Scaling cathode bias resistor

[EFK]

Maybe I've missed it, but is there any definitive 'rule' as to a maximum dissipation percentage for cathode bias push/pull at idle? For example, you always hear the generality that fixed bias should be no more than 70% at idle. Is there such a generality for cathode bias? 70% max at idle also? Some guys seem to be idling these at more like 85/90%. Anyone have any thoughts?

[Wilder Amplification]

The biasing method doesn't dictate the idle plate dissipation. The class of operation dictates that.

Class AB amps are biased no hotter than 70% dissipation whereas Class A single ended as well as Class A push pull amps are biased at 100% plate dissipation (usually slightly under 100% dissipation for a "cushion").

[Chuck H]

I think what the OP is noting is that cathode bias amps generally idle hotter, even in AB, than fixed bias amps. And
he wants to know if there is a rule for dis % in cathode bias as there seems to be for fixed (the old (but arbitrary) 70% rule). And the answer is...

No.

Cathode bias behaves differently than fixed bias in a few ways. One is that voltage rises at the cathode resistor as more current is drawn by the PA. This cools the bias. So, to have a cathode bias amp run at the same bias under nominal current (actual playing) it needs to start out idling hotter than fixed. How much hotter?... Depends. If your amp requires a large cathode resistor there will be a greater rise effect to the voltage at current draw. This would mean you need to bias a little hotter still to end up at the same nominal current when playing. It's best to check this with a scope to see what's happening to the waveform. If I had to ball park it I'd say between 80% and 100% dis. Usually at the 80% end for amps with relatively high plate volts and as much as 100% for amps with lowish Vp. No, that 100% won't mean your amp is class A. Voltage will rise at the cathode and shift the bias cooler into class AB as soon as a note is played. Fact is many cathode bised class A (push pull) amps actually idle hotter than 100% and can still end up in AB as soon as they start to clip.

HTH

Chuck

[EFK]

Thanks guys - I appreciate you both taking the time to respond to what surely must seem like a remedial question. Chuck, you would make an excellent teacher as you have responded over time to a number of my queries and always have the ability to provide information in a very accessible and simplified manner. Thank you! In particular, I was trying to avoid getting into the whole class A vs. AB thing as frankly I don't understand it. I don;t care what class something is operating in, all I care it that (1) it sounds good to me and (2) I am not burning up tubes like firecrackers!

[Chuck H]

Chuck, you would make an excellent teacher as you have responded over time to a number of my queries and always have the ability to provide information in a very accessible and simplified manner.

Thanks. But I don't think I would make such a great teacher. I'm no EE and the reason I explain things as I do is because that's how I understand them. Learning what I know (which is really quite limited) has been a long slow process because when these techs here start talking in that cryptic electronics lingo filled with shorthand (sometimes written with a "latin" font to make it seem even more cryptic) and math that has squigly lines and as many letters as numbers, that vien in my head starts to show.

I see this end of our craft as a discipline much like learning how to read sheet music as well as scale, chord and inversion relationships. It's just a different language that I need to learn if I ever want to take it to the next level. I just haven't prioritized it yet. But I would recommend it to you if you have the time and desire. If you do there is voluminous info to be had in books. Many of which I own and half of what's in them remains illegible to me.

If I have anything to teach, other than "do as I say, not as I do", it would be to educate yourself in the language of this craft. Much of the math is basic algebra but you'll need calculus too (occasionally you'll see over zealous formulas exploiting geomertry and trig, but not usually). Then there's the shorthand and the relationships that are also written in shorthand and then inserted into these equations. Learn this and you'll already be beyond my knowledge via what you can access from books.

2C

Chuck

Just to add... I'm currently working on an attenuator design that can work as a post power amp EQ with wide range BASS, MIDDLE and TREBLE controls. Others have done it but with limited success IMHO. I have a good model on paper that should out perform anything currently available (though I doubt it could be effectively marketed due to expense and demand). Point is, while ohms law can get me through a lot of this project I do not know the formulas for determining the inductor and cap values (as well as some other nuances I'd like to polish up) so I'm stuck using limited modeling tools such as I'm able to find on line that are often not up to the task. I've looked into SPICE but even that begins to confuse me before I can render it useful. So, while I have grand ideas my knowledge is too limited to bring them full circle. The amount of time wasted trying to approximate values and then the trial and error process makes designing very frustrating and expensive.

I've even considered starting a mod and repair service to supplement my income in these hard economic times. But because I am transistor impaired and CAN'T LEARN IT due to my ignorance of the math and lingo I can't do that.

Just a little more to consider as you decide how YOU will approach this "hobby".