Help! No tremolo, Fender 6G3 build

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Diverted, Nov 13, 2017.


  1. Diverted

    Diverted Member

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    [​IMG] Help! I am having trouble with non-functioning tremolo on a Fender 6G3 build I’m working on. I have tried everything and racked my brain, going over and over the schematic and wiring diagram. Nothing.

    Here's what I know:

    I installed a bias pot and it is putting out proper negative voltage to the input of the intensity pot. The amp is biased at about 70 percent dissipation and otherwise is playing very well.
    The center of the intensity pot runs to a .05 cap to ground and also to the junction of the two 220K resistors running to the output grids.
    The first two leads on the speed pot are bridged and feed a 100K resistor to ground. The high side of the speed pot runs to the .02 and .01 caps as shown on schematic and wiring diagram.

    The 1/2 of the 12Ax7 that powers the tremolo has the correct voltages. The cathode (pin 8 side) is at 2.4 volts (spec is 2 volts). The anode (pin 6 side) is at 216 volts (spec 205).

    I have checked and rechecked all wiring and run continuity tests between the various points just in case there was a cold or dry solder connection. There’s continuity everywhere. I’ve checked all caps for problems; all are good. Also checked all resistors, also all good. I checked the pots (250K linear for intensity, 3.5M for speed) and both are good and have the proper range. The tremolo is not grounded; I have not built the pedal yet so the signal is not being grounded out (ran continuity test from tremolo pedal point to ground just to make sure I hadn't accidentally grounded it. No continuity so it's in circuit.

    At this point I am totally stumped. Can anyone give me a tip or two on what to try next?
    I realize the photos probably won’t help much but I thought I’d throw them in anyway. Note that the far left pot is an across the line master volume control, not the Intensity (second pot from left) or speed (third pot from left).

    Thank you much.[​IMG]
     
  2. wyatt

    wyatt Member

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    Typically, bias-vary trem works better at a slightly colder bias, it's why amps like the Brown Deluxe and Blackface Princeton Reverbs didn't get bias pots.

    Try biasing around 60% and see if you get any trem? Assuming the trem circuit works, it may take some trial and error to find the sweet spot of bias/trem. You may end up almost as low as 50%.
     
    Last edited: Nov 13, 2017
  3. Deric

    Deric Member

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    Not sure from original post... Did it work before you installed a bias pot?

    Like wyatt said... bias-vary trem works best with colder bias. Re-set bias to ~50%. Does it work now?
     
  4. HotBluePlates

    HotBluePlates Member

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    Here's the album you tried to link:



    Any a.c. volts on V2 pin 6?

    What's up with the resistor & cap on V2? What are those connected to?
     
  5. Diverted

    Diverted Member

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    Thanks for linking the album! Don't know why it didn't work.
    I'm at work at the moment so can't check AC volts at pin 6. However when I first powered up and checked the bias it was around 56 percent, a little cold, so I ramped it up. No tremolo at that time either.
    That resistor and cap on V2 pin3 are correct. They're going to ground. On the original 6G3 they're on the board but I moved them off to the socket to save room for other stuff.
    Thanks!
     
  6. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    Might be my eyes but the 1 meg resistors in the tremolo oscillator look more like 100 ohm parts to me...
     
  7. swiveltung

    swiveltung Supporting Member

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    Yep, I think you nailed it!
     
  8. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    Looks like there’s one on the one input jack too.
     
  9. Diverted

    Diverted Member

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    I wish that was the case! No, those are 1M resistors. The green multiplier band kind of looks dark because this batch of resistors has a brown background. But they're definitely 1Ms.

    Here's the schematic with changes I made noted. Hopefully it'll show up ... haven't figured out how to directly post photos yet. THanks.

     
  10. Diverted

    Diverted Member

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    I built it with the bias pot in place from the start. Didn't think that would be the issue, as the pot's putting out good negative voltage. But I took the pot out of circuit and wired the bias power supply stock per the schematic. No change.

    Also, no tremolo at 50 percent dissipation. It's somewhere else but I'll be damned if I can find it. Time to rebuild the entire tremolo circuit though I honestly see nothing wrong with it. Huh.
     
  11. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    The three caps in the positive feedback loop on the trem oscillator: the first one (a big OD) looks freakin gigantic for a 10nF, are you sure that's not a 0.1uF?. The little 150M I can see is a 10nF, is the smaller OD a 22nF?

    Also, if your oscillator was running, you'd see that on your meter when you read the plate volts for the oscillator tube; your numbers would have been bouncing around mightily.
     
  12. Diverted

    Diverted Member

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  13. Diverted

    Diverted Member

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    I had a larger cap in one of the spots (.047) to slow the tremolo. Just went ahead and replacd all caps in that section of the tremolo circuit. Now, they're at spec of .02, .01 and .01. No change.
    Yeah, I know what you mean on the fluctuations. I built an earlier version of this amp for a friend of mine a few months ago and it was the first time I had dealt with a bias tremolo. I didn't expect the pulsing and just figured out it was best grounding out the tremolo while biasing because it was impossible otherwise. Don't have to do that now; no bouncing around now :(
     
  14. Diverted

    Diverted Member

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    I'm starting to wonder if one of the pots is bad. Checked them already but that's the only part of the tremolo circuit I haven't gone over a million times. I've traced and retraced it all with nothing.
    For the heck of it, also checked all parts of the tremolo that are not supposed to see ground just to see if they accidentally were. Nope.

    Just ran some continuity tests from the pin side of V2 sockets to their eventual tie in points in the system. All good there too.
     
    Last edited: Nov 14, 2017
  15. HotBluePlates

    HotBluePlates Member

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    When troubleshooting, it's bad to assume anything. Since I don't want to assume, does this mean you've probed the oscillator plate for a.c. volts and read 0vac?
     
  16. Diverted

    Diverted Member

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    That's correct. 0 AC volts on pin 6. Getting 216VDC on the plate.
     
  17. Dan40

    Dan40 Supporting Member

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    Are you using a foot pedal to turn trem on and off? You don't happen to have a grounded rca dummy jack plugged in do you? If so, you need to remove it to start the trem with this circuit.
     
  18. Diverted

    Diverted Member

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    No, foot pedal is not being used. The tremolo circuit's not grounded.
    Actually havent even wired the tremolo to the ground jack yet, so there's no option for turning it off.
     
  19. Dan40

    Dan40 Supporting Member

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    Have you tried a new, strong 12ax7?
     
  20. Diverted

    Diverted Member

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    Yep. I have tried four or five good strong 12AX7s inV2. Two NOS Amperex Bugle Boys, two NOS Telefunkens, a few good RCA pulls, etc. I'm pretty confident they're all good tubes.
     
    Dan40 likes this.

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