackface Tremolux and noticed something off with the layout and schematic. The Fender layout and schematic clearly shows the polarity around the diode symbol...

AB763_Diode_polarity.PNG

but here is what I thought the polarity of a diode looks like...

diode_polarity.png

Which is correct? and why is it shown differently?

any help is appreciated.
Thanks,
Kelly

[kanderson70]

ah... after digging around I found this has been asked before:

https://ampgarage.com/forum/viewtopic.php?t=22144

although... I'm still confused.

[sluckey]

Fender's idea of using polarity signs to mark the diodes is a carry over from selenium diodes. Selenium diodes usually were marked with a + on the cathode to indicate the polarity of voltage that would pass thru a diode if you put an AC voltage on the anode. Diode symbols had not become standardized at this time.

If you apply an AC voltage to the anode (which is the case of an AB763 Twin Reverb B+ rectifier), the diode will only pass the positive portion to the cathode. The negative portion will be blocked. Or, if you apply an AC voltage to the cathode (which is the case of the fender bias circuit), the diode will only pass the negative portion to the anode. The positive portion will be blocked.

It is a bit confusing. Probably best to forget the polarity markings on 50 year old schematics and just remember that the banded end is the cathode.

[kanderson70]

Fender's idea of using polarity signs to mark the diodes is a carry over from selenium diodes. Selenium diodes usually were marked with a + on the cathode to indicate the polarity of voltage that would pass thru a diode if you put an AC voltage on the anode. Diode symbols had not become standardized at this time.

If you apply an AC voltage to the anode (which is the case of an AB763 Twin Reverb B+ rectifier), the diode will only pass the positive portion to the cathode. The negative portion will be blocked. Or, if you apply an AC voltage to the cathode (which is the case of the fender bias circuit), the diode will only pass the negative portion to the anode. The positive portion will be blocked.

It is a bit confusing. Probably best to forget the polarity markings on 50 year old schematics and just remember that the banded end is the cathode.

Thank you sluckey for such a great and descriptive reply. I learn new things from you guys every day. I'll just stick with following the symbols then