Ampeg Gvt15 Effect Loop Hum Update Modification + Signal Leveling.

Discussion in 'The Workbench' started by 67Mopar, Mar 15, 2019 at 5:32 PM.

  1. 67Mopar

    67Mopar Well-Known Member

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    There's a slight difference in volume when I switch the effects loop on. If I reduce the value of this resistor, will it increase the return volume? Top-end is cut a bit as well, so maybe the 1uF cap need to be reduced as well?

    [​IMG]
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    effects loopvol.png

    The return gain resistor can be replaced with a pot.
    Example: 50K pot. Then you can turn it up/down as you need.
     
  3. 67Mopar

    67Mopar Well-Known Member

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    Awesome! Thank you. :dude:
     
  4. 67Mopar

    67Mopar Well-Known Member

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    What about the 100pF cap (C6) in-parallel with the resistor (R7)? Would changing the cap value brighten/darken the signal?
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    The cap limits the high frequency response, taking the cap out = maximum treble response
     
  6. 67Mopar

    67Mopar Well-Known Member

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    Thank you. I suppose I could just clip one side, in leu of removing it entirely.
     
  7. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    What amplifier and before you go modifying for nothing, is there effects in the loop? Which effects?
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    Yes you can clip one side of it.
     
  9. myersbw

    myersbw Well-Known Member

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    Keep in mind that cap is there to reduce hiss. Clip it and you might get a lot more upper freq amplified than you want. To increase the gain, you can also decrease R5, and it's easier to piggyback a resistor to see if you like the result. Take a 50-100K pot...wire the wiper and one end across R5 (make sure it's up full and measure about 50-100K between wiper & end before you tack solder it across R5)...push a signal through and incrementally decrease until you don't hear a volume increase that's about unity when switching the loop in/out.

    That said, do this first...just insert a patch cord at the loop and switch it in/out. Does it change? Note if more or less...use the same pot I described and adjust until you can't hear the difference. That'll confirm whether the loop is functioning at unity gain as compared to the loop being in or out.

    However, pedals differ in impedance and how they match up. One may be fine...another seem weak...and, still another make have a gain boost and not make a difference. I'd leave that cap alone. Pedals are notorious for extra high freq...that's why it's there.

    Oh, and take the gain increase slow when turning that pot. If you drop it to zero, remember the gain ratio = 1+(R7/R5) in this case...drop R5 to effectively zero and you have a high gain amp with all the pretty squeals from oscillation. Reducing R5 preserves the cutoff knee of bandpass that R7 and the cap dictate.

    Cheers!
    Brad
     
    Last edited: Mar 16, 2019 at 11:25 AM
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  10. 67Mopar

    67Mopar Well-Known Member

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    Ampeg GVT15 combo. The volume drop is there, both with effects or using a jumper cord. I use Rockman Stereo Echo [mono setting, of course] in the loop for some slap-back.

    I didn't notice this with my GVT15 head, but I will go back a recheck it. The head has an effects loop hum mod, but I don't recall hearing a difference in the volume prior to the mod. The combo doesn't exhibit the usual hum [heater/effect return trace proximity] found in many GVT15 amplifiers. I'll spray the jacks today, and see if that helps.
     
  11. 67Mopar

    67Mopar Well-Known Member

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    Awesome! I did try using a patch cord, and the volume drop is the same. Could the C6 capacitor be responsible for a perceived the loss of gain via cutting highs?

    Between yourself, ampmadscientist, and Mickeydg5, no problem is unresolvable here on the Marshall forum! :cheers:
     
    Last edited: Mar 16, 2019 at 3:35 PM
  12. JohnH

    JohnH Well-Known Member

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    For something to try, I like Brads suggestion, to splice another resistor across R5 to increase gain at the opamp. No desoldering and its reversible, if you can just splice the new resistor across R5's leads. A good value to try would be around 18k, which in theory should lift the level by 3db.

    I have done this exact mod on my DSL401 for the reverb and FX returns.
     
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  13. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    There is no need to modify the amplifier yet. Why does everyone go straight to that conclusion?

    The GVT15H and combo are the same amplifier, same effects loop.

    Can you show or tell us what the "effects hum mod" is in the amplifier?

    How is the Echo Input Gain set? Is it set as high as possible with no clipping? Is the incoming signal to the Echo set at the point before clipping?
    Are the Echo Volume(s) set to high or maximum level?

    Did you check patch cables or try different cables?

