AB763 - Effects Loop?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by RoadDeville, Jun 30, 2006.


  1. RoadDeville

    RoadDeville Member

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    While some might argue "Why?", where & how would this be implemented?
    Thanks
     
  2. John Phillips

    John Phillips Member

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    Or, if you want it at more pedal-friendly signal levels, immediately before the third gain stage on the Vibrato channel (after the reverb mix section).

    Why? :)
     
  3. danieldroukas

    danieldroukas Member

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    John Phillip's comment is good if you're only running pedals. I like to put mine where Old Tele Man suggested since that's great for running studio gear, but I add a tube circuit to enable me to switch back and forth between studio levels and pedal levals. Adding a circuit like this also allows you to nicely switch between a series and parallel effects loop.

    My next project is a Tweed Twin/Deluxe hybrid with just such an effects loop built in.
     
  4. PaulC

    PaulC Member

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    Or if you're just wanting it for time delay stuff put it in place of the reverb circuit so it's parallel. Just rewire the driver as a buffer, make the freq responce flat, and give it a send level control. Then make the freq responce of the return amp flat by increasing the value of that 3n3 cap and you're done. Works great for mixing in verbs and echos without running your dry signal through the fx unit.

    PaulC
    Heritage amps/Tim & timmy pedals
     
  5. wthiessen

    wthiessen Member

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    I know that this thread is older, but I'm looking to do something similar to my twin (passive loop). Shouldn't the send and return go before the reverb? I was under the impression that reverb is supposed to come after delay and mod effects. Putting it before the PI would have Reverb/Trem > Delay.

    Thanks!
    Will
     
  6. John Phillips

    John Phillips Member

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    This is the other reason why I don't think there's any point.

    The AB763 circuit does not have enough gain to distort in the early stages, so all the distortion comes from the PI and power stage. Since you then can't get the FX loop after the distortion, IMO there's no point in modding the amp.

    It might be worth it if you're going to boost the front end so hard that the first or second gain stage distorts, but that doesn't actually sound very good on these amps anyway.

    The only other reason I could think of is that if you're using a line-level-only processor, it may be better (for noise) to run it at a higher signal level.

    FWIW, inserting the loop after the reverb mix does put it after the reverb, but before the tremolo.
     
  7. wthiessen

    wthiessen Member

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    I should have also mentioned that I have had my amp modded to have higher gain and also reverb/trem in the first channel. That is why I need an FX loop.

    Will
     
  8. wthiessen

    wthiessen Member

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    Can someone tell me if I should being using shielded wires, like on the inputs, to bring the signal to the send and return? Or is the the signal level high enough by this point that it doesn't matter?

    Thanks,
    Will
     
  9. jay42

    jay42 Member

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    Fwiw, the effects loop on the Series II fender amps is after the reverb recovery stage, in front of the PI. By all means, use coax.
     
  10. wthiessen

    wthiessen Member

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    John, where is the 3rd gain stage before the Vibrato channel? Would you be able to point it out on a schematic for me?
    I inserted the loop just before the PI cap. The amp sounds a lot better when the loop is bypassed. When running my DL4 through it there seems to be a loading effect. Would adding a cap after the 220k summing resistor's remedy this?
    My amp tech had suggested using a MXR or similar FET clean boost after the loop effects to bring it back up to the same loudness. Any thoughts on this?

    Thanks,
    Will
     
  11. John Phillips

    John Phillips Member

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    That insert point is at far too high a signal level for the DL-4 (or pretty much any pedal) to cope with. It might work with a +4dB processor, but I'm not certain. The Rivera series has it there, but that's because it uses additional pad and gain stages to match the levels.

    The other point is right before the grid of the third tube stage on the Vibrato channel, after the 3.3M resistor/10pF cap and 470K reverb mix resistor. Break the signal path there, between the eyelet board and the tube socket, and add a 1M resistor to ground on the tube grid itself or the return jack (to stop it floating high if there's a loose cable plugged into the return jack, which will blow the 12AX7).
     
  12. wthiessen

    wthiessen Member

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    Thanks John, I did everything ask instructed, I still have some issues, though.

    1.) When DL4 is off it is noisy (hums), this goes away when it is engaged.
    2.) I have the loop set up using switching jacks so when nothing it plugged in it is automatically bypassed. The 'switching' contact on the send and return are connected together. With nothing plugged in it is very very quiet... probably due to the 1M resistor connected to the input of the 12AX7.
    Any recommendations on how to fix this? Does the 1M resistor need to be switched out?

    Other than these problems the loop sounds great! I was able to use the existing wires around this tube to for send and return.

    Thanks,
    Will
     
  13. John Phillips

    John Phillips Member

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    Are you sure the switching contacts are working? The 1M resistor should make almost no difference to the signal level, since there's already a 220K resistor to ground at that point (next to the 470K reverb resistor on the schematic). In any case, since the DL-4 is 'true' bypass (assuming you have it set that way), the 1M resistor is equally in the circuit when the loop is in use but the pedal is off.

    I would guess that one or other of the switch contacts - I only use the switch in the Return jack BTW, you only need one - is not working.

    The hum when the DL-4 is bypassed may be because it's 'true' bypass - this will make the return stage much more sensitive to acting as an aerial than if it's buffered, since the whole loop of cable is fed from the high impedance of the reverb mix stage instead of the low impedance of a buffer. You can set it to 'DSP bypass', which makes it buffered - I can't remember exactly how, I think you have to turn it on with a couple of the buttons pressed... it's in the manual.
     
  14. wthiessen

    wthiessen Member

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    Thank you John and everyone else for the abudance on help on this topic!

    This question is aimed at effects loops in general:
    Can I use a TRS cable/jack for send/return?
    I would like to cut down on the number of cables that I have to hook up.

    Thanks,
    Will
     

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