Is cathode bias the same thing as "self" bias?

[RDK]

I'm building a project amp. It is cathode bias. I was wondering, is this the same thing as "self" bias? I own a kustom 72 coup and was told it is "self" bias. Are they both the same thing. The reason I ask, my 72 coup gets pretty hot and so far I'd like to stray away from that type of bias.

[Chuck H]

The best answer I have is 'probably'. I can't think of any better way to make a tube self biasing than cathode bias. It should be easy enough to check.

Cathode bias, or any self bias, doesn't need to overdissapate the tubes. You can bias the tubes cold with cathode bias and they would still be self biasing.

[RDK]

Thanks Chuck. Most of the people I hear the term "self bias" are not very educated in guitar amp circuitry. I mean I'm new to it myself but I just want to get things straight and understand things better.

[Tom Phillips]

Self bias and cathode bias are not terms that are specific to guitar amps. However in traditional audio amplifier terms they are the same thing.

[Jazz P Bass]

By having a resistor on the cathode, this in itself will make a properly terminated grid more negative than the cathode.
"Self Bias"
Most of the preamp tubes are setup this way.
Please do not get caught up or confused with all of the "bias" hyperbole, mojo & BS.
On a self bias circuit you want to set the tube on the most linear portion of the tubes conduction curve.

[kleuck]

On a self bias circuit you want to set the tube on the most linear portion of the tubes conduction curve.

This woul lead to use only class A amps......

[jmaf]

This woul lead to use only class A amps......

Why?

[Chuck H]

Exactly. There is a most linear region for setting bias on class AB1 too. If the above statement is specifically meant to mean right between saturation and cutoff then that would indeed be class A. But "most linear" is not a single point. It's relative to operating class.

[kleuck]

Because "the most linear portion of the tubes conduction curve" -providing you do not choose a insanely low B+ or a deliberately stupid Output Xfmer for a given tube- is the center of the curve.

Exactly. There is a most linear region for setting bias on class AB1 too. If the above statement is specifically meant to mean right between saturation and cutoff then that would indeed be class A. But "most linear" is not a single point. It's relative to operating class.

Well, "Linear" for me is a precise word, meaning "where the signal can vary in both direction moving more or less on a virtual straight LINE", am i missing something ? Cause i can't figure how it can be anything but a center-biased tube.

[jmaf]

Because "the most linear portion of the tubes conduction curve" -providing you do not choose a insanely low B+ or a deliberately stupid Output Xfmer for a given tube- is the center of the curve.


Well, "Linear" for me is a precise word, meaning "where the signal can vary in both direction moving more or less on a virtual straight LINE", am i missing something ? Cause i can't figure how it can be anything but a center-biased tube.

Check these curves out:


You can bias triodes such as the 300B almost anywhere you want.....

Edit: It's even clearer on the Vg x Ip plot:

[kleuck]

300B are well known for that, but they are triodes, and i think alone in their category.
I don't know a lot of guitar amps using 300B in PP configuration.

[jmaf]

Jazz P Bass said: "On a self bias circuit you want to set the tube on the most linear portion of the tubes conduction curve."

You replied: "This woul lead to use only class A amps......"

So I showed you one example where it would not for a particular tube for which it's easy to see. It's not a scientific rule that to work in the linear region you gotta be in class A, that is my point.

If you look at curves for tetrodes such as the 6L6 and 6V6 you'll see they get very linear towards 0V on the grid, which is not midway between cutoff and zero.

[Jazz P Bass]

Uhmm.
I was only trying to get the "Self Bias" description across to the OP.
No need to get pedantic about operating class.

[kleuck]

If you want, but it's not "the most linear region" in my book, but that's not important, what is, is that curves are A class curves, if you are not using A class, you do not have a line, as the primary impedance varies from one to two, you can draw two lines to choose a bias point, but the reality will be a curve.



http://www.freewebs.com/valvewizard1/pp.html

Uhmm.
I was only trying to get the "Self Bias" description across to the OP.
No need to get pedantic about operating class.

Well, Chuck H gave the answer
Most of the times, commercial cathode-biased amps are biased very hot, to accommodate most of the tubes you can find i suppose, but they do not have to.

[jmaf]

Uhmm.
I was only trying to get the "Self Bias" description across to the OP.
No need to get pedantic about operating class.

The only reason I quoted you was to make a point for the OP.