Bias Diode Schematic Puzzlement

[NickC]

Greetings,

I am puzzled by the depiction of the bias diode in this schematic for the Fender Deluxe Reverb (easiest to see in the second page layout diagram, upper left hand quadrant):

http://www.ampwares.com/schematics/delu ... _ab763.pdf

The schematic symbol for the diode in the bias circuit is shown with the cathode connected to the 470 ohm bias resistor, and the anode connected to bias cap and bias pot. However, the polarity markings for the diode appear opposite (cathode -V, anode +V). AC from the PT flows to the bias resistor and then the diode, is rectified and negative DC flows to the bias pot.

Is the diode marking in the schematic ambiguous, or is the diode not forward biased? Should the diode be installed based on the orientation in the schematic, or is there more to it? In this case the anode side (banded) connects to the 470R?

I know this must be a rookie (rank amateur) question. The more I google trying to figure it out, the more confused I get. Thanks for your patience.

Best Wishes,
Nick

[Alexo]

The diode is reverse-biased.

If you want positive DC, (as in a typical main power supply rectifier) you would connect the banded end to the filter cap and the non-banded (cathode) end to the power transformer.

In a bias circuit, you want a negative DC supply, so you put the diode in 'backwards,' banded end TOWARDS the PT and un-banded end towards the filter cap (which is also 'backwards, positive end to ground, negative end to diode).

[echuta13]

Also make note of the polarity of the cap used in the bias section. Positive is going to ground.

[NickC]

Thank you Alexo and echuta13.

I am reassured now that my wiring is correct, and now I understand. The way the schematic is presented makes perfect sense to me now.

Again, my thanks to you both!

[Firestorm]

A lot of electronic symbols and terminology predate the discovery or at least the understanding of the electron. Remember when computer monitors were called CRTs (cathode ray tubes). That term dates to 1897. The pointy end of a diode points to the more positive side. But of course, current is actually flowing the opposite way ...

[DonMoose]

The diode is reverse-biased.

If you want positive DC, (as in a typical main power supply rectifier) you would connect the banded end to the filter cap and the non-banded (cathode) end to the power transformer.

In a bias circuit, you want a negative DC supply, so you put the diode in 'backwards,' banded end TOWARDS the PT and un-banded end towards the filter cap (which is also 'backwards, positive end to ground, negative end to diode).

Pardon me while I pick a nit:

The diode is Forward-biased during the negative part of the AC wave being supplied to its cathode (voltage on the bias cap, and anode - is higher - as in less negative - than the voltage on the cathode). While the AC wave is positive, the diode is Reverse-biased and does not conduct.

Hope this helps!

[Super_Reverb]

It might not be clear what reverse-bias means. Silicon diodes are fabricated from N-type silicon (extra electrons, doped with phos, antimony or arsenic) and P-type silicon (extra holes / shortage of electrons, doped with boron). When the voltage on anode (p-type) is ~ 0.6V greater than cathode (N-type), the diode conducts current. This condition is called forward bias.

When the cathode is positive relative to the anode, the condition is called reverse-bias, and only a small trickle of current flows - not measureable w/o expensive equipment.

Hence, the name semiconductors.


cheers,

rob