biasing 12ax7

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biasing 12ax7

Postby pedor on Tue May 25, 2004 1:15 pm

How can i bias a 12ax7 for achiving pure class A operation (and symetrical clliping)!
Is there some simple calculation to know this! I dont have an osciloscope, only a multimiter, and i wanted to correct some gain stages that i know by the sound that are a bit offset!
Knowing the B+ voltage, how can i choose the plate and cathode resistors?
please keep it simple...an excel table would be great!!!!
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Re: biasing 12ax7

Postby johnnysavant on Wed May 26, 2004 4:35 am

I'm not sure I understand. You are using a 12ax7 for an output tube? What are you trying to achieve, a 1 Watt amplifier?
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Re: biasing 12ax7

Postby pedor on Wed May 26, 2004 8:16 am

No!
I just want to optimize the Class A operation of the preamp tubes!
Lets say that i increase the plate resitor of a 12ax7 from 100k to 220k, i should also increase the cathode resistor, right? To keep the tube running under optimal conditions!
I just want to know how to calculate this!
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Re: biasing 12ax7

Postby Baron Von Machinenmann on Wed May 26, 2004 8:40 am

This is a good site for the "hows and why's" :

http://www.aikenamps.com/Equations.htm

:cool:
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Re: biasing 12ax7

Postby DreamTone7 on Wed May 26, 2004 9:40 am

Aikens works in theory....tubes are never optimal, though. Just adjust the cathode resistor until the plate voltage is half the B+...and don't forget to subtract the voltage at the cathode from the plate voltage if the cathode is not bypassed by a cap. Make it easy on yourself and use a pot for the adjustment phase, then sub in resistor(s) of the same value when done. Be sure to check it for drift after you've been cranking it for awhile.
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Re: biasing 12ax7

Postby pedor on Wed May 26, 2004 10:11 am

Thanks i will try that!!!
I was reading Aiken article, but it will take some time for me to absorv all theory...
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Re: biasing 12ax7

Postby chill on Wed May 26, 2004 11:42 pm

Get an Ia/Va datasheet for the tube you are using, or use the JJ datasheet. Measure your supply voltage. Divide by your plate resistor to find current. Draw a line between the supply voltage at zero mA and the current at zero V. This is your load line. Find Va where the Vg = 0 line crosses the load line, this is the lowest point Va will swing during operation. Bias the idle point so your plate voltage is halfway between Vsupply and the Va at Vg=0 crossing the load line. Your cathode resistor is calculated by Vg at the operating point divided by Ia at the operating point (since all the current going through the plate is also going through the cathode).
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Re: biasing 12ax7

Postby DreamTone7 on Wed May 26, 2004 11:50 pm

chilll - He's after even clipping...not the cleanest linear operation close to the zero-signal point. Your method would work fine if he didn't intend to get anywhere near overdrive on that gain-stage. Use this method and one side will clip way earlier than the other.

That being said, many amp manufacturers use this to great advantage...like the Dumble ODS. It uses a duel-overdrive system that uses one stage to clip one side of the signal, and the next stage in line to clip the other side. Thing is that it requires somebody who knows what they're doing to set it up to work right...and you have to do it every time you change the 12AX7 that houses these two gain stages.

THAT being said, many people like uneven clipping in a 12AX7...so it's all in the ear of the beholder.
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Re: biasing 12ax7

Postby chill on Thu May 27, 2004 11:02 pm

Yep, it will result in a bit of uneven clipping because the saturation side (going toward B+ voltage and little/no current) will have more headroom than the hard-clip side (Vg=0)...so put the operating point a little bit closer to the B+ side of the line. Or bias the stage so the grid is about half way between zero and the max Vg for your B+, but you'll still get some asymmetry.
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