How to choose an optimal load line for a cathode follwer buffer?
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manamanam
diyAudio Member

Join Date: Jun 2009
Location: Athens
Quote:
 Originally Posted by manamanam Thank you for your analysis and calculations. These are very helpful. I just cannot understand how do i determine min and max plate currents. The ECC88 datasheet states typical Ia=25mA. Also the follower is not directly connected to a next stage grid, it is just a buffer to drive a pair of active monitors with 10K input impedance.
Actually states Ia=15mA

kavermei
diyAudio Member

Join Date: Jan 2005
Location: Lokeren, Belgium
Quote:
 Originally Posted by manamanam I just cannot understand how do i determine min and max plate currents.
After choosing the quiescent point (-7.5V in this example), you follow the load line from the quiescent point up to the Vgk=0V line. That is your maximum positive Ia swing since you don't drive the grid positive. Because the drive signal is symmetrical around the quiescent -7.5V, its minimum is at -15V. So you can read the maximum negative Ia swing from the intersection of the load line with the Vgk=-15V curve.
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 4th July 2012, 02:46 AM #13 pingfloid   diyAudio Member     Join Date: Jan 2004 Location: Bangkok Hi. Thank you for all your contributions. I am still confused about how to plot the load lines in a white cathode follower headphone amplifier If I plot a line between maximum B+ (220V) and the idle conditions at the cathode resistor of the bottom tube (V= 2.4V, I= 8.9 mA) I’ve got this: That gives an intersection at V=107.2V and I=17.5mA Is it the curve for the tube at the top or at the bottom? Last edited by pingfloid; 4th July 2012 at 02:50 AM.
 5th July 2012, 01:52 AM #14 pingfloid   diyAudio Member     Join Date: Jan 2004 Location: Bangkok I forgot to mention that I wish to learn how to plot the lines in this case because I plan to substitute the output tubes for a couple of 6F8G (the 6SN7 in a big bottle) therefore I have to recalculate the resistors. Any recommendations will be highly appreciated.
 5th July 2012, 03:13 AM #15 Triodethom   diyAudio Member     Join Date: Dec 2009 Location: Newnan GA The output cap can be smaller say a 47uf so you use a film cap sounds better the 12at7 can be run harder ( more current thru it ) What headphone are you thinking of using ?
 5th July 2012, 08:33 AM #16 pingfloid   diyAudio Member     Join Date: Jan 2004 Location: Bangkok Hi Triodethom. I have already bought the output electrolytic caps (Jensen Audiograde 500uF/300V) I am using Sennheiser HD650 (300 Ohm) I find that if I run more current through the first tube, then the signals at the two output tubes are not balanced. I have found that values playing with Orcad, and the result is a nice symetric figure at the output tubes. Anyway, I will keep mixing values to see if I can find another lucky match running more current at the input tube. (measuring from grid to cathode at each output tube) Although I still don't know how this may affect the sound Last edited by pingfloid; 5th July 2012 at 08:57 AM.
Triodethom
diyAudio Member

Join Date: Dec 2009
Location: Newnan GA
Quote:
 Originally Posted by pingfloid Hi Triodethom. I have already bought the output electrolytic caps (Jensen Audiograde 500uF/300V) I am using Sennheiser HD650 (300 Ohm) I find that if I run more current through the first tube, then the signals at the two output tubes are not balanced. I have found that values playing with Orcad, and the result is a nice symetric figure at the output tubes. Anyway, I will keep mixing values to see if I can find another lucky match running more current at the input tube. (measuring from grid to cathode at each output tube) Although I still don't know how this may affect the sound
I would use them as power supply caps. If you go to Broskie site Tube CAD Journal you will see that the output cap can be smaller than 47 uf for the output . That is film size it also saves you some subsonic rumble that muddies the deep bass .

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