Measuring bias

pof@ · 17th November 2011, 12:45 PM

I have a SE amplifier with some sort of autobias. I don’t have any schematic. I would like to check what the bias is without taking the box apart.

I think this:
Tube-Town Online-Store - Bias-Messadapter Oktal, Fertigadapter tt-probe8
has a 1 ohm in series with the cathode. Would this affect any known autobias circuits so my measurements will be incorrect?

SY · 17th November 2011, 12:51 PM

Do you mean bias or idle current? These terms are often interchanged but have VERY different meanings. Idle current can be determined by measuring the voltage from cathode to ground, then dividing by the resistance from cathode to ground. Bias is the voltage between cathode and grid and can usually be measured directly.

pof@ · 17th November 2011, 01:52 PM

Well, the adaptor I'm referring to is described to be used for bias. But in your terms it seems like I would like to test the idle current. What I want to check is that the autobias gives me a good working point - what ever that might be.

artosalo · 17th November 2011, 02:37 PM

If the adapter has 1 ohms resistor in series with the cathode, then it does not affect to any autobias circuit and the result is correct.

Tom Bavis · 17th November 2011, 07:59 PM

Quote:

Originally Posted by pof@

Well, the adaptor I'm referring to is described to be used for bias. But in your terms it seems like I would like to test the idle current. What I want to check is that the autobias gives me a good working point - what ever that might be.

Terminology: You ARE setting bias. But you're MEASURING idle current, which is controlled by the bias voltage. And typically you don't care what the bias voltage is - you want to set an operating point at a specific current.

ChrisA · 17th November 2011, 09:12 PM

Quote:

Originally Posted by pof@

...
has a 1 ohm in series with the cathode. Would this affect any known autobias circuits so my measurements will be incorrect?

Yes the probe does change the bias and the results are not exact. But the error is on the order of about one quarter of one percent. So there is some error but it is trivial, to small to notice with most DMMs

To put this in perspective, in the tube era, before transistors, parts with tolerances better than 10% were rare.

pof@ · 18th November 2011, 06:46 AM

Thanks for making it clear.

17th November 2011, 12:45 PM	#1
pof@ diyAudio Member Join Date: Nov 2008	Measuring bias I have a SE amplifier with some sort of autobias. I don’t have any schematic. I would like to check what the bias is without taking the box apart. I think this: Tube-Town Online-Store - Bias-Messadapter Oktal, Fertigadapter tt-probe8 has a 1 ohm in series with the cathode. Would this affect any known autobias circuits so my measurements will be incorrect?

17th November 2011, 12:51 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	Do you mean bias or idle current? These terms are often interchanged but have VERY different meanings. Idle current can be determined by measuring the voltage from cathode to ground, then dividing by the resistance from cathode to ground. Bias is the voltage between cathode and grid and can usually be measured directly. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

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17th November 2011, 01:52 PM	#3
pof@ diyAudio Member Join Date: Nov 2008	Well, the adaptor I'm referring to is described to be used for bias. But in your terms it seems like I would like to test the idle current. What I want to check is that the autobias gives me a good working point - what ever that might be.

17th November 2011, 02:37 PM	#4
artosalo diyAudio Member Join Date: Feb 2010	If the adapter has 1 ohms resistor in series with the cathode, then it does not affect to any autobias circuit and the result is correct.

18th November 2011, 06:46 AM	#7
pof@ diyAudio Member Join Date: Nov 2008	Thanks for making it clear.

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