AA864 with some tweaks... need some help

Discussion in 'Shock Brother's DIY Amps' started by zeppelinofled69, Sep 15, 2021.

  1. zeppelinofled69

    zeppelinofled69 Tele-Meister

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    Good Afternoon All,

    I have been kicking this project around for a while and it has seen a few iterations of the years, though I have finally settled on a design that I think will stick.

    I have the power amp done, and it's a set of 7591 tubes driven by a 6SL7 using the AA864 LTP phase inverter. The preamp is what I am struggling with... The AA864 normal channel with a typical 12AX7 sounds good and works well, as expected. I also have a 5E3ish preamp stage running through a 6SN7.

    The 6SN7 doesn't seem to be running properly, as the plate voltage on the second triode is down around 55V DC while the plate voltage of the first triode is up around 120, which is what I would have expected.

    When I change the cathode resistor and get one stage right, the other one changes and rinse, repeat.

    I did my best to get a layout together of what is going on. let me know if I have made any glaring mistakes...
     

    Attached Files:

  2. 2L man

    2L man Tele-Holic

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    It looks they have same cathode resistor. Then test if tube halves are different connecting both grids together. If anode voltages stay different, then halves are different. If they come significantly closer each others there is something on grid side which screws the bias?
     
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  3. zeppelinofled69

    zeppelinofled69 Tele-Meister

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    Thank you for pointing me to the direction of the grids. I think I had a very poor ground connection on the tone stack, because adding a dedicated wire to the area and removing the pot casing grounds seems to have fixed the issue on that preamp tube.
     
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