    Did you try cleaning jacks and plugs with an electronics lubricating contact cleaner?
     
    Last edited: Mar 16, 2019 at 5:28 PM
  14. 67Mopar

    67Mopar Well-Known Member

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    I will clean the jacks today.
    I made sure the signals were leveled, prior to comparisons.
    The Rockman echo is set to its highest setting, no clipping at all. Not even close.

    Update:
    I was playing the GVT15 combo in the family room last night, using my alternate Rockman echo (using George L's patch cables), and sure enough, I could hear the same buzz/hum that I was getting from the GVT15 head. If I recall correctly, I didn't always hear buzz/hum through the head (prior to the mod) either. Anyway, when I moved the amp back in the living room (variac set to 117VAC + Furman strip), the buzz/hum is nearly non-existent. I am also not using the George L's patch cords in the living room, so I'll try the same echo (with the George L's cables) in the living room today, and see if I get the same buzz/hum effect.

    Since my tech tapped a signal off of an op-amp, this makes me thing that he rerouted signal to the effects send, away from the heater voltage trace. The heaters supply is AC, then rectified to DC, correct? If yes, why would the loop have a hiss/hum issue?

    I'm looking here...

    [​IMG]

    I will snap some pics of the GVT loop mod later today. I'm sure there are many out there who would like to have it, as the new owners of Ampeg will not release it to non-authorized entities. When I tried to get the loop update from Ampeg, they replied; "Um, well, uh, no... we don't have that in out database..." :rolleyes:
     
    Last edited: Mar 17, 2019 at 3:43 PM
  15. 67Mopar

    67Mopar Well-Known Member

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    Okay... I'm getting a hissing sound from the combo loop, regardless of the patch cables/effects used. The head had more hum, vs the combo which exhibits more hiss than anything. It's not that bad, so I may just leave it as-is. I just don't get why Ampeg would make such a great sounding amp, then drop the ball on the loop?

    I use a Fulltone SSTE with the GVT15 head, so the loop isn't an issue. I wanted the combo to use as a clean amp (with effects) to mate with my Silver Jubilee. I suppose I'll just use the GVT head + cab with the Marshall, and use the combo with the tape echo.

    Once I post pics of the loop update (mod), maybe you guys can figure out an alternate option to tapping signal from the op-amp. The mod my tech did was made per Ampeg's update bulletin. I was thinking... Would covering the heater/effects traces with EMF protective paint stop the hiss/hum?

    I updated the thread title, so that others who are experiencing the same issues can find it in a search.
     
    Last edited: Mar 17, 2019 at 12:18 PM
  16. ampmadscientist

    ampmadscientist Well-Known Member

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    1.If the effects loop has a hum, try disconnect the ground (shield) from the return patch cable at the effects unit output jack end.
    This will break a ground loop if there is one...

    2. Hum can also be caused by the power supply for the effects.

    3. Hum can also be caused by :
    running the input jack effects and running the loop effects from the same single power supply.
    If you are doing this try using 2 separate power supplies.
     
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  17. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Please clarify this.
    How did you make sure the signals were level? What signals did you make level?

    Every amplfiier, every device, every component and every wire has differences whether they look the same or not.
    This could mean the combo may require the ECHO Levels be set to -3dB while the head only need be set at -5dB due to any differences like internal tolerance differences.
     
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  18. 67Mopar

    67Mopar Well-Known Member

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    I balanced echo on vs echo bypass. I also tested the loop using three patch cables direct, no effects, all resulting in the same signal loss, and loop noise. I'm gathering info on the OEM loop update now...
     
  19. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Ok, I get it.

    The preamp tube's filaments are feed DC.
    The effects loop upgrade information would be good to see.

    Like i mentioned, any variance in the components can affect the signal gain/loss. It seems its not your cords or gadgets so it must be in the loop itself since the loop bypass provides a higher signal level. In this case @myersbw had the best solution for dialing in the the entire effects loop. That would make the 10k R5 at U1-B your target. Actually a small trimmer instead may be liked or combination keeping it always variable.

    But first we should see this loop upgrade and how it affects the circuit.

    Also make sure you have good connection/continuity at connections VREF-1 and VREF-2 of that effects loop circuit.

    .
    .
    .

     
    Last edited: Mar 17, 2019 at 2:29 PM
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  20. 67Mopar

    67Mopar Well-Known Member

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    What about the 6V6 filament heaters? Are those DC as well? Pins 2 and 7.
     

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