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Hi Group:
I'm working on an output stage of a FIXED BIAS output stage, UL, using
KT88's. According to the data sheet on the KT88, they recommend separate
cathode
resistors. I've seen values in other designs ranging from 0 ohms to 470
ohms! How is
the value derived? I'm using individual bias pots for each tube. The OP
transformer will be
4.3K @ 43% UL taps. I've read through all the books I have and can't find
any discussions on the
subject. I would like to be able to intelligently work this out.
Any help appreciated.

BTW
All of you have a great Christmas!

RonL



Hi Ron.

A low value R (10 Ohms is common) at 5W. So different designs
have a different potential across the current sensing resistor.
I tried to establish some kind of standard value for my own use,
and choose a value for Rc that will give Vc= 2V

On a KT 88 amp, I used 33 Ohms. With 65 mA of idling current
you get a convenient 2V across Rc

On an EL 34 amp, for 50 mA idle current, I use a 39 Ohm
resistor, which again gives 2V

Best regards

Iain