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Old 25th October 2013, 08:23 AM   #1
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Default Load lines. What?

I've read and read about load lines and I understand the graphs and how to calculate various things to put a dot in the graph, but, where do I WANT the dot? I hear about people placing the line to keep the tube in it's most linear operating point but all the lines looked curved to me.

I've been playing with this nifty calculator. 12AX7/ECC83 High-Mu Dual Triode Calculator

I guess the question is... How do I know what I want? Is it based on the B+ voltage I'm running? Or the gain I need? Or...?
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Old 25th October 2013, 08:44 AM   #2
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They are curved... but it's the points where the said load line intersects them that define how the stage will behave. And the dot, you want it at a low current for this particular tube, but not too low because if you look closely then the points produced by the load line and the curves will be less equally spaced. If you want just a preamplifier stage you might even like it more with a little distortion (brings more "tube magic" if you ask me) but if it's part of an all-tube amp you want it to be "clean" as possible, since each stage will not only amplify that distortion but add it's own as well.
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Old 25th October 2013, 08:55 AM   #3
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In addition to the plate characteristic curves, take a look at the transfer characteristic curves, i.e., Ia vs. Eg1. You want to operate in the region where the slope is the straightest (at a given Ep).
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Old 25th October 2013, 08:58 AM   #4
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It's a hifi amplifier.

Do you mean like I've drawn the green lines?

Click the image to open in full size.
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Old 25th October 2013, 09:02 AM   #5
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Let's say Ep is 150V and Eg1 is -1.5V, look on the transfer characteristic curve for Ep of 150V and see how straight is the line between 0V and -3V.
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Old 25th October 2013, 03:33 PM   #6
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If you don't understand load lines, I would suggest that Norman Crowhurst explains it the best. This article has been reprinted many times and is probably the article from which I learned about them.
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Old 25th October 2013, 05:52 PM   #7
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Keep in mind that the published curves in the data sheets do not always represent what you will get in hand.... Even in the old days the curves were based on "Cherry-Picked" tubes... Also some of the curves were modified by hand to look a bit more linear....
The data sheets are only as good as how tight your operating conditions are...ie, the curves are based on having REGULATED voltages when the were sweeping...
So if you have an un-regulated power supply that sags 50 to 80 volts from idle to full power, then your fooling yourself to design with those curves... you can design the entire amplifier at full power output...
For Class A pre-amps you should be fine....
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Old 25th October 2013, 08:06 PM   #8
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A circuit to stabilize the B+ is fairly simple.
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Old 25th October 2013, 08:32 PM   #9
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Crowhurst finally started making some sense to me after I read this:
How to "screw around" your tube load line

It's a funny read but it walks you through it pretty good. Sometimes it takes a few different approaches before things sink in. Between that article and guidance I received on this board I can predict my pre-voltage gain stage behavior much better.

In a class there would be lab experience and other students to draw from. It's difficult to learn from reading alone. -Fred
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Old 25th October 2013, 09:16 PM   #10
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Quote:
Originally Posted by cerrem View Post
Keep in mind that the published curves in the data sheets do not always represent what you will get in hand.
That's true. However, load lines are usually accurate enough to predict the operating points of the tube and its most linear region. I always create a schematic before I build anything and work out all the voltages and component values theoretically before I start to solder. I am generally surprised and delighted to find that the circuit works as predicted from the math and the load lines. There is generally room for some tweaking and "dialing in", but my experience is that load lines are a critical tool for the tube audio engineer.
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