6AK5 in Triode - Plate Resistance?
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 2nd October 2019, 01:36 AM #1 FlaCharlie   diyAudio Member   Join Date: Feb 2004 Location: Gainesville, FL 6AK5 in Triode - Plate Resistance? I was able to find the curves for the 6AK5 wired in triode but no other info. Does anyone know the plate resistance?
PRR
diyAudio Member

Join Date: Jun 2003
Location: Maine USA
rp is just the slope of the line *around your selected bias point*.
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FlaCharlie
diyAudio Member

Join Date: Feb 2004
Location: Gainesville, FL
Quote:
 Originally Posted by PRR rp is just the slope of the line *around your selected bias point*.
Thanks. So closer to vertical is lower and more toward horizontal is higher?

How did you come up with the figures 3.3k and 9k?

 2nd October 2019, 02:06 PM #4 DF96   diyAudio Member   Join Date: May 2007 By estimating the slope of the curve.
 2nd October 2019, 02:21 PM #5 kward   diyAudio Member   Join Date: Sep 2013 Location: Western USA Take the orange tangent line shown at grid = 0V for example. That tangent line crosses 24 mA and 12 mA on the Y axis. The plate voltage at 12 mA is 66 volts and the plate voltage at 24 mA is 108 volts. Therefore the inverse slope of that tangent line is (108-66V)/(24-12mA) = 3.5k
 2nd October 2019, 08:46 PM #6 FlaCharlie   diyAudio Member   Join Date: Feb 2004 Location: Gainesville, FL Thanks guys!
PRR
diyAudio Member

Join Date: Jun 2003
Location: Maine USA
Quote:
 Originally Posted by FlaCharlie How did you come up with the figures 3.3k and 9k?
_I_ grabbed the orange line and slid one end down to 0V 0mA. Then peeped the V and mA at the other end of the line. Math as the others say; simple division.

This also highlights the fact that rp is NOT a "constant". It is different around every operating point, and for large swings around that point.

 3rd October 2019, 11:00 AM #8 RajkoM   diyAudio Member     Join Date: Feb 2009 The PRR is correct, of course. rp is variable and depends of the operating point. Some posters here on tube forum, find that rp is determined by the curve Vg=0 only, and "everything else is the internal resistance of the tube". I didn't even have the will to discuss with them about it.
 3rd October 2019, 11:20 AM #9 TonyTecson   diyAudio Moderator     Join Date: May 2003 Location: Maybunga, Pasig City you can even get the mu and the gm from that curve.... __________________ planet10 needs your help: Let's help Ruth and Dave...http://www.diyaudio.com/forums/plane...ml#post5010547[B
FlaCharlie
diyAudio Member

Join Date: Feb 2004
Location: Gainesville, FL
Quote:
 Originally Posted by kward Take the orange tangent line shown at grid = 0V for example. That tangent line crosses 24 mA and 12 mA on the Y axis. The plate voltage at 12 mA is 66 volts and the plate voltage at 24 mA is 108 volts. Therefore the inverse slope of that tangent line is (108-66V)/(24-12mA) = 3.5k
OK, here's what I'm trying to figure out . . . a poster on another forum measured voltages in his FX-01 preamp that uses these tubes in triode.

The PS has rails that measure +34v / -34v. He measured 51v plate voltage (plate to cathode). He measured -32.19v cathode to ground with tubes installed and -34.44v cathode to ground with tubes removed. So I figure the cathode is at +2.25v so the grid is at -2.25v.

Looking at the curves, that operating point doesn't even seem to be on the chart. Or maybe it's super close to the X axis?

Working off the -2v curve I get: (60v - 45v) / (0.8ma - 0ma) = 18.75k

Am I doing it correctly? I'm confused because the cathode resistor is 200 ohms and that would indicate (2.25v / 200) a current of 11.25mA.

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