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Vacuum Tube SPICE Models
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Old 16th October 2013, 11:27 AM   #31
jazbo8 is offline jazbo8
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Thanks, that makes sense - the failure of the physical model was the reason for using the empirical approach in the first place! Looking forward to the next installment on the screen and the grid current.
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Old 16th October 2013, 12:17 PM   #32
jackinnj is offline jackinnj  United States
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Here is, then, a comparison of the Weaver and Koren models for the 6V6GT. I used Robert's parameters:

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Old 18th October 2013, 10:10 PM   #33
jackinnj is offline jackinnj  United States
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There was a question a while back -- how do I calculate an ultralinear load line. I don't know if this is right answer, but will post in the expectation that someone has done something better.

The load line is a line tangent to the safe operating area of a tube at a specific operating point. The slope of the line, dV/dI is expressed in ohms. To graphically show this I downloaded the data from the GE datasheet for the 6550 in UL mode. A recommended operating point was Vg=-29.5, Va=450 and RL=3.5K.

As the chart shows, the 3.5K load line just skirts the SOA while the 1.5K line gets you in trouble below 200V, and the 6.5K becomes troublesome above 500V. With Excel it's easy to change the operating point and load lines to come up with alternate solutions.

Going further, one could set up the equations to calculate the output stage THD with various operating points and load lines.

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Old 18th October 2013, 10:46 PM   #34
jackinnj is offline jackinnj  United States
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To calculate an ultra-linear model, the screen voltage must change with plate voltage. The General Electric datasheet uses 40% and a minimum value of 270Volts. The simulated ultra-linear pentode looks like this:

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Old 19th October 2013, 09:35 AM   #35
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Quote:
Originally Posted by jackinnj View Post
A recommended operating point was Vg=-29.5, Va=450 and RL=3.5K.

As the chart shows, the 3.5K load line just skirts the SOA while the 1.5K line gets you in trouble below 200V, and the 6.5K becomes troublesome above 500V. With Excel it's easy to change the operating point and load lines to come up with alternate solutions.
The chart looks great, since the SOA isn't even shown on the GE datasheet. Could you please plot the load lines for the Ra-a=4k for push-pull UL operation? On paper, it seems both Class A and Class B lines are well above the SOA.
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Old 19th October 2013, 01:22 PM   #36
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So you can't just use the pentode parameters if you are operating ultra-linear. This chart illustrates the difference. The pale beige line is the ultra-linear curve, the blue line is ultralinear operation with the pentode parameters.

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Old 19th October 2013, 06:22 PM   #37
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Quote:
Originally Posted by jackinnj View Post
So you can't just use the pentode parameters if you are operating ultra-linear. This chart illustrates the difference. The pale beige line is the ultra-linear curve, the blue line is ultralinear operation with the pentode parameters.
Of course, I used the ultra-linear curves in the GE datasheet... The SOA lines in your graphs are 100W, but the Pdiss for the 6550A is only 42W...

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Old 20th October 2013, 07:44 PM   #38
jackinnj is offline jackinnj  United States
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The datapoint chosen by GE is 450V, Vgs=-29.5, Ia=219mA =~99W. Each tube is conducting 1/2 the time.
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Old 20th October 2013, 07:58 PM   #39
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Are we even looking at the same datasheet? I got mine from Frank's.

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Old 21st October 2013, 01:39 AM   #40
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Here's a guess of the Class A and Class AB1 load lines. I took the Pd down to the UL levels of 34 and 70 watts. In pentode mode the 6550 can do 100W. This need some twiddling of the dials:

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