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How to read Pentode Preamp load lines?

Discussion in 'Shock Brother's DIY Amps' started by Jewellworks, Jan 27, 2021.

  1. Jewellworks

    Jewellworks Tele-Meister

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    im looking at the plate characteristics data sheet for the 6SJ7. the one that has the plate current on the left and screen current on the right. i figured the gain i wanted by using a 60k resistor, 1650 umhos came out to 99. when i drew the load line, 295 B+, and guesstimating a cathode resistor of 1.5k, it came out to about 4.45ma. but im REALLY not understanding how to read the grid voltage curves against my load line. so i cant figure out how to draw my cathode load line.
    and with the screen current on the right... how does that play into the lines?


    Screenshot_20210127-223516_Adobe Acrobat_1611805161857.jpg
     
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  2. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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  3. robrob

    robrob Poster Extraordinaire Ad Free Member

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  4. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The Tung-Sol datasheet has a less cluttered chart.
     
  5. Ten Over

    Ten Over Tele-Holic

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    The solid lines are for the control grid (g1) and the dashed lines are for the screen grid (g2). The negative numbers are the g1-to-cathode voltages in both cases.

    6SJ7 Lines.JPG

    You draw the load line from 295 plate volts to 4.9 plate mA for a 60K plate resistor and then choose an idle point somewhere around the middle of the useable range. The intersection of the -3.5 g1 curve and the load line is pretty darn close to the middle, so we will run with that. The plate current is 2.2mA and the plate voltage is 163V.

    Now we need to know what the g2 current is when the plate voltage is 163V and the g1 voltage is -3.5V. We are going to have to draw our own g2 line at -3.5V because they only give -3V and -4V. The intersection of the -3.5V g2 line and 163V plate voltage line is at a g2 current of 0.6mA.

    The total cathode current is the sum of the g1 current and the g2 current which is 2.2mA + 0.6mA = 2.8mA. The required cathode resistor is -3.5V divided by 2.8mA which is 1.25K. I would go with the readily available 1.3K.
     
    Last edited: Jan 29, 2021
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  6. Jewellworks

    Jewellworks Tele-Meister

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    OMG THANK YOU for explaining this so completely! i had half your explanation correct, but i didnt get the screen g2 portion correct. i just drew my line over from 2.2 on the left, to the 1.5 on the right. so from there, i was off on my numbers. i guessed the dashed lines were the screen g2 numbers, but i didnt "get" that if you use -3.5 for one scale, you have to use if for the other as well.

    that said, my supply voltage is slightly off from the estimates above, but i can readjust. it comes out to 273. and when i measured the voltage on my plate, it only came out to 29! that doesnt seem right at all!! i used a 750 on the cathode. it may be that far off due to my miscalculation with the g2 current.
    if my center bias is 163 (or thereabouts) shouldnt i read that on the plate? (btw, ive got 80 on my screen.)
     
  7. Jewellworks

    Jewellworks Tele-Meister

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    What am I doing wrong? I've got a supply voltage of 250. Plate resistor of 62k. Cathode resistor of 1.3k. from what I see on the chart, I should have an idle of 163v or so... I've got 45v on the plate. That's a whopping 118v discrepancy!
    Ive got 96v from plate to cathode.

    My screen has the same supply, 149k resistor, .5uf cap to ground, and 92 volts on the screen.

    This is driving me nuts
     
  8. Ten Over

    Ten Over Tele-Holic

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    It's hard to have 45V from plate to ground and 96V from plate to cathode. What is the voltage across the 62K? What is the voltage across the 149K? What is the voltage across the 1.3K?
     
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