Cathode bias review

[sdorer]

I forgot why aned would appreciate a quick reviewof the following. Why does a 6v6 cathode biased amp use a 250 0hm resistor if both cathodes are biased but a 500 ohm resistor if they are wired independent of each other?

[David Root]

Because two 500 ohm resistors in parallel equals 250 ohms.

[vibratoking]

I forgot why aned would appreciate a quick reviewof the following. Why does a 6v6 cathode biased amp use a 250 0hm resistor if both cathodes are biased but a 500 ohm resistor if they are wired independent of each other?

So, I'm gonna jump in and teach you to fish instead of giving you a fish. This is BASIC stuff and is not something that you should forget because you should be able to figure it out with only basic knowledge.

Look at method 2 in this link:
http://www.wikihow.com/Calculate-Series ... Resistance
You should know this, and all the other methods on that page, if you plan on playing with any kind of electronics.

Now think about what the voltage is on the top side of those parallel resistors using V=IR. In your example, you want the cathode(s), or top side, voltage to be the same in order to achieve the same bias conditions assuming matched tubes. How are you gonna do that?

[tubeswell]

And along the lines of what vibratoking said:

V = I x R

where:

V = Voltage (in Volts)
I = Current (in Amps)
R = Resistance (in Ohms)

Some worked examples:

1 Volt = 1 Amp x 1 Ohm

2 Volts = 2 Amps x 1 Ohm (or = 1 Amp x 2 Ohms)

20 Volts = 20 Amps x 1 Ohm (or = 1 Amp x 20 Ohms, or = 0.1A x 200 Ohms, or = 0.08 Amps* x 250 Ohms, or = 0.04A** x 500 Ohms) etc

*80mA which is about the total tube current of two 6V6s combined
**40mA which is about the tube current through one 6V6

[Jana]

Question: Okay, fine and dandy with all the formulas and stuff. How does that make the big resistor like a small resistor?

Answer: Valentine's Day.

[Structo]

These threads are always so biased.

[johnnyreece]

These threads are always so biased.

Ba-dum CHHHH!

[Colossal]

These threads are always so biased.

Ba-dum CHHHH!

Ha, exactly.

And these threads always seem to degenerate into feedback, sometimes smooth, but sometimes harsh if the debate is grossly imbalanced and there's unpleasant effects from distortion of the truth.

[romberg]

I forgot why aned would appreciate a quick reviewof the following. Why does a 6v6 cathode biased amp use a 250 0hm resistor if both cathodes are biased but a 500 ohm resistor if they are wired independent of each other?

It is to keep the bias voltage the same. The bias voltage is developed by current flowing through the cathode resistor. So suppose each tube is drawing a current I. And suppose they share a single cathode resistor R. So, then the voltage of the cathodes will be elevated by:

V = (2 * I) * R

Now if you use an independent resistor for each tube the current (for a single tube) will still be the same. And if you keep the resistance the same your bias voltage will drop by half (Vbias_to_low = I * R). So, you double the value of R to keep the bias voltage the same.

Mike

[roberto]

Vir bonus dicendi peritus