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Old 21st July 2011, 02:23 PM   #1
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Default 12au7 600 ohm output

hi there, i wanted to check a 12 au7 cathode follower output stage, what is a good cathode resistor for driving a 600 ohm load? does it matter or should it be quite low? would 100k do? 68k? 47k? 22k? 10k? 600 ohm? sorry if this seems stupid but id like to be sure.
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Old 21st July 2011, 02:54 PM   #2
DF96 is offline DF96  England
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You are asking a significantly incomplete question. HT voltage? Quiescent current? Bias method? Peak signal voltage? Why ECC82/12AU7?

The quick answer is that this valve won't drive a 600 ohm load, unless you are happy with significant attenuation and some distortion.
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Old 21st July 2011, 04:07 PM   #3
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HT 200v and its being driven by the other half of the ECC82/12AU7 with a 220k plate load biased for by a 3.3k resistor and 47uF cap, and because its all i have available and i know they can drive the load, and attenuation is fine, distortion is ok depending on how much, 600 ohm is a worst case scenario and its much more likey to be driving 1.2k-5k loads. (some local NFB may also be used)
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Old 21st July 2011, 04:19 PM   #4
DF96 is offline DF96  England
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An ECC82 CF driving 600ohms would only put out half the signal put in, so not a very effective buffer. I would say 5K is the minimum load resistance, 50K would be better.

Cathode resistor could be somewhere around 10k? It depends on the anode voltage of the previous stage, assuming you are using direct coupling.
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Old 21st July 2011, 05:19 PM   #5
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halving the signal sounds fine for what im planning, maybe even a bit more, 10k cathode resistor, thought so. would i get away with 20k seeing as i have no 10k 5w resistors around?
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Old 21st July 2011, 05:20 PM   #6
SY is offline SY  United States
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Is this for an MI amp or hifi? If the latter, be aware that the distortion will be quite significant.
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Old 21st July 2011, 05:37 PM   #7
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its kind of a guitar/mic preamp, so im not worried about some distortion, its more for testing bits and pieces, tone stacks, gain stage voicings etc. so i can build stuff without building PA sections to go with it and it can be changed fairly quickly.
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Old 21st July 2011, 05:40 PM   #8
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but im looking for a lowest possible impedance out for some recording interfaces, that seem to have 600 ohm inputs.
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Old 21st July 2011, 05:41 PM   #9
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Why not use an ecc88 / 6dj8? Same pin out, better suited.
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Old 21st July 2011, 05:44 PM   #10
SY is offline SY  United States
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Bingo. For low impedance, you want high transconductance. 12AU7/ECC82 might make 2000 umhos on a good day, downhill, with a tailwind. ECC88 is about 5 times better.
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