Help with negative bias voltage

[J Rindt]

Electronics is pretty new to me. I was wondering if somebody could explain the AA864 bias circuit to me. When I look at the schematic, it shows a + and - on either side of the diode (I thought the cathode and anode were the other way around) that is in series with the 1W 470 ohm resistor. I am assuming the initial current flow is through that 470 resistor and then just the positive half of the sine wave goes through the diode.
At that point, does the 25/50 cap charge to its capacity (approximately the voltage of the transformer tap) and then does it discharge through the 15k resistor and the 10k pot? Is that what the path of travel looks like?
I guess with the + side of the 25/50 cap at ground, the voltage across the 15k resistor and 10k pot would be negative when compared to ground. Is that what is happening?  The + and - on either side of the diode are a little confusing to me. I realize this must be pretty basic knowledge for a lot of you guys, sorry about that. I cannot find a good, beginner level explanation of this.
Thank You

http://www.el34world.com/charts/Schematics/fender/BASSMAN_AA864.pdf

[JayB]

Electronics is pretty new to me. I was wondering if somebody could explain the AA864 bias circuit to me. When I look at the schematic, it shows a + and - on either side of the diode (I thought the cathode and anode were the other way around) that is in series with the 1W 470 ohm resistor. I am assuming the initial current flow is through that 470 resistor and then just the positive half of the sine wave goes through the diode.
At that point, does the 25/50 cap charge to its capacity (approximately the voltage of the transformer tap) and then does it discharge through the 15k resistor and the 10k pot? Is that what the path of travel looks like?
I guess with the + side of the 25/50 cap at ground, the voltage across the 15k resistor and 10k pot would be negative when compared to ground. Is that what is happening?  The + and - on either side of the diode are a little confusing to me. I realize this must be pretty basic knowledge for a lot of you guys, sorry about that. I cannot find a good, beginner level explanation of this.
Thank You

http://www.el34world.com/charts/Schematics/fender/BASSMAN_AA864.pdf

For every positive wave form, there is negative wave form as well. Let's say with your bias tap, you got 50Vdc. The center tap would be the other end of it with -50Vdc. Think of the current running the opposite way. Your just filtering the -Vdc portion of it.

Wait for PRR, he'll get scientific with it.

[FYL]

Yes, bias is negative re. ground. It limits the current naturally drawn by the tube(s): no bias voltage, max current drawn and a red plating tube; too much bias, very little current and a tube in cut-off.

Bias also conditions how the tube(s) work in a push-pull power stage: high bias voltage/low idle current, the amp is class B, idling for one half of the cycle, then conducting on the other half. Transition isn't instaneous, hence crossover distorsion and other nasties. More bias - which means lower bias voltage, -30V is lower than -40V ! - and you're in AB, with tubes conducting for more than half cycle and nearly no crossover distorsion. A lot of bias, the tubes conduct for a full cycle, you're in class A.

This said, te Fender circuit used in the AA864 is pretty standard : an app. 50V bias tap on the PT, 470R in series as a load, then a diode - remember, we're dealing with negative voltages, hence the anode/cathode orientation, a 25µ smoothing/reservoir cap with positive to ground (we're still dealing with a negative voltage) and a 10K pot + 15K R acting as a voltage divider.

[sluckey]

In the early days of silicon diodes, a plus symbol (+) was often used to designate the cathode of the diode in drawings as well as on the body of the diode itself (possibly a carry over from the selenium diode notation). It was confusing. That's why that convention did not stick and become an industry standard. Just remember these few things and you'll be able to keep it straight.

1. Current flows from negative to positive in a circuit.
2. Current flows from cathode to anode inside a diode.
3. Forward bias means the voltage applied to the anode of a diode is positive in respect to the cathode voltage. The diode will pass current.
4. Reverse bias means the voltage applied to the anode of a diode is negative in respective to the cathode voltage. The diode will not pass current.

[J Rindt]

"1. Current flows from negative to positive in a circuit.
2. Current flows from cathode to anode inside a diode.
3. Forward bias means the voltage applied to the anode of a diode is positive in respect to the cathode voltage. The diode will pass current.
4. Reverse bias means the voltage applied to the anode of a diode is negative in respective to the cathode voltage. The diode will not pass current."

OK...... bare with me here. During the negative swing of the wave, the cathode is negative which makes the anode positive with respect to the cathode. In this mode the diode will conduct the negative half of the sine wave, and the cap will charge to the value of the transformer tap. Then the cap will discharge through the 15k, and the bias pot; which smooths out the ripple from the diode and provides a relatively straight line of DC to the grids of the power tubes.
Do I have this right now?
Thank You

[sluckey]

Good 'nuf.