newbie questions about 300B tube amp
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greg_e46
diyAudio Member

Join Date: Nov 2008
Location: Athens
Quote:
 Originally Posted by tomchr Does this make sense?
Yes it does make sense but if I do that the maximum current is about 240+ mA!!
So if the quiscient current is 80 mA the Q-point is not at the middle... right? and if this is right why does it happend? I think that in class A amplifiers the Q-point must be at the middle..

 22nd June 2012, 11:28 PM #12 12E1   diyAudio Member     Join Date: May 2012 Location: Near Huntingdon, Cambridgeshire, UK If your bias point is with the current at 80mA then the current can swing plus / minus 80mA maximum (and in reality a bit less than that). So the peak current flow would be just under 160mA. I think you assumed that the grid would go to 0 volts. To swing the current from 0 to 160mA the grid will not need to go that far - with a 2.5k load line biased for 80mA idle, it would go no more positive than about -30 volts for 160mA current. If you do take the grid more positive than -30 then you will simply introduce lots of distortion since the bias point would then not be at the middle of the current swing. Last edited by 12E1; 22nd June 2012 at 11:31 PM.
greg_e46
diyAudio Member

Join Date: Nov 2008
Location: Athens
Quote:
 Originally Posted by 12E1 If you do take the grid more positive than -30 then you will simply introduce lots of distortion since the bias point would then not be at the middle of the current swing.
Ok I see what you mean.. But how can I be sure that the grid voltage wil not reach 0 or -10 volts?

 26th June 2012, 08:06 PM #14 greg_e46   diyAudio Member   Join Date: Nov 2008 Location: Athens anyone??
costis_n
diyAudio Member

Join Date: Aug 2008
Quote:
 Originally Posted by greg_e46 so after reading some articles I have more questions about the schematic. I found EH 300B datasheet and tried to draw the load line and calculate the bias point. in the picture below I set the plate voltage at 360v (80% of Vplate max). With 880Ω resistor I have about 69volts dropped across cathode resistor and the bias current is ~80 mA. so the max current is 150mA. But if I draw the load line I don't have 2.5kΩ load as the schematic shows. So I suppose I am thinking-doing something wrong... Right?
Not correct. If you drop 69 vols on the cathode resistor, this means that your HV (high voltage) will be 360-69 V, because ths is what the 300B "sees" between anode and cathode. So the center of the load line will be at the intersection of 291 volts with the 69Volts grid curve.

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