PR oscillation - optimzing the ROI on a fix (group question)

Discussion in 'Amp Tech Center' started by LudwigvonBirk, Jun 13, 2019.

  1. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Trying to do a fix in the lowest number of focused steps, if possible. Here's the patient:
    • New Princeton Reverb build (Hoffman circuit, not my first).
    • Everything sounds really good, except for what sounds like "ringing" noise on G string, notes B and C (3rd and 4th fret). Doesn't happen with any other notes afaict.
    • Doesn't do this with same guitar on a very nearly identical amp, same guitar settings and amp settings.
    • Reverb cables unplugged and verb switched off, no difference. Trem circuit switched off, no difference.
    • Treble and bass dialed up and down, no difference.
    • Occurs on input 1 and input 2.
    • Swapped out all tubes from the "good amp", no difference.
    • Bias settings are super-close between the two amps.
    • Speaker is new, Canabis Rex 12. Have not tried a different speaker but have a few spares of different types.
    So I'm thinking this is a lead dress thing that I should look at first. My plan is to yank the chassis, get it up on blocks, inject an "offending" signal with a sig gen, and scope that from the speaker-out. Hopefully I will see something in the waveform at the offending freq that shows a squirrelly waveform. If so I'll then chopstick the wires attached to V1.

    Would you do something else first?
     
    Last edited: Jun 13, 2019
  2. dan40

    dan40 Tele-Afflicted Silver Supporter

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    Does it do it with the nfb disconnected? Positive feedback can cause some strange symptoms in a new build and it does not always scream or howl like we are use to hearing. If lifting nfb works, swapping the OT primary's or the grid wires at pin 5 should solve it.
     
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  3. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Update:

    Took the amp to my weekly jazz gig tonight (the above problem probably would not be heard over the drums and horns, I could chase it later I THOUGHT). But unlike at my house just a day earlier, at the gig the amp was suddenly way way way quieter and had some distortion going on that was also brand new (I got a compliment on it, ha). I had a spare EH 12AX7 and during a set break I tried that spare tube in V1 (preamp)- no change. Trem was working so I didn't think it was a bad V4 (on a PR, half of V4 is trem, half is PI). But I tried a tube swap in V4 and...

    ...problem seems totally fixed. No fizzy notes like before, amp instantly became loud and clean, sounded great for 2 hrs. On-site hotfix, gotta like it when that happens.

    Will do some more careful checks soon but this seems to have been a simple case of a failing tube. Oddly that "problem tube" worked fine in the same position in a different (nearly identical) amp a few days go.
     
    Last edited: Jun 13, 2019
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  4. Andy B

    Andy B Tele-Afflicted Gold Supporter

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    Not that unusual. The same tube in a different spot or a different amp may be quite normal.
     
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  5. clintj

    clintj Friend of Leo's

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    Check your circuit over closely. You may have a squirrely ground somewhere that was not letting some signal go to ground as designed, aka a voltage divider not dividing. The added gain from that could be enough to provoke an oscillation, and transporting the amp could have caused the connection to be remade temporarily.

    Or, transporting the amp has caused something to find a ground path for some signal or knocked something loose.

    Sent from my Pixel 2 using Tapatalk
     
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  6. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Thanks Clintj. I also thought something could have jarred loose moving the amp. Since I did the tube swap in V4, the problem seems solved, so I'm not going to crack open the amp again unless further problems arise. V4 is (half of it) for the phase inverter on this amp btw.

    Just for grins over coffee, I fed a bunch of different test tones into the amp (1.5 mV and then 200 mV), looking for hair on the waveform. I set the amp volume just shy of the breakup point (easily visible on the scope). Zooming in I was not able to see any odd-looking fuzz or anomalies in waveform.
     
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