Cathode vs Fixed

[Jammin'John]

No

No cell phone and they all live in different towns.
2 think I work first shift,2 second shift,2 third shift.
All their parents are dead.

JJ

[Enzo]

#2, #4, and #5 all subscribe to Cosmo and have it sent to your address. The mailing labels won't peel off.

[tubeswell]

I agree with you Enzo. The worst I got was when I ditched the model before this one after I had test driven the newer model. The soon to be ex, somehow chucked two full glasses of Gin at my rapidly retreating figure as I ducked, and each narrowly missed my head shattering on the wall behind me. And I thought, 'okay there's a lesson here..."

[chocopower]

Hi there!!

Finally, i had cathode biased my Pro-reverb Blackface clon..

after calculations, i´m running my TAD 6L6WGC at 20.5 watts of plate dissipation...

in a Fixed bias circuit it be a little hot to my taste, but... in a cathode bias amp? Can i go to 80% or 90% as said in a lot of interner places?


thanks!!

[Enzo]

Don't confuse bias with class of operation. A lot of class A amps are cathode biased, but that does not mean all cathode biased amps are operating in class A.

In class A the current flows all the time, so they run a lot higher dissipation. Just because you made your amp cathode biased doesn't change the drive and signal levels.

Is that what you had in mind?

The tube has no idea where its bias comes from, all bias means is the voltage relationship of the grid to the cathode. Now having said that, if we have say 400v on the plates and fixed bias, then there are 400v across that tube. On the other hand in a cathode bias amp, the voltage at the cathode is subtracted. In other words if there is 400v on the plate and 25v on the cathode, then there is really only 375v across the tube.

[chocopower]

Hi!!

Yes i know my amp is still an AB class.

Just looking in the internet y read:

In a fixed bias amp, when a signal is apliqued, bias voltage raise. This is the reason for the 70% rule.

In a cathode bias amp the input signal dont afrect the bias. This way you can go to 80% or 90% plate dissp. with safe...

is this wrong?

[Manic]

http://www.aikenamps.com/Why70percent.html
http://www.aikenamps.com/Biasing.html
http://www.aikenamps.com/ClassA.htm
http://www.duncanamps.com/technical/lvbias.html

I read and re read these regularly...

[Ptron]

If the cathode is bypassed with a large cap, isn't the primary difference between cathode and fixed bias simply lower headroom? Not saying that that's not exactly what we're going for, but if you eliminate the inherent negative feedback developed across the cathode resistor, isn't that pretty much what your left with?

I just re-read this thread and realized I was thinking class A when I reasoned this. In class AB, the bypass cap would charge up to some higher voltage during with greater signal levels, changing the bias of the tubes, and dissipate back down to quiescent levels based on the rc time constant of Ck and Rk. Thus sag, No?