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-   -   Linearity (http://www.diyaudio.com/forums/tubes-valves/178425-linearity.html)

 rsumperl 3rd December 2010 11:23 AM

Linearity

How can one tell how linear a tube is or will be? Is it determined by looking at the data or is it determined by looking at the graphs? I'm thinking about playing with the triode and pentode sections of 6AF11's and 6BD11's for a pre-amp.

Thanks,
Ray

http://www.scottbecker.net/tube/sheets/123/6/6BD11.pdf

http://www.scottbecker.net/tube/sheets/123/6/6AF11.pdf

 erin 3rd December 2010 12:24 PM

For class a pre-amp you need to bias the tube to be running in its most linear area.
You need to work out the load line.
Check out this website:
What is the best load line for a tube

 DF96 3rd December 2010 12:33 PM

You can get a rough idea from the graphs. Bear in mind that these are intended for choosing biassing, not calculating distortion. They are just an average of several samples, so any particular valve will not follow them exactly but won't be off by much more than about 20%. The only way to find distortion accurately is build it and measure it.

 hidnplayr 3rd December 2010 12:53 PM

you can calculate the distortion for a given loadline.

Of Loadlines, Power Output and Distortion. (also read part 2, 3 etc while you're at it ;)

 artosalo 3rd December 2010 01:19 PM

Based on my practical experience I agree with DF96.

You can get an idea of the tube's linearity from the graphs, but just an idea.

Let's take a pentode as an example. The curves are usually given with one, two or max. three fixed g2-voltages. So we do not know the values between those. Or below or above.

In case of triode, by fine-tuning the cathode resistor you have big effect to distortion of the stage.

I usually do so that after I have built the amplifier or some part of it, I optimize the stages one by one. I use audio generator to feed the planned input level and analyze the output with a distortion analyzer. I search for the minimum distortion ( = optimum linearity) by fine-tuning the component values obtained with graphical planning methods.

This method is superior compared to design a stage just using given graphs.

Actually I see that a good HIFI-amplifier can not be built without such procedure. ( except those guys who can do all this by just listening ;)) .

 rsumperl 3rd December 2010 01:50 PM

Looking at the 2 tubes I am thinking about, any initial guesses whether they will work ok for audio?

Quote:
 Originally Posted by artosalo (http://www.diyaudio.com/forums/tubes-valves/178425-linearity-post2386021.html#post2386021) Based on my practical experience I agree with DF96. You can get an idea of the tube's linearity from the graphs, but just an idea. Let's take a pentode as an example. The curves are usually given with one, two or max. three fixed g2-voltages. So we do not know the values between those. Or below or above. In case of triode, by fine-tuning the cathode resistor you have big effect to distortion of the stage. I usually do so that after I have built the amplifier or some part of it, I optimize the stages one by one. I use audio generator to feed the planned input level and analyze the output with a distortion analyzer. I search for the minimum distortion ( = optimum linearity) by fine-tuning the component values obtained with graphical planning methods. This method is superior compared to design a stage just using given graphs. Actually I see that a good HIFI-amplifier can not be built without such procedure. ( except those guys who can do all this by just listening ;)) .

 artosalo 3rd December 2010 02:16 PM

Quote:
 Originally Posted by rsumperl (http://www.diyaudio.com/forums/tubes-valves/178425-linearity-post2386058.html#post2386058) Looking at the 2 tubes I am thinking about, any initial guesses whether they will work ok for audio?
Most obviously they will work. I do not see any reason why not.
Ofcourse much depends on the circuitry you plane to use.

What sort of pre-amp you plan to build ?

 DF96 3rd December 2010 03:44 PM

From the graphs you can get a rough estimate of 2nd, and an even rougher estimate of 3rd. You get the best estimate for the one you are probably least interested in.

You are sometimes given explicit graphs of 2nd and 3rd order products for remote cutoff RF valves (in Philips/Mullard datasheets), in terms of maximum input for 1% distortion, but you would not normally use these for audio except in a compressor.

 rsumperl 3rd December 2010 03:58 PM

Looking to build a pre-amp for a magnetic phono cartridge. I wanted to compactrons.

Quote:
 Originally Posted by artosalo (http://www.diyaudio.com/forums/tubes-valves/178425-linearity-post2386097.html#post2386097) Most obviously they will work. I do not see any reason why not. Ofcourse much depends on the circuitry you plane to use. What sort of pre-amp you plan to build ?

 artosalo 3rd December 2010 04:38 PM

Quote:
 Looking to build a pre-amp for a magnetic phono cartridge. I wanted to compactrons.
I will make some calculations and give you a suggestion where to begin.

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