Ok, being honest here and never having used a set of pentode load lines I have some questions if I may. I need to end up with 150volts on the plate of the 6SJ7. I printed out the load lines and need to make sense of them so any help would be appreciated. My B+ feeding the 6SN7 2nd tube of the Aikido is going to be 300vdc with 10mA going thru that stage. So the B+ feeding the plate resistor on the 6SJ7 will be 300 also. So I need to drop 150 volts and have the 150 going to the grid of the 6SN7's top section.
I'm going to try to attach some curves in the hopes someone can explain what the top and bottom graphs are.
I have drawn a line hopefully I am correct in its placement. I started the line at 300VDC and tried to intersect a load line at 150 volts hopefully the plate voltage and a current of 4mA.
I don't understand the horizontal lines. I can do load lines on triodes they are different. Please excuse my stupidity. 37500 plate resistor?
I'm going to try to attach some curves in the hopes someone can explain what the top and bottom graphs are.
I have drawn a line hopefully I am correct in its placement. I started the line at 300VDC and tried to intersect a load line at 150 volts hopefully the plate voltage and a current of 4mA.
I don't understand the horizontal lines. I can do load lines on triodes they are different. Please excuse my stupidity. 37500 plate resistor?
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Ah, OK. On your PDF, on the lower graph:
1. Draw a 30K load line.
2. Draw a box that touches the origin (0,0) and your load line at 150V DC.
3. Note your grid bias here, then use the curves to determine screen current.
4. Add screen current to plate current, then use V=I*R to calculate cathode resistor value.
5. B+ is 300V and Vsg is 100V, so take 200V=screen current * screen dropping resistor and fill in your screen current to calculate the screen dropping resistor.
6. Size the screen grid bypass cap appropriately (0.47uF will be fine for most of these low current pentodes).
1. Draw a 30K load line.
2. Draw a box that touches the origin (0,0) and your load line at 150V DC.
3. Note your grid bias here, then use the curves to determine screen current.
4. Add screen current to plate current, then use V=I*R to calculate cathode resistor value.
5. B+ is 300V and Vsg is 100V, so take 200V=screen current * screen dropping resistor and fill in your screen current to calculate the screen dropping resistor.
6. Size the screen grid bypass cap appropriately (0.47uF will be fine for most of these low current pentodes).
You drew the 30K load line and uploaded it here...
OK, now that you know the grid is at -2V, you can look at the dashed lines and see how much screen current the tube will draw with 100V on the screen, -2V of bias, and 150V on the plate. The screen current labels are on the right edge of the graph.
OK, now that you know the grid is at -2V, you can look at the dashed lines and see how much screen current the tube will draw with 100V on the screen, -2V of bias, and 150V on the plate. The screen current labels are on the right edge of the graph.
You have to be more precise than that, it's more like 2.5mA.
You have 2V of bias and 5mA of plate current and 2.5mA of screen current. V=I*R, so 2=.0075*R, R=266 is your cathode resistor.
Screen voltage needs to be dropped to 100V from 300V, so a 200V drop at 2.5mA, 200V=.0025*R, so the screen dropping resistor is 80K.
You have 2V of bias and 5mA of plate current and 2.5mA of screen current. V=I*R, so 2=.0075*R, R=266 is your cathode resistor.
Screen voltage needs to be dropped to 100V from 300V, so a 200V drop at 2.5mA, 200V=.0025*R, so the screen dropping resistor is 80K.
The curves you posted had 100V on the screen and I'm lazy.
Generally speaking, lowering the screen voltage will increase gain, tend to lower plate current, etc. Raising screen voltage will decrease gain, tend to lower plate current, etc. These are generalities that can be gotten around though.
For your preamp, you need very little gain, so I would suggest running the screen at near max voltage (100V is close enough to 125V).
Generally speaking, lowering the screen voltage will increase gain, tend to lower plate current, etc. Raising screen voltage will decrease gain, tend to lower plate current, etc. These are generalities that can be gotten around though.
For your preamp, you need very little gain, so I would suggest running the screen at near max voltage (100V is close enough to 125V).
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