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Post subject: Interesting Information Learned about Power Tube Bias
Posted: Sat Jun 15, 2019 10:44 am
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Thanks to the people at Eurotubes I learned some very interesting facts about biasing and thought I’d share it here just in case there are any other forum members that, like me, lack technical knowledge in this area.

Last year I bought one of the Pro-One bias probes from Eurotubes. This measures both voltage and current so allows you to do the ohms law calculations to set the correct bias. In my case it is a Fender DRRI.

Prior to getting the Pro-One I had been using a simple octal bias probe and a multimeter which allowed me to measure only the current.

However, I recently swapped the power tubes in my amp and decided to use the Pro-One on one tube and the probe on the other. This showed a significant difference between the current in each tube. Thinking the tubes were mis-matched I switched the devices and found that the tubes were matched but the Pro-One and probe were consistently giving different readings. Across the entire range of the bias control the probe was showing a current approximately 10% higher than that showed on the Pro-One. As I did not have another multimeter to compare with I had no idea which device was correct. So I emailed the good people at Eurotubes.

It turns out that both devices were, in all probability, showing the correct current; but they were measuring different things!

The Pro-One measures plate current using pin 3 whereas the simple probe uses pin 8 (fortunately this was in the user instructions but not all simple probes are the same) and measures the cathode current (which is the combined plate and screen currents) which will be greater.

Problem solved but that raised another question: which current should be used to set the bias, the plate current or the cathode current?

The advice I was given was that whilst both can be used the more accurate is the plate current because the purpose of setting the bias is so as not to over-dissipate the plate.

So what does this mean in practice? Probably not a lot but if you are using a simple probe then it might be worth your while to know which pin it uses. The 10% variance in measured current will mean a 10% difference in bias so if the tubes are biased to, say, 65% dissipation at 20mA using the simple probe, the “real” dissipation will be more like 58% at 18mA.

Of course if you do not use a probe or you bias your tubes using your ears and what sounds good then none of this relavant!


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Post subject: Re: Interesting Information Learned about Power Tube Bias
Posted: Sat Jun 15, 2019 11:25 am
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Nice info amigo. I usually set my BIAS to what will give me the cleanest sound and longest lasting tube life which is usually the recommended setting by the amp manufacturer. I only use a Multi Meter with the BIAS Point on the board of my Hot Rod Deluxe. I got my info on how to do it also from Eurotubes which is where I buy all of my tubes.

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Post subject: Re: Interesting Information Learned about Power Tube Bias
Posted: Sun Jun 16, 2019 2:46 am
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desertbluesman wrote:
Nice info amigo. I usually set my BIAS to what will give me the cleanest sound and longest lasting tube life which is usually the recommended setting by the amp manufacturer. I only use a Multi Meter with the BIAS Point on the board of my Hot Rod Deluxe. I got my info on how to do it also from Eurotubes which is where I buy all of my tubes.


This test point TP38 on 1 ohms resistor is not "accurate" .
The 63 mv you read = 63 ma
63 ma at 431 volts ( schematic) could be different for tubes life or tone than if you have 450 or 400 with different tubes manufacturers
You must read plate voltage too.


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