Fender 6G3 - problem diagnosis

Discussion in 'Amp Tech Center' started by Steve-0-, Nov 2, 2019.

  1. Steve-0-

    Steve-0- TDPRI Member

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    Hi All, hoping for a bit of advice before diving in.....

    The patient is my 6G3 deluxe. I've rebuilt it since getting it as a basket case....
    - all electrolytics done
    - both transformers changed
    - bias adjustment added
    Sounds and plays really nice.

    However.....
    1) As of a few days ago, I noticed the tremolo is no longer doing anything. I can hear a pulse when it's turned up and I'm plugged into the normal channel but nothing when into the bright channel.
    2) When plugged into the normal channel with the volume off, but with the bright volume up, there is a bleed through of signal to the speaker. Not much, but noticeable. Normal channel works fine.

    So..... obviously needs a good looking at but any suggestions from you wizards as to where to start.
    I'm comfortable with safety precautions etc.
    Thanks a load in advance.

    Steve
     
  2. slider313

    slider313 Tele-Afflicted Silver Supporter

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    Is this an original or a clone? Do you have any chassis photos?


    There are three oscillating caps off of pin 6,7 and 8 of V2. I would start there.

    [​IMG]
     
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  3. bcorig

    bcorig Tele-Afflicted Silver Supporter

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    God! I still marvel at the depth of knowledge on this site!!
     
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  4. Steve-0-

    Steve-0- TDPRI Member

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    Hi,

    Thanks for the quick responses.

    It is an original, yes. But like I said, transformers have been swapped out due to abuse in a previous life.

    Don't have any chassis photos yet as I haven't opened it up. Will post some when I get a chance.

    Thanks for the pointers..... I've found them so will have a look there first.

    Cheers

    Steve
     
  5. corliss1

    corliss1 Friend of Leo's

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    Step one of any amp troubleshooting is a voltage chart. Even a simple tube swap could get things going.
     
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  6. slider313

    slider313 Tele-Afflicted Silver Supporter

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    Early brown Deluxe amps ('61 to very early '62) had a mix of yellow Astrons and Mallory blue molded caps. The old yellow Astrons are famous for leaking dc voltage.
     
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  7. Steve-0-

    Steve-0- TDPRI Member

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    Thanks guys. I'll start with a full voltage check, particularly around v2 and see what it looks like. While I could believe the tremolo issue may be a dodgy tube, the normal to bright signal leakage seems more sinister.

    Mine is a 62 and I don't remember seeing any yellow Astrons. Coukd be wrong though.

    Cheers,

    Steve
     
  8. dan40

    dan40 Tele-Afflicted Silver Supporter

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    The 6g3 modulates the bias voltage on the power tubes to create the tremolo effect. Since this happens at the end of the signal chain (power amp), it's very strange that the tremolo works when using the normal channel but not the bright channel. This type of tremolo works much better with a cooler bias setting so be sure that the bias isn't set too high on the power tubes.
     
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  9. BobbyZ

    BobbyZ Doctor of Teleocity

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  10. BobbyZ

    BobbyZ Doctor of Teleocity

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    You said you replaced the electrolytics, Does that mean all the cathode bypass caps, the 25uf-25volt ones?
    Nothing worse than chasing trem issues only to find out the cathode bypass cap is old as dirt. :)
     
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  11. BobbyZ

    BobbyZ Doctor of Teleocity

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    1653.gif Well it uploaded, never know what the internet will do here.
     
  12. Steve-0-

    Steve-0- TDPRI Member

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    I'll be opening it up this afternoon so will be able to have a proper check. I'm sure I did but honestly can't remember as it was a good few years back.
    If they need changing out, what would you recommend? I can get Sprague atoms at the right spec that would do the job.

    In response to some of the other points...... the trem for an work on either channel. You can hear a faint pulsing when the intensity is up when plugged into either channel but nothing done to the sound.
    Also, the signal bleed happens when plugged into either normal or bright. If the other channel volume is up you hear something. Is this just typical of old fender input circuits?

    Thanks,

    Steve
     
  13. Wally

    Wally Telefied Ad Free Member

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    Steve, as dan40 notes, the bias vary trem is dependent on a certain bias level for the power tubes. If the current draw is too high, the trem will be weak or even non-functional. Your power tubes may have drifted?
    There is a common cathode bypass circuit for the two input preamps. Perhaps this causes some bleed??
     
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  14. BobbyZ

    BobbyZ Doctor of Teleocity

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    For Ecaps I use F&Ts were ever they fit. Normally only use Sprauge for cathode bypass spots. Thier blue filter caps are over priced and don't have the best reputation anyway.
    F&T does make a double 25uf-25volt cap but it's HUGE! Too bad because it'd be nice for old Fenders that used doubles. Luckily I looked at the dimensions before ordering those. (normally not that smart)
     
  15. Steve-0-

    Steve-0- TDPRI Member

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    Right guys...... I've had the beast opened up.

    Firstly, the cathode bypass caps were changed with the rest of the electrolytics. Bias cap was also done.

    Checked the bias..... it had drifted significantly. The JJs were pulling between 11.5 and 12 watts. Ouch. This has been reset (8.5 and 9 watts now) and all voltages round the circuit are within range. Why would these have drifted so much?

    Have changed all preamp tubes for known good ones.

    The problem is still there. When the trem intensity is turned up, you can hear a pulsing/clicking type sound that varies in speed and intensity as it should - it just doesn't affect the guitar sound.

    So..... What's next?

    Cheers,

    Steve
     
  16. corliss1

    corliss1 Friend of Leo's

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    Because JJ tubes suck??? Because a resistor changed value after getting hot?

    Do you have anything besides a meter? A scope would let you see the the trem section oscillating and then you could follow that. However, if you hear the pulse in the speaker you know it's working all the way to the output section. It's either too weak to effect the signal or not getting mixed in with the signal. Did we try swapping the trem driver tube just for fun?

    How loud is the pulsing clicking sound? Is it very subtle or is it pronounced?
     
  17. Steve-0-

    Steve-0- TDPRI Member

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    Hmmmm......

    Tubes could be in question here. Just re checked the bias...... plate voltage has dropped to 270ish from about 380ish and the cathode current through the roof. Both tubes though.

    No..... I only have a meter but could maybe get hold of a scope by end of the week.

    Definitely hear the pulse in the speaker. Have swapped all preamp tubes with known good ones including the trem driver.

    The pulsing is fairly subtle. Becomes more noticeable when you turn the intensity up. Drowned out when you start playing though. Could it be the connection to the pedal? I don't have a pedal for it so nothing connected.

    Cheers,

    Steve
     
  18. corliss1

    corliss1 Friend of Leo's

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    That's especially odd if the tubes are changing that quickly. Do you have another set of power tubes to try? I'm not saying that's the issue, but if the voltages/current fluctuates with two sets of tubes, something is up. Or it won't, and you have a bad set of tubes and maybe other issues.
     
  19. Steve-0-

    Steve-0- TDPRI Member

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    Yes it is...... I've checked a couple of times since and no further change.

    Don't have any spare 6V6s as the original goosed OT killed the other set before it was replaced.
     
  20. Steve-0-

    Steve-0- TDPRI Member

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    Hmmm...... Can't decide if i did something stupid before. Plate voltages and current back where they should be.

    Have I maybe got a problem in the bias supply?
     
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