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Old 28th September 2013, 07:20 PM   #1
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Default Pentode SPICE

With some success for triode models, I thought I'd take the information from Koren, Leach, Rydel etc. and expand the Microsoft Excel Solver routine:

Here's a first approximation using the minima of log squashed error and all the data points:

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When the anode voltage data points Vak<=50 are removed, the SPICE parameters become more useful:

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I'm all ears to anyone who can improve this.
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Old 30th September 2013, 07:14 AM   #2
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For another check on the accuracy, could you please plot some transfer characteristics (Eg1 vs Ip at constant Eg2 like in the datasheets)?
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Old 30th September 2013, 01:21 PM   #3
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Quote:
Originally Posted by jazbo8 View Post
For another check on the accuracy, could you please plot some transfer characteristics (Eg1 vs Ip at constant Eg2 like in the datasheets)?
Do you mean hold Vak constant and plot Ip vs Eg1?
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Old 30th September 2013, 01:47 PM   #4
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Quote:
Originally Posted by jackinnj View Post
Do you mean hold Vak constant and plot Ip vs Eg1?
Yes. I was going to do them myself, but my laptop died a horrible death this morning Since the pentode is most likely to be used as a power amplifier, wouldn't the mismatch of "knees" cause significant error for output power and distortion calculations?
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Old 30th September 2013, 06:18 PM   #5
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Here's how the curves fit, with the parameters used to get best fit in the previous chart:

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Optimizing using solver can make it appear better:

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But the new parameters result in this mess:

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Old 1st October 2013, 06:20 AM   #6
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The second one matched perfectly but made the plate characteristic much worse... The solver was doing a good job, but the Koren pentode model has much to be desired (as we have been complaining all these years)...
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Old 1st October 2013, 12:08 PM   #7
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Really have to get into the physics and dig out the books from the 1920's through 1950's. We're looking at the graphed data in a 2-dimensional plane, where it's really a 3-dimensional surface.
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Old 2nd October 2013, 07:44 AM   #8
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So what is really needed is plot surface(s) optimization, that's why tube models are much more difficult to do right than transistor models.
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Old 2nd October 2013, 11:39 AM   #9
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Triodes aren't that difficult because you only have 2 unknowns.
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Old 4th October 2013, 06:08 PM   #10
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I took Rydel's parameters and compared them with the "limited" solver solutions:

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