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Old 9th October 2012, 09:02 PM   #1
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Default Data sheet and load line question

Looking at THIS data sheet, on the second page an operating characteristic "S" is listed with the units of ma/volt. I'm wondering what that is? Is it the slope of the operating line? If so, that would be rather steep, no?

AND

Looking at THIS datasheet, second page under the RF amp section, plate voltage and current figures are given. Are these their suggested operating points, or the Va at zero current and Ia at zero voltage that define the load line?

I'm a little puzzled by the 6GM8 plate characteristics. It seems difficult to find a pair of load lines that work well with a cathode follower circuit. If anyone can point to a schematic with such a circuit, I'd like to study it.

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Old 9th October 2012, 09:14 PM   #2
DF96 is online now DF96  England
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The links don't work, but S is usually transconductance.

The plate voltage and current figures are probably suggested operating points.
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Old 9th October 2012, 09:18 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

A/V is the inverse of V/A = resistance, = conductance in Siemens.

rgds, sreten.
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Old 9th October 2012, 09:55 PM   #4
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Here are the links again. Sometimes the dr tube links don't work.

http://www.drtube.com/datasheets/ecc86-philips1958.pdf

http://tubedata.milbert.com/sheets/035/6/6GM8.pdf

Last edited by Captn Dave; 9th October 2012 at 09:58 PM.
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Old 9th October 2012, 10:02 PM   #5
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Quote:
Originally Posted by DF96 View Post
The plate voltage and current figures are probably suggested operating points.
That would be my assumption but a 25 volt operating point would be at the right margin of the plate characteristic chart, right? How would one operate at that point without leaving the chart? I'm missing something.
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Old 9th October 2012, 10:24 PM   #6
DF96 is online now DF96  England
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Yes, you would be just off the edge of the chart. Makes drawing load lines a bit difficult! However, RF designers might not bother with load lines but just use the recommended operating point. Nothing horrible happens off the chart.
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Old 9th October 2012, 10:33 PM   #7
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Okay, but at a 30v max plate, think how steep would the load line need to be to get there? I suppose there is no harm in that.

I've been looking for the Vgk max number on this tube. Obviously the 1 volt rule of thumb is of no use here. it looks like you would need to swing right to the zero line to get much use out of this tube.
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Old 10th October 2012, 10:09 AM   #8
DF96 is online now DF96  England
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For RF the load would be a choke or IFT so quiescent plate voltage equals supply voltage. The load line would be drawn from 50V (twice the supply rail) to twice the quiescent current. The half-way point on this line is the quiescent point.

At low supply voltages the valve runs with low voltage on the grid so you just have to accept some grid current and design your circuits for lower impedances.

Last edited by DF96; 10th October 2012 at 10:11 AM. Reason: add comment on Vgk
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Old 10th October 2012, 04:45 PM   #9
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I did not know that! Interesting.

Thank you for that explanation.
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