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#1
Posted to rec.audio.tubes
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Cathode resistors in Ultra Linear output??
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 |
#2
Posted to rec.audio.tubes
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Cathode resistors in Ultra Linear output??
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#4
Posted to rec.audio.tubes
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Cathode resistors in Ultra Linear output??
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 |
#5
Posted to rec.audio.tubes
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Cathode resistors in Ultra Linear output??
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 |
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