Preamp help

Discussion in 'Amp Tech Center' started by Johnman, Jul 30, 2021.

  1. Johnman

    Johnman TDPRI Member

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    Having issues with preamp voltages
     
  2. Jon Snell

    Jon Snell Tele-Holic

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    Hmm. I wonder what on.
    A schematic and description or list of voltages would help as there are many designs out there and we don't know which one you have chosen.
     
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  3. Johnman

    Johnman TDPRI Member

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    It’s a1968 Marshall superlead build
     
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  4. Johnman

    Johnman TDPRI Member

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    V1

    Pin 1 229v

    Pin 2.1mv

    Pin 3 1.34v

    Pin 4 3.06v

    Pin 5 3.05v

    Pin 6 300v

    Pin 7 .04 mc

    Pin 8 0

    V2 pin 1 418v

    Pin 2 .4 mv

    Pin 3 418.8v

    Pin 4 3.10v

    Pin 5 3.10v

    Pin 6 423v

    Pin 7 423v

    Pin 8 423v

    V3

    Pin 1 446v

    Pin 2 373v

    Pin 4 3.11v

    Pin 5 3.11v

    Pin 6 443v

    Pin 7 373v

    Pin 8 419v
     
  5. Jon Snell

    Jon Snell Tele-Holic

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    V2 doesn't look at all right.
    Firstly a bit of history since 1959;
    Screenshot 2021-08-01 at 07.54.45.png
    The cathodes of the valves must sit no more than 3volts above the grids and the grids of V1 and the first half of V2 must be around zero volts. The second half of V2 has a grid voltage of between 100 and 200volts as does the cathode.
    V3 is off as well!
    If this is a Marshall amplifier and has just gone wrong, look for burnt resistors caused by faulty valves.
    If home made, check your work.
     
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  6. Johnman

    Johnman TDPRI Member

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    V1

    Pin 1 210v

    Pin 0v

    Pin 3 1.24v

    Pin 4 3.06v

    Pin 5 3.05v

    Pin 6 274v

    Pin 7 0v

    Pin 8 2.29v

    Pin 9 3.10v

    V2 pin 1 105.6v

    Pin 2 .004v

    Pin 3 49v

    Pin 4 3.10v

    Pin 5 3.10v

    Pin 6 108.6v

    Pin 7 108.7v

    Pin 8 108.5v

    Pin 9 3.1v

    V3

    Pin 1 269v

    Pin 2 29v

    Pin 3 47.8v

    Pin 4 3.11v

    Pin 5 3.11v

    Pin 6 266v

    Pin 7 31..5v

    Pin 8 47v

    Pin 9 3.05v
    I had a missing ground and this is the voltages as of right after soldering the missing ground for v2
     
  7. Jesco

    Jesco TDPRI Member

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    V2 pin 3, 6, 7, 8 still look off. I would double/triple check your wiring.
     
  8. Wally

    Wally Telefied Ad Free Member

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  9. Johnman

    Johnman TDPRI Member

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    If I wanna have more bias current adjustment what should I do?
     
  10. Wally

    Wally Telefied Ad Free Member

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    So you got the preamp and PI straightened out? What are your V2 voltages now? What about V3?
     
  11. 2L man

    2L man Tele-Holic

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    Schematic above is impossible to read but if you want higher bias current, reduce the resistor value which is drawn above adjust potentiometer.
     
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