Neg Bias Voltage

[J Rindt]

It has been several years since i worked on an amp. I have forgotten a lot. Including how the neg bias is generated.
For starters.......
In the schem below, i am having a hard time figuring out the current path of travel.
In the tranny, when the center tap to ground is positive, and the tap to the diode is negative, is the current going up through that diode, charging that cap and returning to the tranny down through that 15k resistor to the center tap.?


https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_aa165_schem.pdf

[sluckey]

yes

[PRR]

Erase the stuff that isn't bias. But it seems you got it.

In the tranny, when the center tap to ground is positive, and the tap to the diode is negative, is the current going up through that diode, charging that cap and returning to the tranny down through that 15k resistor to the center tap.

[J Rindt]

Not so fast
 Now when the polarity of the tranny flips, the CT is neg and the diode tap is positive.........current is going up through the  15k, it cannot return through the diode, it is reversed biased, is that right.?And what is that cap doing at this point.?Thank You

[J Rindt]

Not so fast
 Now when the polarity of the tranny flips, the CT is neg and the diode tap is positive.........current is going up through the  15k, it cannot return through the diode, it is reversed biased, is that right.?
And what is that cap doing at this point.?
Thank You

[PRR]

Half the time, nothing happens. The diode is back-biased. The capacitor holds the charge. It decays, but not much, before the next active half cycle.
https://en.wikipedia.org/wiki/Rectifier#Rectifier_output_smoothing

....only upside down.

[J Rindt]

Half the time, nothing happens. The diode is back-biased. The capacitor holds the charge. It decays, but not much, before the next active half cycle.
https://en.wikipedia.org/wiki/Rectifier#Rectifier_output_smoothing

....only upside down.

Right.....OK.

So what is making the voltage  negative.?Is it just the ground reference across that cap or diode, that makes the meter say -50 instead of +50........ or whatever the voltage is.?

Thank You So Much for helping me with this BTW
BOTH of you

[WimWalther]

So what is making the voltage  negative.?

The polarity of the voltage is a direct consequence of the orientation of the diode in the bias supply circuit. If the cathode (line) is the output, the voltage will be positive. But in the bias supply, the Anode (triangle) is the output, so the voltage is negative.

[J Rindt]

So what is making the voltage  negative.?

The polarity of the voltage is a direct consequence of the orientation of the diode in the bias supply circuit. If the cathode (line) is the output, the voltage will be positive. But in the bias supply, the Anode (triangle) is the output, so the voltage is negative.

I have always seen those polarity marks in schematics, but never knew what they meant.
Now i know, but i must be missing some basic understanding of electronics that makes that so...... why the output is Pos or Neg based on what side of the diode you pull it from.

[shooter]

https://www.electrical4u.com/diode-working-principle-and-types-of-diode/

[WimWalther]

Now i know, but i must be missing some basic understanding of electronics that makes that so...... why the output is Pos or Neg based on what side of the diode you pull it from.

The key to this is the fact that the diode is connected to the output of a transformer, which is supplying an AC voltage.

As you probably know, an AC voltage is constantly changing (alternating) in polarity. At any given moment it may be in the negative or positive phase.

Depending on how the diode is oriented, it will either block the negative phase and pass the positive phase, or (as in the case of a bias supply) it will block the positive phase and pass the negative phase. In this sense, a diode behaves like a check-valve for electrical currents.

These periodic negative half-cycles are then passed along where they are accumulated (and integrated..) by the filter capacitor, which acts as a reservoir from which the negative (now DC) bias voltage is drawn.

[tubeswell]

So what is making the voltage  negative.?

The bias supply rectifier diode is connected backwards (reverse-biased diode) so it works 'in reverse' -  It lets the pulse of current on the negative part of the A/C cycle pass, but blocks the positive pulse (so you only have negative voltage being filtered by the filter caps (which also must be mounted in reverse polarity - positive terminal grounded - negative terminal connected to the diode).

[WimWalther]

The bias supply rectifier diode is connected backwards (reverse-biased diode)

"Reverse-bias" doesn't correctly describe the situation.

The bias supply diode is acting as a half-wave rectifier, and like any such diode, it is forward-biased ("on") 50% of the time and reverse-biased ("off") 50% of the time.

