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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    There have been a few Hammond reverb amp conversions on here. I elected to remove the negative feedback, I'd been advised that I would like this and it seemed to be good on one or other amps that I tried. Anyone else done this or anything similar?

    I added a standard ECC83 gain stage and although it works great there is far more gain than I need. Reinstating the neg feedback, possibly making it switchable seems a good start. However, when I replaced the original patch, with its original resistor the amp started making a horrendous low frequency thumping sound. I have never moved so fast in my life pulling the power out.
    What could that be?
     
  2. symbiotic

    symbiotic TDPRI Member

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    I want to jump in here quickly to test my own knowledge. Someone else will hopefully confirm or deny.

    When you added another gain stage, you've added another point at which the signal reverses phase. I think that if the "negative" feedback comes back into the circuit at this stage or earlier then you have actually created positive feedback and will need to swap the OT primary connections to the power tubes.

    About having created more gain than you need - another option would be a voltage divider after your new gain stage to knock back the signal voltage by some relatively large percentage.
     
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  3. since71

    since71 Tele-Meister

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    Thanks, my added gain stage is before the feedback loop and reinstating the feedback is just that, I put it back exactly where it was which is in the AO 44's original gain stage. The other issue, of dampening down the gain - is it as simple as a voltage divider? I'm not an authority at all, just common sense and time on my hands but I'm wary of doing anything that alters the fabulous tone I'm getting. Unfortunately the preamp volume and master volumes are on a hair trigger. Its incredibly loud and hard to control.
     
  4. symbiotic

    symbiotic TDPRI Member

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    Yes the feedback is inserted at its original location but, where is the new stage in relation to that?
     
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  5. since71

    since71 Tele-Meister

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    before it. It is literally so. Its even in a different housing.
     
  6. dan40

    dan40 Tele-Afflicted Silver Supporter

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    Most likely positive feedback as symbiotic noted above. You will either need to swap the OT's primary wires at the power tube sockets or swap the grid wires leading from the phase inverter to the power tubes. Either pair of wires may be swapped from one socket to the other and this should restore the amp to normal operation.
     
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  7. since71

    since71 Tele-Meister

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    Thanks Dan40. If, as seems likely from a trawl through other threads, I have positive feedback, surely what I need to do is move the wire from the OT pos to OT neg. And as this goes straight to the speaker socket those wires need to be reversed. If I measure the voltage at the output that should tell me which is the right one? I haven't altered any of the set up of the original amp. Well, I did remove the transistor first stage and changed all the caps but the valve second stage, PI and output are as stock. Nothing else was disturbed.
    Apart from my wife and neighbours.
     
  8. symbiotic

    symbiotic TDPRI Member

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    Aha! Removing a single stage of transistor amplification within the loop would also create the positive feedback condition, wouldn't it? You've changed the number of inverting stages within the feedback loop by one, changing the relationship of the injected feedback to the signal there.
     
  9. since71

    since71 Tele-Meister

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    ermm... Yes, Symbiotic. But forgive me, the transistor stage is NOT within the loop (see the schematic of the original AO 44. Hammond AO44 schematic.jpg
     
  10. Snfoilhat

    Snfoilhat Tele-Afflicted

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    You may have inadvertently returned the NFB to that triode's grid instead of the cathode. Recheck the socket pin numbers?
     
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  11. since71

    since71 Tele-Meister

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    The triple check is always a good thing. So, I here I go again although I'm underselling it by calling it triple, and, no, I didn't mix up the grid and cathode. I did alter the wiring to the speaker socket. Now the + is going to the jack tip. I don't think this makes any difference, does it, providing you maintain the speaker phase henceforth and its only ever going to one 8ohm celestion. Then I tapped the + to the cathode as on the diagram using the original resistor which measures 77K. As the amp warmed up a strange ominous beating began, different to before but I didn't hang around to enjoy it.
    This has got me checking every component on the build, all worked and is working fine its just my attempts to reinstate the NFB with the aim of dampening down its boldness.
    Time I produced an actual schematic diagram of the whole thing.
     
  12. since71

    since71 Tele-Meister

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    This is the whole thing: Rob'sAO44conversionschematic.jpg
     
  13. dan40

    dan40 Tele-Afflicted Silver Supporter

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    Have you swapped the plate leads yet? The simplest test for positive feedback condition in an amp that utilizes nfb is to remove the nfb temporarily and see if the problem disappears. Your amp seems to perform properly without the nfb but begins to oscillate when nfb is reintroduced so swapping the plate leads from the OT's primary will likely solve your issue.
     
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  14. since71

    since71 Tele-Meister

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    Thanks, this sounds promising. I'll try this in the morning. Do I have to also swap the direction of C10 + R18? Please forgive the dumb question.
     
  15. since71

    since71 Tele-Meister

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    I swapped the plate leads and tested the amp. All good. Next I connected the NFB. Switched on, horrendous high frequency out of the speaker (4Kz?) - switched off. Made cup of tea.
     
  16. Digital Larry

    Digital Larry Tele-Afflicted

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    I'm not really a tube amp expert, but after scanning through this...
    a) You're saying "I put it back the way it was before and now it's making this low frequency oscillation".
    b) Everyone assumes you actually did NOT do this
    c) You changed the phase of the NFB signal and it's even worse now
    d) So maybe your original issue has nothing to do with the NFB phase
    e) something DID change though... anything else you can think of?
     
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  17. zook

    zook Friend of Leo's

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    Lose the second triode in the Princeton schem and go directly to the triode after the transistor stage.
     
  18. zook

    zook Friend of Leo's

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    Check this
     

    Attached Files:

  19. since71

    since71 Tele-Meister

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    haha yes, well spotted. The plate leads I swapped this morning were so obviously 50 years old.
    I changed all the caps including the big can and removed the NFB......... The ground scheme got some attention too. The PT ground ran to the can anchorage I moved it to one of the PT bolts and used it as the main ground rather than the stuff at the other end of the chassis which grounded to tagstrip bolts. Didn't strike me as optimum. And the amp does work.
     
  20. since71

    since71 Tele-Meister

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    That would certainly tame the distortion, if that's what you are suggesting.
    As I understand it, (which means I've sat through a lot of Uncle Doug's YT vids) each time you add a stage the phase of the signal changes. As my rebuild has one more gain stage than the original then the phase at the output would be different from the original.
    Am I onto something?
     
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