Hammond AO 44 negative feedback

Discussion in 'Shock Brother's DIY Amps' started by since71, Aug 12, 2019.

  1. since71

    since71 Tele-Meister

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    Thanks for this! It shows a single triad pre amp as you suggest. But whats all that about, removing C10/R18? I can't find another PP amp that has these components. What are they for?
    And the NFB is as stock
     
    Last edited: Aug 14, 2019 at 12:20 PM
  2. dan40

    dan40 Tele-Afflicted Silver Supporter

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    Sorry that swapping the plate leads did not work. Positive feedback normally causes a loud squeal when you power up the amp but in some cases it can cause other weird noises to occur. Since the amp is now squealing after the plate lead switch, I would go ahead and move them back to their original spots.

    When you added the extra gain stage, did you decouple it with a resistor/capacitor in the b+ line?
     
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  3. since71

    since71 Tele-Meister

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    Plate lead switching:
    1. There was no squeal after I decoupled the NFB, regardless of which way round the leads are connected.
    2. the 10k resistor and small cap between the plates. What are they for?
    3. I will swap the leads back. Getting shot of (2.) will make it much easier

    B+ in first stage:
    This is one of those questions I was always hesitant to ask:- B+ is what me and my advisers have always called HT? whatever you call it for the extra stage has 100k resistors for the plates but there is no cap, other than the can of course. What cap would be appropriate ?

    I really appreciate all this, incidentally and I'm grateful for all the contributions
     
  4. dan40

    dan40 Tele-Afflicted Silver Supporter

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    B+ is an old term from the days when tubes were run from batteries but it is still used today. The b+ rail would be everything down the line from the rectifier to the last preamp stage.

    In many cases, when adding an extra stage, it is necessary to decouple it with a suitable dropping resistor/filter cap...for example 10k/10uf. Some amps will use use a single resistor/cap combo on multiple gain stages and others will need more nodes to prevent interaction with the different stages.

    I'm not certain that a simple amp like this would need the extra decoupling node so hopefully one of the more knowledgeable members will weigh in with some thoughts.
     
  5. since71

    since71 Tele-Meister

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    May have misunderstood.... there .02 caps as the circuit shows
     
  6. since71

    since71 Tele-Meister

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    Certainly it works as is
     
  7. dan40

    dan40 Tele-Afflicted Silver Supporter

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    It does function but it is not working properly. The oscillation may be caused by adding in the gain stage without the extra decoupling node. Simple preamps can get away with a single node but higher gain designs will need the extra decoupling to prevent cross talk between the stages. This is where the theory gets a little over my head so hopefully one of the more knowledgeable members will chime in with some thoughts.
     
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  8. zook

    zook Friend of Leo's

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    They most were likely there to stabilize the circuit for the reverb. I removed them since most guitar amps don't have anything in tat part of the circuit
     
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  9. Wally

    Wally Telefied Ad Free Member

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    Since you asked... as I understand it, High tension voltage is the AC secondary to the rectifier. The rectifier changes the AC HT to DC for the B+ rail.

    I have been watching this thread...here’s hoping you find the problem. Designing circuits is not my forte.
     
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  10. D'tar

    D'tar Friend of Leo's

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    Is it picture time yet? I love picture time!:)
     
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  11. zook

    zook Friend of Leo's

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    Here's one I recently did. I have since move a couple controls to the front and added a tone pot. BTW, the bulb that was in the original amp was covered with red nail polish and used as a pilot.

    These are the pics in the PDF file I previously up loaded.
     

    Attached Files:

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  12. since71

    since71 Tele-Meister

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    IMG_0457.JPG IMG_0457.JPG Just HAVE to show you my light bulb! A torch bulb (like Vox AC30) mounted in a craft paint lid suspended on a 3/4" grommet dangling from 2 random points by copper wire fixed with epoxy. What could be simpler?
    And the chassis: IMG_0455.JPG

    And the remote gain stage:
    IMG_0838.JPG
     
  13. ArcticWhite

    ArcticWhite Tele-Meister

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    Hard to see any detail in that chassis. Can you take a better picture? Better light and higher resolution perhaps?
     
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  14. since71

    since71 Tele-Meister

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  15. Wally

    Wally Telefied Ad Free Member

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    ...
     
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