1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

Any idea what is this?

Discussion in 'Shock Brother's DIY Amps' started by smuc, Jan 23, 2021.

  1. smuc

    smuc Tele-Meister

    Posts:
    127
    Joined:
    Nov 22, 2020
    Location:
    Austria
    Hi fellow amp builders!

    My 6V6 Bassman/Plexi is exhibiting some strange behavior. Even when set to pristine clean, the bass notes have a distorted, buzzy resonance to them. As I go up the fretboard, the resonance becomes less noticeable.

    On the scope everything looks ok except on the junction of the PI coupling caps and the power tube grid leak resistors. I thought it was a cap gone rogue but even after replacing the cap the problem persists. This funky waveform appears only in a narrow signal range.

    Any ideas?

    Here's the buzz recorded through the Palmer PDI-03:


    And the waveform
    IMG_5566.JPG
     
  2. Tom Kamphuys

    Tom Kamphuys Tele-Holic

    Age:
    41
    Posts:
    668
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
    Oscillations!

    Do you have grid stopper?
     
  3. smuc

    smuc Tele-Meister

    Posts:
    127
    Joined:
    Nov 22, 2020
    Location:
    Austria
    Yes! I cannot remember by heart without opening it but I am pretty sure I put the 1.5K.

    Why would they appear only on one side of the PI? And any ideas on how to solve them?

    Funny thing is, on the scope they go away as soon as I reduce the master a bit (grid leaks are a dual gang 100K pot), but the buzz happens even when I set the master to 50%. Which makes me think that the buzz and oscillations actually might not be related.
     
  4. Tom Kamphuys

    Tom Kamphuys Tele-Holic

    Age:
    41
    Posts:
    668
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
  5. RollingBender

    RollingBender Tele-Afflicted Vendor Member

    Posts:
    1,448
    Joined:
    May 14, 2011
    Location:
    SW Minnesota
    Don’t know what that scope trace tells us but the design would look cool on a t-shirt! ;)
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.