Unpleasant distortion in 6G2 Princeton

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

  1. Bob Arbogast

    Bob Arbogast Tele-Afflicted Silver Supporter

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    With the tremolo intensity very high, you may be pushing the output tubes into (or far toward) cutoff. Perhaps that causes an unpleasant, non-linear response from the output stage.

    I don’t recall . . . Did you strap a diode across the intensity pot? That prevents the opposite problem of reducing the bias voltage to where the output tube grids start passing current, with its issues.
     
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  2. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    Thanks.

    I did end up putting the diode across the Intensity pot, so that is in there.

    It could just be the extreme intensity of the trem pushing it into distortion. I may just spend some time checking on all the resistors, particularly in the trem circuit.

    To be honest, I'm not too sure how to check capacitors, but I planned to look it up.
     
  3. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Good recordings. Super playing too. :) One thing I'm wondering. I would support trying a 470K PI grid stopper for sure, since it's a simple, logical step. But AFAIK, despite the theoretic factors, it's not a common mod in 6G2 builds. So is that actually distortion per se? And maybe in the trem downslope, where in my 1st-grade picture of stuff, I mighta thought distortion would be dying out. Stuff that can cause static? Not sure. Bad caps (leaking DC)? Bad pots? Dirty tube pins or loose socket springs? Of course I have no idea if any of those could make noise only in a brief part of the trem cycle.

    I'm guessing you don't have a scope, and if you did I'd have zero idea of how to use it here -- but someone would.
     
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  4. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    You're far too kind!

    I'm trying a few things, but won't be able to test the results for a bit.

    I reflowed a few solder joints. Everything seemed fine though.

    I measured all the pots resistance. Matched the schematic.

    I cleaned the intensity pot. It wasn't crackling but maybe it's connection was poor high up on the dial.

    Most importantly, I measured all the resistors in circuit. Most were within tolerances except 2. One of those wasn't wildly drifted, but one was...

    Screenshot_20210413-130338_Drive.jpg

    The 1000 ohm on the left side of the board was measuring about 1650 ohms.

    20210413_130127.jpg

    The 56k ohm highlighted on the right, though, measures about 1.4k ohms.

    20210413_124923.jpg

    That one seems to have a couple of drops of goop on it. Something sorta solidified.
     
  5. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    That 56k is part of the NFB loop, I think. It looks a long way out of spec, and I'd change that next.
     
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  6. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    And while you have the iron out, do the 1000r as well, and upgrade it to at least 2W.
     
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  7. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    Yep just out grabbing some resistors.
     
  8. dan40

    dan40 Friend of Leo's

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    Many of those resistors are in parallel to other parts of the circuit. To truly measure them accurately you must unsolder at least one leg and lift it up away from the eyelet. This isolates the resistor and allows for an accurate measurement. The 1000 ohm resistor on the far left of the board is the dropping resistor for the screen grids of the 6v6's. Reading a little high here won't cause the distortion issue you are hearing but it will ever so slightly lower the voltages to the screen grids and preamp.

    Adding the 470k grid stopper would be a worthy mod. I assuming you already have 1.5k's on the 6v6 grids as the amp did come stock with them.
     
  9. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    I just learned this.

    The 56k resistor was fine, so I'll put it back in. The 1000 ohm was a fair bit out of spec but maybe I'll try the 470k ohm mod and see what it does here.
     
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  10. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    That 1K should probably be replaced based on age/workload alone. As per @dan40, that's one of the hottest resistors on the board, and a nice 2-3W metal oxide would be much better for the work it’ll continue to do.
     
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  11. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    Well this may be the solution... I couldn't push the volume much due to the hour but in a brief test the distortion may be gone... or far less, anyhow.
     
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  12. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    Bah, it's still there. Might be less than before but still quite audible.

    Listen to 6G2 distortion #3 by AlbertaGriff on #SoundCloud
    https://soundcloud.app.goo.gl/DExGj

    I might see if subbing out the caps one by one in the trem circuit do anything.
     
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  13. theprofessor

    theprofessor Poster Extraordinaire

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    Sorry your exorcism hasn’t been as successful as you had hoped! You’ll get it! (To quote something from an unrelated realm, maybe this “kind only comes out by prayer and fasting”)
     
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  14. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    Ha I'm not that good at either or those. But I'm patient. At this point I think I've eliminated all potential problems except for capacitors.
     
  15. theprofessor

    theprofessor Poster Extraordinaire

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    Ha! Well patience is good! If the 470k resistor helped at all, it does make me think it might have to do with the capacitor(s) to that phase inverter section. Again, I'm hardly an expert...
     
  16. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    How about providing a voltage chart to see if there are any abnormalities.
     
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  17. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    That's a good idea as well. I might have a chance tomorrow night.
     
  18. ArcticWhite

    ArcticWhite Tele-Afflicted

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    I'm in the middle of converting a 1947 Webcor PA into a guitar amp. I was having static and breakup problems with the volume so I swapped in a new PI tube and power tubes, with no change.

    Then I started swapping out all of the coupling caps, and old carbon comp resistors in the signal path to metal film.

    I ended up doing every one of them. No success.
    Then I checked the 6AT6 Preamp tube and the rectifier on my tester.
    Both were bad.
    So I did all that work for nothing.

    Have you checked all your tubes, including rectifier?
     
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  19. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    The only one I haven't swapped was the rectifier... I figured it wouldn't affect tone but you're right that I should try it first.
     
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  20. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    Just checked the rectifier too, no luck.
     
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