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

wrote:

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

Just use 10 ohms, 5 watt, wire wounds.

The 10 ohms is only there to be able to monitor Ik for each output tube.
Fixed bias amps need negligible value cathode resistors to work properly,
and minimise current feedback which is unwanted in output stages.

Patrick Turner.

Thanks Patrick!
That was my conclusion too. I DO plan on metering the Ik's and wanted to
incorporate
the Rk's for that purpose but wasn't sure why some other designs use other
values.

Thanks


"Patrick Turner" wrote in message
...


wrote:

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

Just use 10 ohms, 5 watt, wire wounds.

The 10 ohms is only there to be able to monitor Ik for each output tube.
Fixed bias amps need negligible value cathode resistors to work properly,
and minimise current feedback which is unwanted in output stages.

Patrick Turner.

wrote in message
news
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

Hey Thanks Iain!
Glad someone in the group still wants to talk about audio every now and
then!




"Iain Churches" wrote in message
...

wrote in message
news
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