[pdf64]

Yes, reverse biased = diode doesn’t conduct.
Forward biased = diodes conducts.

[tubeswell]

Wim and pdf64 are mincing with my words.


‘Reverse-bias’ = conducts ‘backwards’ current forward 50% of the time (where the ‘direction’ of current is relative to ground potential and ‘Backwards Current’ is current).

[shooter]

I'll add some country-boy understanding;
A diode will pass current when the anode is more positive than the cathode by the "break-over" voltage, typically .6-1.2vdc

[J Rindt]

Im sorry, you guys must think i am an idiot. 
Let me start with  this............ why do diodes have those polarity marks in a schematic...... Pos on the cat and Neg on the anode.?

[sluckey]

Let me start with  this............ why do diodes have those polarity marks in a schematic...... Pos on the cat and Neg on the anode.?

That is a holdover from the days of selenium rectifiers. That's just how selenium rectifiers were marked back then. Kinda confusing ain't it? Just accept it. I just ignore the polarity markings when present. The important things to remember is that current can only flow in one direction. And that direction is into the point of the arrow (cathode to anode, just like a tube).

[J Rindt]

Let me start with  this............ why do diodes have those polarity marks in a schematic...... Pos on the cat and Neg on the anode.?

That is a holdover from the days of selenium rectifiers. That's just how selenium rectifiers were marked back then. Kinda confusing ain't it? Just accept it. I just ignore the polarity markings when present. The important things to remember is that current can only flow in one direction. And that direction is into the point of the arrow (cathode to anode, just like a tube).

Right... OK
For that to happen in this case... correct me if i am wrong... when the neg wave hits the secondary winding, the polarity of the winding has to be Neg at the diode cathode and Pos at  the center tap.?

[WimWalther]

Wim and pdf64 are mincing with my words.
‘Reverse-bias’ = conducts ‘backwards’

But rectifier diodes don't conduct "backwards", that would defeat their purpose. The diode in the (negative)  bias supply conducts in precisely the same manner as any other.

(Zener diodes are an exception, though they're not normally used as rectifiers.)

[sluckey]

when the neg wave hits the secondary winding, the polarity of the winding has to be Neg at the diode cathode and Pos at  the center tap.?

correct

[J Rindt]

when the neg wave hits the secondary winding, the polarity of the winding has to be Neg at the diode cathode and Pos at  the center tap.?

correct

OK Thank You
Bear with me here.Again, correct me if i am wrong.
 To the diode it is just current flowing that raises to a voltage of.... lets just say 50 volts.
But the diode does not know, that on the primary side of the tranny, this came from a "Negative Wave"
The diode just knows that the current is going the right way and the voltage at the cathode got above 0.7 volts, so it turned on.What is making this voltage negative where the diode is concerned.?

Do you understand my confusion.?
Suppose the Pos Wave on the primary had caused that same polarity on the secondary winding so that  current would have flowed during the pos wave instead of the neg wave.Would the voltage from ground to anode have been positive.?
I must be missing something very basic somewhere

[sluckey]

I must be missing something very basic somewhere

I believe so. You need to understand Ohm's Law. And then you need to understand that current can only flow through a diode in ONE direction. For example, looking at the attached pic, the diode will be forward biased (switch ON) when the AC present at the bias tap is negative. The diode acts like a switch and allows current to flow up through the diode, turn left through the bias pot, turn down through the 15K resistor to ground and back to the PT center tap. This is the completed circuit. As the current travels through the pot resistor a voltage drop is created based on the formula E = I x R, and the polarity of that voltage drop will be as shown. Just accept it. This is always true. As the same current travels on through the 15K, another voltage drop occurs, but the voltage will be different because the resistance is different. The resistors and the current flow are what determine the voltage.

Now let's say the AC at the bias tap swings positive in respect to the CT (ground). Now the diode will be reverse biased and the switch is off (open). Obviously no current can flow now. This whole AC cycle repeats 60 times per second in the USA.

That's all I got to say about this. If you ain't got it by now, you just need to go back to schoolin'.   

