12AX7 cathode resistor values - diyAudio
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Old 10th November 2011, 07:09 PM   #1
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Default 12AX7 cathode resistor values

Hi, I must be missing something...

I have used very low cathode resistor values, down to ~380 ohms on a 12AX7 preamp tube-second stage, with what I think are very pleasing distortion/gain results.

But I have read that much lower than 800 is bad and will 'hiccup'.

Do my ears deceive me? Or maybe the book I have does...

Thanks,

Daniel
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Old 10th November 2011, 11:07 PM   #2
Minion is offline Minion  Canada
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If it sounds good to you then it sounds good , I generally leave the Cathode resistor at 1k-1.5k and use different values for the Plate resistor (330k ,220k, 150k, 100k) ......

You can also adjust the value of the Cathode bypass cap to get different tonal variations , the Bypass cap with the cathode resistor form a filter which is probably the effect you are hearing when adjusting the cathode resistor(it also adjusts the bias so it is probably clipping more with a low cathode resistor) but I prefer to keep the Tube biased to optimize gain and adjust the cathode cap to get a different tone , a lower value to cut some of the low end and a higher value for more low end ......


Cheers
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Old 11th November 2011, 10:41 AM   #3
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The cathode resistor sets the bias for the valve, as long as you're under the maximum current for the valve you should be fine.
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Old 20th November 2011, 09:45 PM   #4
DF96 is offline DF96  England
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It might depend on what you are driving it with. Low cathode resistor means low bias voltage which means greater risk of grid current on signal peaks. This could cause a bias shift in a coupling capacitor.
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Old 21st November 2011, 03:23 AM   #5
jjman is offline jjman  United States
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Some guitar amps use the opposite approach for the same result. Hi value cathode resistor to bias cold enough to reach cutoff clipping. "Cold clipping." No risk of grid current issues and still get asymmetric clipping.
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Old 21st November 2011, 10:18 AM   #6
DF96 is offline DF96  England
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Yes, that would make the clipping level stable rather than depending on what happened in the previous few seconds.
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