Fixed Bias Equation

[plexi50]

I have a fixed bias using a 250 ohm resistor. I have 355 VDC on the plates of a pair of 6L6GC tubes

The bias voltage is 27.00 VDC. Is this per 6L6GC tube making a total of 54.00 VDC ???

[jeff]

No the bias voltage is 27V.
Each tube's grid is -27V relative to the cathode.

The current for both tubes is going through that resistor
V=IR
27=I*250
I=27/250
I=.108mA for two tubes
or 54mA each tube(plate and screen)

about 10% of that is screen current so each plate's current is ~48.6mA

The tubes are running at 328V(Plate-Cathode:355V-27V)
48.6mA*328V=15.94W

[plexi50]

Im missing something.

[Fresh_Start]

Not "fixed bias".  Cathode bias.

Jeff's calculations look correct to me.  The combined plate & screen grid currents of both tubes are going through that 250 ohm resistor.  Calculate the current and divide by 2.  

16 Watts (the way he calculated it) is barely above 50% of max plate dissipation.  Most guys ignore the fact that screen grid current shows up at the cathode too, so using 54ma per 6L6 you get 19 watts or 64% of max.  Might be a bit cool for cathode biased.

I haven't done a cathode biased 6L6 amp so don't know where the sweet spot might be.

Chip

[plexi50]

Well some thing is weird in my world today. I have an amp that sounds great with 27VDC on the cathode but no grid voltage is showing on my meter. Zero The 6L6GC tubes have that nice blue aura in them and they are too hot to touch.  I get no pin 5 grid voltage. I have several DVM's. Is this one of the side effects of the world ending last month? I will have to do this again tommorrow. I have no pin5 grid voltage

[VMS]

Biasing is all about making voltage difference between grid (G1) and cathode. The aim is to make cathode voltage more positive than grid voltage.

In 'fixed-bias' cathode is connected to ground which means 0V and therefore we have to make negative voltage for the grid.

In 'cathode-bias' grid is connected to ground through grid leak resistors which means that the grid voltage is 0V and therefore we have to make positive voltage to cathode. This is done with right size cathode resistor.

Here is some better explanations:

http://www.harpamps.com/micKqanda/What-is-Bias.html

http://www.aikenamps.com/WhatIsBiasing.htm

[jjasilli]

Let's start over:

I have a fixed bias using a 250 ohm resistor. That's cathode bias, not fixed bias.

I have 355 VDC on the plates of a pair of 6L6GC tubes Your schematic shows 6V6 tubes.

The bias voltage is 27.00 VDC. That's the cathode voltage.  I think that by definition it is not bias voltage.  Bias voltage is derived from a negative voltage supply and applied to the grid of the tube.

Is this per 6L6GC [sic] tube making a total of 54.00 VDC ??? No.  Each 6V6 power tube cathode has +27VDC on it.  Each grid is at -0- volts. So, as stated above, you could say that the cathode is 27VDC more positive than the grid, which is another way of saying that the grid is 27VDC more negative than the cathode.  (See Jeff's & Fresh_starts posts above).

The max plate dissipation for 6V6 is 14W; for PP you probably want 90% of that = 12.6W.  So 16W is quite hot.  If you swapped-in 6L6's then max plate dissipation is 30W X 90% = 27W, so 16W is way cold.

[plexi50]

Does the capacitor play any role in the setting of the voltage at the cathode?

[jeff]

Fixed bias. I totally missed that. I heard "resistor" and was trying to answer if it was 27V or 54V and didn't even catch that. Blanked out for a sec.

The point of bias is to make the grid negative relative to the cathode.
In fixed bias, a negative voltage is applied to the grid and the cathode is grounded.
In cathode bias, the grid is at 0V and a positive voltage is developed on the cathode because of the current through the resistor.

Whether the grid is at 0V and the cathode is at 27V or the grid is at -27V and the cathode is at 0V, the tube sees that the cathode is 27V more positive than the grid, and the grid is 27V more negative than the cathode.  

[jeff]

Does the capacitor play any role in the setting of the voltage at the cathode?

