Calculating cathode resistor for cathode biasing.

[Alan0354]

I decided to use cathode bias power amp using either 6V6 or 6L6. This is my calculation. Please advice:

1) For 6V6 at 400V between plate and cathode and assuming cathode to grid is 35V. For 10W per tube, idle current is 10W/400V=25mA per tube. So for two tubes is 50mA. The cathode resistor is 35V/0.05A=700ohm.

2) For 6L6 at 400V. Assuming cathode to grid is 45V. For 20W, idle current is 20W/400V=50mA. So the pair use 100mA. Resistor is 45V/0.1A=500ohm.

Thanks

[jazbo8]

See this post.

Also for Class AB, Pda should be limited to less than 70% of the maximum plate dissipation.

[Alan0354]

See this post.

Also for Class AB, Pda should be limited to less than 70% of the maximum plate dissipation.

Thanks for the info, I read through the whole thread. I still have question. In the thread, OP measured Vgk=-28V for 29mA current per tube. But in my measurement, For 6V6, When I get 25mA per tube, the bias is about -35V. So there is a difference in reading. I am running at plate to cathode of 400V. So for max of 12W per tube, running at 70% will be 8.4W. Current I=8.4W/400V=21mA.

For Cathode to grid =35V, R=35/0.042=833 ohm!!! That's very high. What am I missing?

[Chuck H]

I suppose that resistance should be divided by two for a shared cathode resistor. So, 416.5 ohms. But...

It sounds like you wish to bias in class A. If you bias at 12W per tube you won't be biased in AB and the 70% suggestion doesn't apply. But, even if you figure your resistance for biasing in class A the number will still look high. And it probably would be in practice. Not sure why, this just seems to be the case more often than not WRT cathode/class A bias configuration. I honestly wouldn't recommend running 6V6's at 400Vp to cathode. It's too high. Certainly too high for reliable class A operation. I've seen a couple old Fender Vibro Champs with 390Vp to cathode biased a bit over class A and they can eat tubes if you run them into clipping. With the wide range of tubes out there, some more fragile than others, I'd be inclined to either lower the Vp or stick to biasing in AB.

[potatofarmer]

Hot bias alone does not an operating class make! I'd like to see a load line for class A 6V6s with 400 volts on the plate.

High plate voltage + cathode bias means you've got to push the bias point warmer to try to avoid crossover distortion and pray that the increase in plate current pushes the cathode voltage high enough to prevent over-dissipation. You know that, what the hay.

[Chuck H]

I was generalizing. As is often done and, I think, acceptable. A tube biased at it's max dissipation is generally said to be biased class A. And indeed you're right to stipulate the balance of forces. Especially problematic with the combination of high Vp and a hot, cathode bias arrangement. I've had plenty of trouble with more modest endeavors than what Alan is proposing.

[JoeM]

Go to the old board and search Fisher bias.

[Chuck H]

Go to the old board and search Fisher bias.

I did that. And now I need to ask, why?

I wasn't sure how far I was supposed to read before finding something relevant, but I ran out of time.

Link?

[jazbo8]

But in my measurement, For 6V6, When I get 25mA per tube, the bias is about -35V.

Something is not right here, how did you measure the current? What is the screen voltage? The figures are "off the charts" if Eg2=250V, they do not even show up on the datasheet.

[JoeM]

I did that. And now I need to ask, why?

I wasn't sure how far I was supposed to read before finding something relevant, but I ran out of time.

Link?

I'll have to find the link or info, but theres a very cool way some old fisher amps had a variable cathode bias method.

[Alan0354]

I suppose that resistance should be divided by two for a shared cathode resistor. So, 416.5 ohms. But...

It sounds like you wish to bias in class A. If you bias at 12W per tube you won't be biased in AB and the 70% suggestion doesn't apply. But, even if you figure your resistance for biasing in class A the number will still look high. And it probably would be in practice. Not sure why, this just seems to be the case more often than not WRT cathode/class A bias configuration. I honestly wouldn't recommend running 6V6's at 400Vp to cathode. It's too high. Certainly too high for reliable class A operation. I've seen a couple old Fender Vibro Champs with 390Vp to cathode biased a bit over class A and they can eat tubes if you run them into clipping. With the wide range of tubes out there, some more fragile than others, I'd be inclined to either lower the Vp or stick to biasing in AB.

I am actually trying to run at 21mA. With 400V, that gives W=400V X 0.021A=8.4V. This is 70% of max dissipation of 6V6 ( 12W). I am not trying to run class A. In fact from my testing and listening, high bias current tends to make the amp sounds more "in your face", stiffer. I know Fender Deluxe RI bias for 25mA and running at 400V.

From calculation of a pair of 6V6 at 21mA each, total current is 42mA. The grid to cathode is assumed 35V, so cathode resistor is 35V/0.042A=833ohm.

If bias for 25mA each R=400V/0.05A=700ohm.

[Alan0354]

Something is not right here, how did you measure the current? What is the screen voltage? The figures are "off the charts" if Eg2=250V, they do not even show up on the datasheet.

I actually measure the current of the 6V6 using a bias probe. The circuit is the typical Fender Deluxe. Screen grid is about 2V below the plate voltage ( drop through the choke).

[Alan0354]

I'll have to find the link or info, but theres a very cool way some old fisher amps had a variable cathode bias method.

I cannot find anything particular. If you have a link, please give me the link.

[jazbo8]

You need to subtract the cathode voltage from the plate voltage to get Vpk, so 400 - 35 = 365V, now re-run your calculations. Here is a handy calculator to save you the trouble in the future.

[pdf64]

Also 35V on the cathode seems rather high?
It's often ok to run the tubes at a static plate dissipation approaching 100%, as it generally tends to reduce under signal, and also due to the inherent limiting / regulation that cathode bias provides.

From Aiken http://www.aikenamps.com/index.php/t...ord-on-biasing
'Cathode-biased class AB amps are usually exempt from the "70% rule", because their cathode voltage rises when a signal is applied, effectively reducing the bias, and shifting the amp further into class AB operation. This means you can bias them hotter than a normal fixed-bias class AB amp and the tubes will still survive. Having said that, you have to experimentally determine how hot you can bias them by finding out how far the bias shifts during signal flow.'