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Unpleasant distortion in 6G2 Princeton

Discussion in 'Amp Tech Center' started by AlbertaGriff, Apr 11, 2021.

  1. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    Hi all,

    I know I've been asking a lot of the forum but I need a bit more help.

    After doing the cap job on my '62 Princeton, I noted the trem didn't work at high volumes and there was an unpleasant distortion in the decay of notes when using the trem. After checking the bias, I noticed it was down around 30%. I installed the bias adjust and got the bias up to around 63%.

    Now the trem works GREAT! But the unpleasant distortion remains.

    I've tried to record it. First I'm at volume near 6 with trem on 10. The distortion is audible to me, but not so much in the recording. Then I turn trem off so you can hear the normal overdrive. Then at the end I lower volume to around 3.5 and turn the trem back on. This is where you can really hear the unpleasant distortion.

    Listen to Brownface Princeton distortion by AlbertaGriff on #SoundCloud
    https://soundcloud.app.goo.gl/aeHNo

    What I've done to the amp:
    -Replaced bypass caps
    -Replaced filter cap can
    -Replaced bias cap
    -Installed bias adjust pot and set bias to 63%

    20210408_224827.jpg 20210410_131440.jpg 20210408_201152.jpg
     
    Last edited: Apr 11, 2021
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  2. theprofessor

    theprofessor Poster Extraordinaire

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    lathoto and AlbertaGriff like this.
  3. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    I could certainly try that! Thank you.
     
  4. dan40

    dan40 Friend of Leo's

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    Does this problem only occur with the trem switched on? Does the overdriven tone sound fine with the trem off?
     
  5. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    It's 'sort of' hard to tell. I don't think I hear it with the trem off.

    Definitely at lower volumes, the distorted tone is only there when the trem is on, and at a high setting.
     
  6. dan40

    dan40 Friend of Leo's

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    That's strange. I'm guessing that this problem didn't exist before the cap job and the bias pot mod. Have you tried rotating through different valves to rule those out first? A list of all voltages throughout the preamp and power amp may help to narrow down the possibilities.
     
  7. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    Actually it was there prior to the rebias. I just hoped it would go away once the bias was set better.

    Prior to the cap job I can't really say, I didn't play it too much.
     
  8. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    Maybe I'm hearing more here than there really is...

    I compared to my 5F11, and I could hear some of the distortion I'm talking about here with the 6G2, but probably a third or half of the volume.

    Also, I was just playing at the lower volume, with trem intensity maxed, and I didn't hear any of the distortion. Could be that it only happens after bringing the volume back down from nearly cranked...

    I'll dig a bit more.
     
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  9. theprofessor

    theprofessor Poster Extraordinaire

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    If it's only happening with the tremolo engaged, I'd tend to think that the bias is still too high for a bias-vary tremolo amp. My 6g2 would distort when the tremolo engaged, until I got the bias down a bit more. With these amps, you've got to balance the sound of the tremolo (distortion) and the overall sound of the amp. Try something like 55% mpd and see what happens.
     
  10. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    Yeah that's a good suggestion.
     
  11. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    I went down to between 50% and 55% and unfortunately the problem remains.

    I'm wondering if it's actually present when the trem is off, but is just less noticeable because of the rest of the signal. Wondering if it's a bad cap or resistor somewhere along the line.
     
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  12. theprofessor

    theprofessor Poster Extraordinaire

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    Well shoot! Sorry that didn’t fix it!
     
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  13. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    Don't apologize, I appreciate the thought and suggestion. It was definitely worth a try.
     
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  14. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    Here's another recording. Volume is around 4. Starts off with no trem, it's an idea of the clean tone. Then I crank the trem to 10. The distortion is quite audible after the chords... it's like a ghost note in the decay.

    Listen to 6G2 Princeton Distortion #2 by AlbertaGriff on #SoundCloud
    https://soundcloud.app.goo.gl/43Dsc
     
  15. theprofessor

    theprofessor Poster Extraordinaire

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    I'm no expert, but I'm going to think a bit more on this. One good thing, though: the clean tone of that amp is fantastic!
     
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  16. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    Yeah the tone overall is great, but the distortion on the trem is quite noticeable when I dig in.

    The sound reminds me a bit of a problem I had with my SF Bassman, which ended up being a bad screen resistor on one of the power tubes.
     
  17. lathoto

    lathoto Tele-Meister

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  18. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    They are JJs and they are reasonably paired. I tried briefly with another pair of JJs and had the same problem.
     
  19. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Did you put in that 470K grid stopper @theprofessor suggested?

    It could be a bad part somewhere along the line but...
    Essentially the tube gain stages are the same as a 5E3, but the 5E3 uses a 12AY7 and has the two volume pots and one tone pot bleed off over 90% of the signal. This thing isn't bleeding off anywhere near that. Ime you are slamming it even with the volume at 3.5 on the dial. That blocking distortion pops up it's ugly head. That 470K grid stopper should be in place imo. Maybe drop the .02uF cap before the PI to .0033uF. Maybe drop the 220K screen grid stoppers down to 130K to 180K. Maybe try a 12AY7 but the tube won't get you there alone.
     
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  20. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    I haven't yet tried the grid stopper yet but I will sometime this week. I've got the kids at home on self-isolation from daycare so time is a luxury.
     
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