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?
Last edited:
You are drawing 75K load lines, is that the plate load resistor you ordered? (you mentioned before that you bought the parts according to the schematic that showed 20K)
I'm sorry for the confusion. I bought parts to put together the 6AU6/5787 line stage.
I haven't bought anything to put together the 6SJ7/6SN7 line stage. Trying to learn how to figure out the 6SJ7.
I haven't bought anything to put together the 6SJ7/6SN7 line stage. Trying to learn how to figure out the 6SJ7.
Last edited:
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).
Draw one. You have 300V of B+, so put a dot on the x axis at 300V and 0mA. With a 30K plate load, the most current you can draw through a 300V power supply is 10ma. Put a dot on the Y axis at 0V and 10mA. Connect these lines.
Open the PDF you sent me. Scroll down to the plate curves at the bottom.
Put a dot on the Y axis at 10mA. Put a dot on the X asix at 300V. Connect them with a line.
The 8th grade students I tutor in Algebra could do this, so can you.
Put a dot on the Y axis at 10mA. Put a dot on the X asix at 300V. Connect them with a line.
The 8th grade students I tutor in Algebra could do this, so can you.
OK, that's a 30K load line. On that load line where you reach 150V on the plate, put a dot.
What is the grid voltage there?
What is the grid voltage there?
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.
don't know
If you stayed on the dot and looked right it would be 2.5mA Don't think I'm doing this right.
If you stayed on the dot and looked right it would be 2.5mA Don't think I'm doing this right.
Last edited:
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.
Plate current and screen current =.0075mA 2.0/.0075= 266.66666
200v/.0025= 80k
300-150/30k=.005 plate current
I made it harder than it was and thanks to you spoon feeding it I think I understand it.
200v/.0025= 80k
300-150/30k=.005 plate current
I made it harder than it was and thanks to you spoon feeding it I think I understand it.
Thank you Sir for your patience and understanding. This shows that maybe an old dog can still learn. Just a bit slower.
Is there a way to calculate gain on the tube?
Is there a way to calculate gain on the tube?
Last edited:
Just when I thought I had this all figured out yet another question came to mind. How did we figure to use 100volts on the screen instead of say 75? Is there a ration of grid to plate voltage?
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).
I guess I should learn to proof read what I write. Is there a ratio between screen voltage and plate voltage? I read something that suggested a 3:1 ratio.