Not in setting the voltage but in keeping it constant when a signal is applied to the grid. At idle the voltage will be the same cap or no cap.
When you apply a signal to the grid, you allow more or less current to flow. As you draw more or less current through the 250 resistor according to V=IR the cathode voltage will go up or down. The cap takes the signal ripple out of the voltage and keeps the cathode voltage constant.

Think of a preamp tube, preamp tubes are cathdoe biased.

As more current is drawn through the cathode resistor the voltage on the cathode increases and the grid to cathode voltage changes. With no cap as the grid voltage increases, more current goes through the cathode resistor so the cathode voltage increases too. Now the difference between grid and cathode is not 1V even though you're going from -2V to -1V on the grid. Because the voltage is changing on the grid and the cathode that 1V change does not appear to the tube to be a 1V change. Negative feedback. Going from -2V to -1V on the grid, with a cap keeping the cathode voltage constant, the change in the relationship between the grid and cathode is 1V: more gain.

With a smaller cap the higher frequencies are grounded, or to be more correct the cap can't charge and discharge quick enough to keep the voltage constant at lower frequencies so, the treb is accented.

But, does using a small cap boost treble? You can think of using a small cap as canceling negative feedback
on high frequencies, boosting treble, or think of a small cap as introducing negative feedback on lower frequencies lowering gain on low frequencies.

It's a little different with a PP output because as one tube draws more the other draws less.

[phsyconoodler]

Quote:"I have no pin5 grid voltage "

Good.You have 27v on the cathode.The grid is negative to the cathode.You could lower the cathode resistor a bit until you get those tubes cooking away at 25-30 watts and it might sound even better.Only one way to find out.Do it.

Go to the Weber Bias Calculator and plug your figures into the formula for cathode biased amps.The schematic you showed is for blending EL84's and 6V6's.Because they draw differently they need to have that sophisticated switching/cathode cap arrangement to prevent the tubes from being too hot.I never did like the Mesa Blue angel myself.Not bad sounding but who needs all that extra crap inside an amp.IMHO.
  When I cathode bias an amp I like to use a cap/resistor for each tube.Then I can lift the caps to get lower power.Squishy and sweet sounding.i like the tone better with a cap/resistor for each tube also.Seems more present and a bit punchier.Me like.

[jeff]

If you use seperate cathode resistors they should be double the restance of sharing one.
Ex. One shared 250ohm or two seperate 500ohm.

I like to use a cap/resistor for each tube.Then I can lift the caps to get lower power.Squishy and sweet sounding.i like the tone better with a cap/resistor for each tube

You could get away with using only one cap.

[plexi50]

Ok heres what i have done. I did away with the EL84 tubeas and all parts and 1 meg blender pots associated with the original mix circuit. I am now using only the 6L6GC tubes. I like the headroom better.  In the next few minutes i am going to lower the cathode resistor and try to get the cathode voltage up on the 6L6GC tubes. I dont want to loose any plate voltage though. May have no choice in that matter

I have just looked at the Weber Cathode bias calculator and i have the following results:


How many tubes share the same cathode resistor? (2)
 
Voltage measured across cathode resistor               (27)

Actual resistance of cathode resistor                        (250)
 
DC voltage measured between plate and cathode. (Actual Plate Voltage)  (360)
 
Plate dissipation per tube (Watts)                             (18)
 
Approximate plate current in each tube (in milliamps) (51)

[plexi50]

I just raised the cathode resistor to 300 ohm. I now get the following:

How many tubes share the same cathode resistor? (2)
 
Voltage measured across cathode resistor               (28.95)

Actual resistance of cathode resistor                        (297)
 
DC voltage measured between plate and cathode. (Actual Plate Voltage)  (362)
 
Plate dissipation per tube (Watts)                             (17)
 
Approximate plate current in each tube (in milliamps) (46)

Im going to lower the cathode resistor to 200 ohm next

[phsyconoodler]

Quote:"You could get away with using only one cap."

