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AB763(ish) woes

Discussion in 'Shock Brother's DIY Amps' started by RollingBender, May 2, 2020.

  1. RollingBender

    RollingBender Tele-Afflicted Vendor Member

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    I’ve been fighting this thing for about 2 weeks now! Here’s the build thread... https://www.tdpri.com/threads/seems-like-i’m-doing-this-backwards.1015555/

    in the course of trying to track down why this thing has such an excessive amount of gain, I have been trying various tricks and while doing so, I tried putting a resistor in between the phase inverter grid and the final gain stage coupling cap. This knocked the gain down considerably, actually too much, so I figured I’d clip in a pot wired as a variable resistor to figure out what value I would need to accomplish the desired amount of gain. In doing so, I (accidentally) discovered that with the whole preamp (theoretically) disconnected from the phase inverter, I’m still passing guitar signal. This signal responds to everything upstream, including volume, tone and reverb controls.
    I put on my jewelers magnifiers and turned on the best lighting and I cannot find anywhere that signal could be getting through. Any pointers on where I should look?
     
  2. separateness

    separateness TDPRI Member

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    A schematic would be helpful, to know what is AB763 and what is "ish." Cannot help otherwise. I did not see one in that other thread.
     
  3. DADGAD

    DADGAD Friend of Leo's

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    The output stage gain is set by the feedback divider resistors. An 820 ohm resistor from the speaker output to a 47 ohm resistor to ground at the PI. Output stage gain is 820 divided by 47.
     
  4. RollingBender

    RollingBender Tele-Afflicted Vendor Member

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    30BB72AE-3CFB-4EA0-8DCB-D2D235905704.jpeg

    27DF0539-D8C7-4707-86BA-172DC4E2F671.jpeg
    Differences are...
    One tube reverb. Same pickup and insertion points, same mixing network.
    1/2 tube oscillator circuit (Princeton)
    6BQ5 output tubes
    EZ81 rectifier

    The preamp is as shown on the schematic. Power section has same layout but some different value components.

    I also disconnected the NFB for this testing.

    so basically, I have it disconnected at the two pages of the schematic but still passing input signal.
     
    Last edited: May 2, 2020
  5. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Something in the ground scheme that isn't really reflected in the schematic? The power supply is one thing that links the preamp and the power amp. Are all the filter caps' grounds in good order?
    '
     
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  6. tubeswell

    tubeswell Friend of Leo's

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    What Snfoilhat said. If the preamp filter cap ground isn’t connected to ground, the preamp supply resistor acts like a common plate resistor for all the preamp stages and the stages on the prior filter cap node (and so on), so the signal will go right through (and may even sound a bit mushy)
     
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  7. Wally

    Wally Telefied Ad Free Member

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    Subscribed.
     
  8. RollingBender

    RollingBender Tele-Afflicted Vendor Member

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    Solved!!!

    I was missing 1/2 of two different voltage dividers. The first one was the R2 of the divider coming off the reverb pot. The second one (not on the schematic but I put it in) was the R1 coming off the 3rd gain stage coupling cap and going into the phase inverter.

    I messed around with those 2 plus the voltage divider of the NFB circuit and thusly tamed the overdrive, reverb and noise. Got me a wonderful sounding amp now.
     
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