[J Rindt]

I must be missing something very basic somewhere

I believe so. You need to understand Ohm's Law. And then you need to understand that current can only flow through a diode in ONE direction. For example, looking at the attached pic, the diode will be forward biased (switch ON) when the AC present at the bias tap is negative. The diode acts like a switch and allows current to flow up through the diode, turn left through the bias pot, turn down through the 15K resistor to ground and back to the PT center tap. This is the completed circuit. As the current travels through the pot resistor a voltage drop is created based on the formula E = I x R, and the polarity of that voltage drop will be as shown. Just accept it. This is always true. As the same current travels on through the 15K, another voltage drop occurs, but the voltage will be different because the resistance is different. The resistors and the current flow are what determine the voltage.

Now let's say the AC at the bias tap swings positive in respect to the CT (ground). Now the diode will be reverse biased and the switch is off (open). Obviously no current can flow now. This whole AC cycle repeats 60 times per second in the USA.

That's all I got to say about this. If you ain't got it by now, you just need to go back to schoolin'.   

Oh Yeah
Thank You
That one red line you drew on the schem.... direction of current through a resistor and polarity of the voltage drops.... makes it clear.
Thanks for hanging in there with me. 

[J Rindt]

Just a trivial question.
Some parts get a name..... slope resistor, grid stopper, clipping diode, etc etc.
Is there a name for that resistor in the bias schem.?
That 15k to ground.
Thank You

[sluckey]

The proper name is "Fifteen Thousand Ohms". The nickname is 15K.   

[WimWalther]

Just a trivial question.
Some parts get a name..... slope resistor, grid stopper, clipping diode, etc etc.
Is there a name for that resistor in the bias schem.?
That 15k to ground.
Thank You

If there is a standard, purpose-based name for that part, I'm not aware of it.

But what it is, is the ground-side resistor in a single-pole (or single-tap) voltage divider. The voltage accumulated by the cap is divided between the two resistors in a 10:15 (or 1:1.5) ratio.

[J Rindt]

The proper name is "Fifteen Thousand Ohms". The nickname is 15K.   

Good Grief

He will be here all week Ladies and Gentlemen   

[J Rindt]

Just a trivial question.
Some parts get a name..... slope resistor, grid stopper, clipping diode, etc etc.
Is there a name for that resistor in the bias schem.?
That 15k to ground.
Thank You

If there is a standard, purpose-based name for that part, I'm not aware of it.

But what it is, is the ground-side resistor in a single-pole (or single-tap) voltage divider. The voltage accumulated by the cap is divided between the two resistors in a 10:15 (or 1:1.5) ratio.

Right  Right
Thank You 

[PRR]

I'll add some country-boy understanding;

Here's another low-theory analogy.

I rented a cabin on the sea. Tide came in, tide went out. There was a dam and a pond behind it.

Put a One-Way Valve in the dam.

If you put it one way, the pond drains down (negative) to the lowest low tide. If you put it the other way, the pond rises to the highest high tide.

Tide-rectifiers are easy to see. Remembering which-way in electricity is tougher. The conventional rectifier "arrow" does not point in the direction of the electrons. Just memorize a basic rectifier, don't try to figure >| or +/- marks on diodes.

The "Fifteen Thousand Ohms" is really part of the adjustment pot. Imagine combined with the 10k to make a 25K total pot. But if you dial it past half-way your bias is too close to zero and your tubes fry in moments. You can make a mark on the pot but who looks?? You can add a barrier to stop the knob pointer but you know someone will force it. Splitting the pot into a 10K variable section and a 15K fixed section (I'd say "stopper") gives you -70V to -42V, the good bias range, without the dangerous -42V to zero range. (And if that's not right, a different 12 cent resistor will give a different range.)

Why has nobody noticed the four hundred and seventy ohm resistor? And why is it a whole Watt?

[WimWalther]

FWIW, I call the 15K part a 'backstop' resistor. No matter where you set the pot, it insures that the pot wiper is never closer than a 15K voltage drop to ground. In other words, the bias voltage is never zero.. but always within some sane range (even if it's too cold or too hot).

ETA: Ideally, the 3 resistors in a bias supply divider (1st resistor, Pot, backstop) are selected so that only the useful range of voltage falls across the pot. For instance, if the ideal bias voltage is -15V, you select the parts to drop the range from -10V to -20V across the pot.

This gives enough range of adjustment that even the "coldest" or "hottest" tubes can be set to the desired bias current.

[sluckey]

He will be here all week Ladies and Gentlemen   

Are you not entertained?