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AC15 lite - parasitic oscillation? Funky (bad) noise problems

Discussion in 'Shock Brother's DIY Amps' started by Ronno25, Apr 15, 2021.

  1. Ronno25

    Ronno25 TDPRI Member

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    Please help me identify and fix some noise issues I'm having with my AC15 build. Thank you in advance!!

    I built an ac15 lite modeled after a build by Sluckey. I used the transformers and chassis from an AO-39 Hammond. The amp is wired point to point.

    Tube Compliment: 5Y3, 2x EL84's, 12AX7, and EF86 preamp.
    The amp is fitted with a volume, cut, and 6-way rotary tone selector.

    The problem(s):
    1. The volume and Cut pots make popping sounds at multiple places when sweeping through their range.
    2. There is a dead spot where the amp becomes totally silent when moving the volume and cut to specific points on the sweep.
    3. There is a loud high pitched whistle that comes into play when the cut pot is past half way.
    4. There is a loud TV static sound the comes in and out in the final half of the volume and cut.
    5. The rotary switch pops loudly when switching settings. I am using a make before break switch.
    Take a listen here:

    These issues largely go away when Cut is at it's lowest setting. I've probed the various ground points in the circuit with alligator clips to ground. This made no difference to the noise issues. The only thing I've done that helped the noise issue is to remove what I think is one of the power tube grid leak resistors by shorting the circuit to ground right before it (I've circled this resistor in green on the schematic I included). This basically removed all of the problems previously listed except a little bit of pop on the sweep of the pots.


    What I've tried:
    1. Swapping each tube out individually. No change.
    2. Chopsticking. Found a little bit of crackle when poking the B+ wire going to the reservoir cap.
    3. Alligatoring each ground point directly to chassis.
    4. Connecting ground points to a common point.

    Questions:
    1. What type of problem am I dealing with here? I'm at a loss, really. Ground issue? Parasitic Oscillation? Bad parts?
    2. What is the solution? Or at least what can I try that may be a solution?
    3. Can I simply remove one of the power tube grid leak resistors, or at least lower the value? Or will this cause problems?

    You guys absolutely rock! I'd still be working on my first build if it wasn't for the awesome help from the generous people on this forum.

    hammond_ao39.jpg.png
    IMG_4319.JPG IMG_4320.JPG IMG_4321.JPG
     
    Last edited: Apr 16, 2021
  2. Ronno25

    Ronno25 TDPRI Member

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    Not sure if anyone is listening but I'm thinking I'm going to give up on fixing the point to point and just rebuild it on an eyelet board. Here is a clip of the sound of the amp actually being played through:
     
  3. gusfinley

    gusfinley Tele-Afflicted

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    That definitely sounds like parasitic oscillation. The cut control should knock those frequencies out, so it makes sense that they go away. Watch out for places that high-level signals couple back into low-signal areas or the amp. I use lots of shielded wire in my amp builds to keep this from happening - especially on the inputs to tubes.
     
  4. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Besides the shielded wire mentioned above, you could try grid stoppers on the ltp grids with a resistance of, let's say, 10k mounted directly on the tube sockets.

    A lot of amps have a capacitor (47pF) between the plates of the LTP. Should also limit HF gain.
     
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  5. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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  6. DrPepper

    DrPepper Friend of Leo's

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    Have you tried swapping the OT leads to the power tubes, might be where the parasitic oscillation is coming from.
     
  7. SerpentRuss

    SerpentRuss TDPRI Member Ad Free Member

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    Have you tried putting a small value cap on a lead to ground and clipping it in the circuit in various places on the signal path to see if you can make the symptoms stop? This has worked for me before in locating where to attack the oscillation issue.
     
  8. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Swapping OT leads is a cure for positive feedback, making it the desired negative feedback. This amp does not have feedback.

    Positive feedback does result in oscillation, it's not parasitic though.
     
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  9. maj34

    maj34 Tele-Holic

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    Before I put on my thinking hat, for situations like this, I often like to dab all the soldering joints with a hot iron, applying some new solder if I don't like the look of the joint. I've resolved issues this way. No sense in thinkin' if you don't have to. :)

    After that I'd probably take out a multi-meter and test that all ground points are connected and show 0 ohms. After that, I'd pull the tubes out and start connectivity: Verify your schematic by making sure that everything that should be connected is connected and test that things that shouldn't be connected are not.
     
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  10. Nickfl

    Nickfl Friend of Leo's

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    You might try a small value snubber cap across the plate resistor of the ef86, 47pf or so.

    Also, you grounding scheme is problematic. There are a number of places where you have a connection to the chassis and then also ran a wire connection back to a star ground. Do one or the other.
     
  11. mrriggs

    mrriggs Tele-Meister

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    I second that. A grid stopper on the phase inverter is what cured the oscillation issues with my latest point-to-point build.

    Your location says Oregon. If you're in a part of Oregon that's close to Vancouver, WA then I'd be happy to lend a hand if you brought it by. I've got oscilloscopes and what-not to take some of the guess work out of it.
     
  12. Ronno25

    Ronno25 TDPRI Member

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    Hey everyone - Thanks so much for all the feedback! It had been a while since I had posted and no one responded so I hadn't been checking the forum for responses. Looks like they came in a flurry.

    With regard to my strange ground scheme: I had initially only had the ground points going to chassis, but grounded them via the wire to see if that would resolve any problems. Probably doesn't make sense, but that's why it looks the way it does. Either way, it didn't help.

    For the grid stoppers on the phase inverter: are we talking about on both pin2 and pin7? Or, just one of those?
     
  13. Ronno25

    Ronno25 TDPRI Member

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    Yep, I'm in Portland. Thanks so much for the offer. I'll definitely like to take you up on this if the grid stopper doesn't get things working. I've already ordered all the parts I would need to rebuild the circuit on turrents but I'd rather not go that route if I don't have to.
     
  14. Ronno25

    Ronno25 TDPRI Member

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    Okay - so I added 10k grid stoppers to the PI. This actually made the noise issue worse. I have yet to try the 47pf cap on the plates of the phase inverter and will do that as soon as I can get the cap. I'll keep everyone posted.
     
    Last edited: May 4, 2021 at 4:14 PM
  15. Ronno25

    Ronno25 TDPRI Member

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    I added a 47pf cap to the plate of the PI. This removed most of the oscillation noise. However, when I plugged my guitar in and played a few chords the noise came back, albeit slightly different. Can I use a higher value pf cap?
     
  16. Ronno25

    Ronno25 TDPRI Member

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    Yikes, well I tried a 100pf, 220pf, and 500pf. Still no luck! :mad:
     
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