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Post subject: Generic Procedure To Bias An Adjustible Fixed Bias Amp
Posted: Mon Aug 19, 2013 8:20 pm
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Here is the generic procedure for properly biasing the power output tubes of an adjustable fixed biased amp:

In order to properly set the power tube bias, you need to measure both the plate current flowing through the tube as well as the plate voltage present at the tube for the measured current. You then multiply those two values together and compare the result to the rated maximum output for that tube. In the case of a 6L6GC, 30 watts is the maximum rated output for a single tube. In any amp, you adjust the plate current so that the product of plate current and plate voltage is between 50% and 70% of the max power. Measuring cathode current is safer and easier than measuring plate current and is close enough for government work and actually gives you a little safety margin to avoid biasing the tubes too hot. As you adjust the cathode current using the bias adjustment pot, the plate voltage will change. Increasing the cathode current will cause the plate voltage to decrease. Decreasing the cathode current will cause the plate voltage to increase. Each time you adjust the cathode current, you need to remeasure the plate voltage and recalculate the percent of maximum plate dissipation until you reach the desired bias setting.

The best and safest way is to measure both cathode current and plate voltage using a bias probe and a DVM. A good bias probe (the one I use) is this:

http://www.amp-head.com/product_info.ph ... ucts_id=70

Weber sells a bias probe that includes the meter:

https://taweber.powweb.com/biasrite/br_page.htm

There are many You Tube videos available that show the proper way to bias amps in general and for specific amps. Although specific to the Hot Rod amps, this video is a good one and the basic principles are the same:

http://www.eurotubes.com/eurotubes-Hotr ... lts-mA.htm

Finally, you might want to read these to learn more about amp biasing:

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

http://www.aikenamps.com/Why70percent.html

Keep in mind that while biasing an amp, the amp is powered on and you are exposed to high voltages which can injure or kill. This is a good read also:

http://www.aikenamps.com/SafetyTips.html

Good luck, have fun, and be careful if you decide to do this yourself. With the proper tools and procedures as outlined, biasing an amp is very safe.

Note: There are other ways to calculate power tube bias. A Google search will provide plenty of links.

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Post subject: Re: Generic Procedure To Bias An Adjustible Fixed Bias Amp
Posted: Tue Aug 20, 2013 3:37 am
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bluesky636 wrote:
Here is the generic procedure for properly biasing the power output tubes of an adjustable fixed biased amp:

In order to properly set the power tube bias, you need to measure both the plate current flowing through the tube as well as the plate voltage present at the tube for the measured current. You then multiply those two values together and compare the result to the rated maximum output for that tube. In the case of a 6L6GC, 30 watts is the maximum rated output for a single tube. In any amp, you adjust the plate current so that the product of plate current and plate voltage is between 50% and 70% of the max power. .


Do you know Weber Bias calculator ?

http://www.webervst.com/tubes1/calcbias.htm


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Post subject: Re: Generic Procedure To Bias An Adjustible Fixed Bias Amp
Posted: Tue Aug 20, 2013 2:56 pm
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Location: Natural Bridge, Virginia
stratele52 wrote:
bluesky636 wrote:
Here is the generic procedure for properly biasing the power output tubes of an adjustable fixed biased amp:

In order to properly set the power tube bias, you need to measure both the plate current flowing through the tube as well as the plate voltage present at the tube for the measured current. You then multiply those two values together and compare the result to the rated maximum output for that tube. In the case of a 6L6GC, 30 watts is the maximum rated output for a single tube. In any amp, you adjust the plate current so that the product of plate current and plate voltage is between 50% and 70% of the max power. .


Do you know Weber Bias calculator ?

http://www.webervst.com/tubes1/calcbias.htm


Yes, I have referenced it many times in posts on calculating bias. It is what I use to calculate plate current knowing cathode current, but not really necessary to do a good job as the difference is small and using cathode current provides a reasonable safety margin for those less experienced or using less accurate tools.

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Post subject: Re: Generic Procedure To Bias An Adjustible Fixed Bias Amp
Posted: Mon Oct 28, 2013 5:12 am
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Looks like this is the tool to do it .. even for beginners. :P

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Post subject: Re: Generic Procedure To Bias An Adjustible Fixed Bias Amp
Posted: Mon Oct 28, 2013 5:59 am
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Mine is double one ; this one is for 6L6 tubes size and I have the probe for EL84 / 6AQ5 tube too.

www.amp-head.com/product_info.php?cPath ... ucts_id=70


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Post subject: Re: Generic Procedure To Bias An Adjustible Fixed Bias Amp
Posted: Mon Oct 28, 2013 11:13 am
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Joined: Mon Apr 05, 2010 2:37 pm
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Location: Natural Bridge, Virginia
stratele52 wrote:
Mine is double one ; this one is for 6L6 tubes size and I have the probe for EL84 / 6AQ5 tube too.

http://www.amp-head.com/product_info.ph ... ucts_id=70


Yup. That's what I use along with a good DMM. No amps with EL84s yet. Maybe my next build? :lol:

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Post subject: Re: Generic Procedure To Bias An Adjustible Fixed Bias Amp
Posted: Mon Oct 28, 2013 11:42 am
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On my own build I alway put 1 ohms precison resistor . But on some customer amps like Peavey Classic 30 too much job or some vintage Vox which I don't what to change the look I use this EL84 bias prob.


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