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Old 29th November 2012, 07:21 PM   #11
Elvee is offline Elvee  Belgium
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Note that U2 works with a slightly positive bias; this is because the valves operate with less than 50V Vak, which is really short.

However, this is not detrimental to the performances since U2 operates in a quasi stasis mode (except maybe for the input impedance): the THD remains below 0.007%

If you want to avoid that mode, you can reduce the string current, by increasing R1 for example.
But this will also increase the output impedance. Depends on where your priorities are.
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Old 29th November 2012, 08:19 PM   #12
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Quote:
Originally Posted by Elvee View Post
Here they are:
I do not know how to plot output impedance with ohm-scale.
Can you assist a little ?
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Old 29th November 2012, 09:42 PM   #13
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Hello Elvee

How do I include your triode.lib into Ltspice to do the simulation of your vacbufecc82 circuit ?

Thank you

Bye

Gaetan
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Old 29th November 2012, 10:41 PM   #14
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Hello Elvee

I finaly succeed to include your triode.lib into Ltspice to do the simulation of your vacbufecc82 circuit.

Now simulation are working.

Thank you

Bye

Gaetan
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Old 30th November 2012, 07:20 AM   #15
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by artosalo View Post
I do not know how to plot output impedance with ohm-scale.
Can you assist a little ?
First, you have to plot a quantity expressed in ohm. The most obvious form is U/I, in this case the current and voltage from the stimulus source and the node of interest, but it can be a more complex mathematical expression (right-click on a trace name to enter it). LTspice makes a dimensional analysis and plots the result in the correct units.
Next, you have to change the left axis from decibel to linear: Plot settings --> Manual limits.
That's it
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Old 30th November 2012, 08:03 AM   #16
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Thanks. I am closer to the solution.
How should the current generator I1 (=load) be configured ?
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Old 30th November 2012, 10:25 AM   #17
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Quote:
Originally Posted by gaetan8888 View Post
Hello Elvee

I finaly succeed to include your triode.lib into Ltspice to do the simulation of your vacbufecc82 circuit.

Now simulation are working.

Thank you

Bye

Gaetan
Are you able to plot the output impedance ?
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Old 30th November 2012, 01:53 PM   #18
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by artosalo View Post
Thanks. I am closer to the solution.
How should the current generator I1 (=load) be configured ?
No function, no current, it just has to be the unique AC source on the sheet: make it AC = 1
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Old 30th November 2012, 02:43 PM   #19
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I have done it, but the results differs from the one achieved with traditional method,
i.e. using two different load resistances and calculating : (U1-U2)/(I2-I1) = Zout.
The result with your method is (@ 1kHz) 73,8 ohms and with the above method 55 ohms.

I make some other simulations.
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Old 30th November 2012, 02:47 PM   #20
Elvee is offline Elvee  Belgium
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With the Bode method, the signal is infinitely small. If you use a substantial voltage and low load resistances, non-linearities could come into play and influence the results.
What are the parameters you use?
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