6v6 2204 sounds a little off

Discussion in 'Amp Central Station' started by jasonvilla, May 18, 2020.

  1. jasonvilla

    jasonvilla Tele-Meister

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    Hi all, I've built a 2204 circuit powered by JJ6v6s and it sounds a bit TOO distorted. But in sort of a fizzy/non pleasing way. I cant figured out what's going on. It's built with a 300-0-300 PT. Classic tone Deluxe Reverb OT and Choke. I have a 250ohm resistor after the choke, because the screens were reading higher than the plates. The heaters are elevated. And the first two dropping resistors are lower than stock values - Initially because the PT is putting out less power than a normal 2204, so I wanted my preamp voltages up. But I further lowered them to try and reduce distortion. Ive played around with fizz caps and even raised some preamp cathode resistors. I've reflowed things, swapped coupling caps, and swapped filter caps. Nothing seems to get rid of it, so it must be something else Im missing...
    These are my voltages:
    Screen Shot 2020-05-17 at 9.07.52 PM.png
    I also JUST got my first oscilloscope(always wanted one). Ive never touched one before, so Im still figuring it out. I (believe) I have a 360mv 1k sine wave going to pin 7 of V1.
    Screen Shot 2020-05-17 at 9.25.49 PM.png

    This is the first gain stage after the .22uF coupling cap on V1b

    Screen Shot 2020-05-17 at 9.29.05 PM.png

    This is after the second gain stage on V1a with the volume set to 7
    Screen Shot 2020-05-17 at 9.32.18 PM.png

    And this is the same spot with the volume on 10
    Screen Shot 2020-05-17 at 9.32.32 PM.png

    Does this look right to you guys so far?
    At the very least Im having a lot of fun playing around with this scope!
    Thanks everyone
     
  2. Paul G.

    Paul G. Friend of Leo's

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    Schematic you used and maybe some voltages?
     
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  3. jasonvilla

    jasonvilla Tele-Meister

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    I believe this is everything except the elevated heaters. All the voltages I've taken so far are in the OP. Thank you!
    Screen Shot 2020-05-18 at 10.01.54 AM.png
     
  4. Commodore 64

    Commodore 64 Friend of Leo's

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    Your first cathode bypass cap is 680n? I have a similar circuit with 6L6, and I used 22uF. I attached my schematic (ignore the -4.5v on the grid of V2a).

    Your scopes look like mine. Well that's a sweet scope you got. Mine's an older CRO. Damn, I'm jealous. Love that!

    I can get square wave top and bottom out of this circuit pretty easy, even before the PI. There is goo gobs of gain on tap.
     

    Attached Files:

    Last edited: May 18, 2020
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  5. Commodore 64

    Commodore 64 Friend of Leo's

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    Injecting: 500mv signal, at about 50% volume, 50% on TMB

    Out of gain stage 2:
    [​IMG]

    Out of Gain stage 3:
    [​IMG]




    Out of my PI:
    [​IMG]
     
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  6. Commodore 64

    Commodore 64 Friend of Leo's

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    A thought occurred to me. My power section is UL so I attributed it to that, but my circuit sounds gnarly in a bad way through 4x10 (jensen Mod 10) and a 15" Emi Legend. I ultimately stuck the chassis (Bassman 10) back in the original cab, which has 4, 10" Carvin BASS speakers. And that's where it sounds best.
     
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  7. Paul G.

    Paul G. Friend of Leo's

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    I don't see an issue here. Maybe you just need different speakers?
     
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  8. dan40

    dan40 Tele-Afflicted

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    Is it distorting too early or does it only distort when pushing the volume up? Are you positive that all of the components in the preamp are the correct value? Did you meter the resistors for correct value when installing them? If all of the values are correct, the bad distortion could be caused by a lead dress issue in the preamp but we will need some pics of the wiring to offer suggestions.
     
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  9. printer2

    printer2 Poster Extraordinaire

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    Try changing the 470R in the PI to 820R.
     
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  10. jasonvilla

    jasonvilla Tele-Meister

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    Thank you so much Commodore!! That's really awesome to see your preamp on the scope. It looks like our first two gain stages are pretty similar, but then the amp starts to differ a bit.
    I've always wanted a scope and it was my birthday last week so I thought might as well. It's been fun to be able to SEE things. It hasn't helped me find my unwanted distortion, yet, but im hopeful.
     
  11. jasonvilla

    jasonvilla Tele-Meister

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    Distortion onset seems correct with the volume pot. I double checked the color bands, but I did not meter everything before install. The distortion just sounds fizzier than I was expecting. Not super pleasing. I realize that probably means nothing and is super subjective.
     