Naw! I use two and the negative end of the caps go to the switch.When the switch is on they form a virtual center tap to ground.When the switch is off they are out of the circuit.
  Works perfectly and NO pops.
Plexi,
  You are headed in the right direction.200 ohm or even smaller is where you need to be to get those tubes working harder.Do a tone test with each setting so you can determine if hot is where you like the tone.It may not be,so don't just assume that hot is better.

[jjasilli]

Im going to lower the cathode resistor to 200 ohm next With cathode bias you wind up mostly chasing your tail, so to speak.  There are many posts and much literature on this.  At first it seems that a larger resistor will limit current.  But the cathode & plate are eventually tied together.  So a larger cathode resistor "boosts" the plate voltage, which in turn affects the current pulled through the cathode resistor.  You keep winding up around the same bias point more or less.  You could keep yourself busy for hours; I did!  

[plexi50]

Yes thats all i did today was chase resistors. So i have converted it to a fixed bias using a 10K panel mounted pot. After all my work today it turns out that my bias pot is no good. It was in a pile of stuff i had. I ordered 3 new ones. What a day

[Fresh_Start]

Yes thats all i did today was chase resistors. So i have converted it to a fixed bias using a 10K panel mounted pot. After all my work today it turns out that my bias pot is no good. It was in a pile of stuff i had. I ordered 3 new ones. What a day

Now I'm confused.  

Fixed bias is when you apply a filtered, negative voltage to the power tube grids (through 220K "grid return resistors") and the power tube cathodes are grounded (IOW at zero volts DC).  

The schematic you posted is cathode biased - the power tube grids are at -0- volts DC and the voltage at the cathodes is raised by inserting a resistor between the cathodes and ground.

Converting to fixed bias requires a well-filtered, negative voltage supply to start with.  A new pot isn't going to make the conversion.

Maybe the confusion comes from the fact that "fixed bias" circuits often include a pot to adjust the negative voltage applied to the power tube grids, while "cathode bias" allows the power tubes to find their own bias point based on the voltage applied to the plates & screens plus the value of the cathode resistor(s).  Sometimes "cathode biased" circuits are referred to as "self biasing".  The term "fixed bias" is confusing.

Respectfully,

Chip

[plexi50]

Yes it is confusing. I removed the cathode biasing resistor and caps. Pin 8 to 1 ohm and to ground. I went with the marshall bias having a supply voltage 100K / 4007 and caps along with a 10K bias pot. I figure i might as well have the ability to swap 6L6GC and 6V6 if i choose.

[plexi50]

I ended back where i left off with a 200 ohm resistor and the following. Sounds good enough for me. Plenty toneful and LOUD. Decided to forget the bias pot and stick with the cathode biasing. A big part of getting the bias where i want it is i used an 18 watt 290-0-290 PT here and the plate voltage drops quite a bit from 360VDC to 345 VDC when using 6L6GC tubes biased at 21 watts per tube.  I love the 6V6 tone but lacks headroom. This PT is better suited for the 6V6 tubes
 
Voltage measured across cathode resistor               (25.50)

Actual resistance of cathode resistor                        (200)
 
DC voltage measured between plate and cathode. (Actual Plate Voltage)  (345)
 
Plate dissipation per tube (Watts)                             (21)
 
Approximate plate current in each tube (in milliamps) (60)

[jeff]

i used an 18 watt 290-0-290 PT  This PT is better suited for the 6V6 tubes

Your pulling almost double the current using 6L6 vs 6V6. Sure you're still within current rating of transformer specs?
Bigger tube usually needs a bigger transformer.
Just double checking, I'd hate to see you fry the transformer.

[plexi50]

Yeah it looks like i better just stay with the 6V6 tubes.  The transformer is rated at 120ma DC. With 6V6 tubes is the following:

Voltage measured across cathode resistor               (19.60)

Actual resistance of cathode resistor                        (250)
 
DC voltage measured between plate and cathode. (Actual Plate Voltage)  (360)
 
Plate dissipation per tube (Watts)                             (13)
 
Approximate plate current in each tube (in milliamps) (37)