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Old 4th August 2013, 06:41 PM   #1
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Default 6550 plate resistance in triode mode? Calculate?

For some penthodes (e.g. EL84) a value for the plate resistance in triode operation can be found in the datasheet. Does anyone have an idea what this would be for a 6550? Is there an easy way to estimate/calculate?
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Old 4th August 2013, 06:49 PM   #2
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Plate resistance is fairly easy to calculate if you have the troide curves.
Think of plate resistance as the slope of the line with more vertical equaling lower plate resistance.
Notice you have voltage on one axis and current on the other. R=Change of Voltage/Change in Curent.
Find some change of voltage and change of current over a small seciton of the curve.
It is also helpful to notice that plate resistance is not constant and can change depending on operating point.
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Old 4th August 2013, 09:47 PM   #3
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Often the triode configured Mu is published in the datasheet. Since this doesn't change much with V or I, it can be used to calculate the triode rp from the pentode gm (which does change noticeably with current, approximately by current to the +1/3 power, so you extrapolate gm from the data sheet to your op point). Then:

Mu = gm . rp or rp = Mu/gm

(Obviously, rp must also vary with current as the -1/3 power to get Mu constant.)
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Old 4th August 2013, 10:14 PM   #4
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Some datasheets give 'inner mu', which is g2 to g1. Triode mu will be similar to this. Triode gm will be bigger than pentode gm because you get g2 current too; multiply pentode gm by (Ia+Ig2)/Ia or for a rough estimate just add 20%.

IIRC there is also 'outer mu' which is anode to g2 (i.e. for g2 drive) but few datasheets give this.
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Old 5th August 2013, 12:43 AM   #5
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Depending on operating point, 700 to 1400 ohm. Use Ra 3,5...4,5 k.
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