12AX7 cathode resistor values

dscottguitars · 10th November 2011, 07:09 PM

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

Minion · 10th November 2011, 11:07 PM

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

Nigel Goodwin · 11th November 2011, 10:41 AM

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.

DF96 · 20th November 2011, 09:45 PM

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.

jjman · 21st November 2011, 03:23 AM

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.

DF96 · 21st November 2011, 10:18 AM

Yes, that would make the clipping level stable rather than depending on what happened in the previous few seconds.

10th November 2011, 07:09 PM	#1
dscottguitars diyAudio Member Join Date: Mar 2011	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

11th November 2011, 10:41 AM	#3
Nigel Goodwin diyAudio Member Join Date: Jul 2005 Location: North Derbyshire	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. __________________ Nigel Goodwin

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10th November 2011, 11:07 PM	#2
Minion diyAudio Member Join Date: May 2006	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

20th November 2011, 09:45 PM	#4
DF96 diyAudio Member Join Date: May 2007	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.

21st November 2011, 03:23 AM	#5
jjman diyAudio Member Join Date: Jan 2009	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.

21st November 2011, 10:18 AM	#6
DF96 diyAudio Member Join Date: May 2007	Yes, that would make the clipping level stable rather than depending on what happened in the previous few seconds.

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