  12. jasonvilla

    jasonvilla Tele-Meister

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    This would cause the PI to distort less, correct? I think my fizziness is coming from the preamp.
     
  13. Luthier Vandros

    Luthier Vandros Tele-Holic

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    They make non-fizzy marshalls?? I keed! What speakers are you running? Nice job, OP, and sweet scope.
     
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  14. jasonvilla

    jasonvilla Tele-Meister

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    I think some of my issue was a microphonic power tube that I sent back and have since replaced. I also have replaced the 68k input resistor on V1 because it was popping when I tapped it. ...Im not sure where the problem was so I replaced the input jack (with a real Cliff jack), ran a new shielded cable, and put in a new 68k resistor. I haven't hot glued the shielded cable yet because Im out of hot glue! But distortion is a bit fuzzy. Maybe thats how its supposed to sound...

    So I put a 260mv 1khz sine wave through it and tried to figure out what the clean output power was - I took the RMS voltage, squared it, divided by 8, and only got around 10watts. I could be doing that wrong. Im still new to this scope stuff :)

    Screen Shot 2020-05-22 at 9.32.52 PM.png
    Preamp volume is on like .5/1 and master is at 8.
    Screen Shot 2020-05-22 at 9.33.48 PM.png
    Guts. Unfortunately this was much cleaner before I started unsoldering, soldering, unsoldering, soldering... :(
    Screen Shot 2020-05-22 at 9.34.05 PM.png
    Screen Shot 2020-05-22 at 9.34.09 PM.png

    The amp in it's home:
    Screen Shot 2020-05-22 at 9.39.38 PM.png
    And here's the crazy waveform when the preamp is dimed.
    Screen Shot 2020-05-22 at 9.34.16 PM.png


    I've replaced a lot of parts on this. I've gone back to stock values on most components as well, so the above schematic is different than where Im currently at.
     
  15. jasonvilla

    jasonvilla Tele-Meister

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    Thank you :) It is a new WGS ET-65, Ive also played it through an older WGS ET-90.
     
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  16. Commodore 64

    Commodore 64 Friend of Leo's

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    [​IMG]

    I can do that too! Diming the preamp. Though I thought that was when I had stray DC all over the place from a bad switch (bootstrapped the CF and put it on switch on teh front panel (200DC on the switch, was leaking to the chassis). I've since fixed that, so let me go out and see if I can still get that pattern.

    In the meantime, check for DC on your preamp grids.
     
    Last edited: May 23, 2020
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  17. jasonvilla

    jasonvilla Tele-Meister

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    Lol, are those tele blanks holding up your scope???

    So im getting -4.5vdc in a spot I was expecting to see any dc. Is that right? That $4 cap better not be leaking!
    Screen Shot 2020-05-23 at 2.43.05 PM.png Screen Shot 2020-05-23 at 2.43.49 PM.png


    Also when I was playing with the scope and dummy load last night my amp started to sing and it freaked me out!! But after searching the web it sounds like this is a pretty normal occurrence haha.

    Wouldn't you expect this amp to put out more than 10 clean watts? Or did I figure the wattage incorrectly? Thanks!
     
  18. jasonvilla

    jasonvilla Tele-Meister

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    Did some more checking. It only has that negative voltage if the pre amp volume is turned up AND there is a signal being injected to the input. Otherwise it reads normal.
     
  19. dan40

    dan40 Tele-Afflicted

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    Have you tried readjusting any of the wires leading to the sockets and pots yet? Because of the smaller chassis size and the placement of the sockets, I am betting that the fuzzy distortion is caused by some crosstalk in the wiring. This issue can sometimes be cleared up with some careful rearranging of the lead dress. Playing a looper pedal through the amp while you try moving the preamp wiring with a chopstick can be a great help in finding the issue. The wire leading to the presence pot can be particularly sensitive and needs to be kept as far from the other tone control wires as possible.
     
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  20. dan40

    dan40 Tele-Afflicted

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    If you take a look at Commodore 64's Bassman schematic he posted above, he is also seeing the same 4.5vdc at the grid of v2a. Your cap is most likely just fine. I will mention though that several builders have reported issues with those black silver mica caps in the past few years. Several reports of caps being bad in brand new builds. It seems they are very sensitive to heat and can be easily damaged if a heat sink isn't used when soldering. IIRC, the problem they were experiencing was crackling and popping noises so they may not be related to your issue.
     
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