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 3rd December 2010, 11:23 AM #1 rsumperl   diyAudio Member     Join Date: Oct 2009 Location: Cascade, Idaho 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
 3rd December 2010, 12:24 PM #2 erin   diyAudio Member   Join Date: Nov 2006 Location: http://www.acmuller.net/con-dao/daodejing.html 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 Or google "tube load line"
 3rd December 2010, 12:33 PM #3 DF96   diyAudio Member   Join Date: May 2007 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.
 3rd December 2010, 12:53 PM #4 hidnplayr   diyAudio Member     Join Date: Sep 2008 Location: Flandres 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 __________________ Real tubes have top-caps
 3rd December 2010, 01:19 PM #5 artosalo   diyAudio Member     Join Date: Feb 2010 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
diyAudio Member

Join Date: Oct 2009
Looking at the 2 tubes I am thinking about, any initial guesses whether they will work ok for audio?

Quote:
 Originally Posted by artosalo 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
diyAudio Member

Join Date: Feb 2010
Quote:
 Originally Posted by rsumperl 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 ?

 3rd December 2010, 03:44 PM #8 DF96   diyAudio Member   Join Date: May 2007 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
diyAudio Member

Join Date: Oct 2009
Looking to build a pre-amp for a magnetic phono cartridge. I wanted to compactrons.

Quote:
 Originally Posted by artosalo 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
diyAudio Member

Join Date: Feb 2010
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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