Tube Buffered FX Loops...Which to use?

Discussion in 'Shock Brother's DIY Amps' started by XTRXTR, May 7, 2021.

  1. XTRXTR

    XTRXTR Tele-Meister

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    I have a 2204 DIY that I plan to MOD with an FX loop
    First I found this and added it to my schematic:
    upload_2021-5-7_3-43-57.png
    But then I found this Merlin schematic
    upload_2021-5-7_3-45-45.png
    Which I like because it is placed between the treble on the tone stack and the MV leaving the MV as the actual MV. Also it is made for high gain which I need as well as clean.

    In any case there are many Tube Buffered FX Loop designs I have seen online. I have a Digitech GSP1101 which I use for delay and reverb.

    GSP1101 Loop specs:
    Loop Input:
    Impedance: 16K Ohms unbalanced or 20K Ohms balanced
    Nominal input level: +4 dBu or -10 dBV
    Maximum input: +23dBu (Level switch at +4 dBu)

    Loop Output:
    Impedance: 600 Ohms
    Nominal output level: -10 dBV (Unity gain from guitar input in BYPASS)

    Electronic SNR: ≥ 110dB

    If you had to choose which of the loops I added above which do you think is better for the effect unit I use? Or is there a better choice I am not aware? Maybe some impedance matching suggestions if the Tube FX loop is not already designed appropriately.

    I know there are solid state loops I could use but I won't because I want to build a tube buffered one. Its more about the building I enjoy than it is the merits of solid state or tube. I am a Gold Star Tube amp'er however.

    Anyway thanks for input. upload_2021-5-7_3-43-57.png upload_2021-5-7_3-45-45.png
     
  2. Guitarteach

    Guitarteach Doctor of Teleocity

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    Watching this... been toying with an fx loop idea for my JMP 50w master lead
     
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  3. XTRXTR

    XTRXTR Tele-Meister

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    44 years on 2204 MKII tube amps always had the delay or reverb in front, just cuz thats what you did. I'm old school, Page and Jimi, That lick on Since I've Been Lovin' You, holy what?! or all that crazy wild stuff on Still Rainin, Still Dreamin. Anyway, I guess its time to see what an effects loop does. Just found I have a LFO on this unit too 0.05Hz to 10Hz. I had a tremolo in my schematic as a MOD, maybe I should see what the LFO will do through the FX Loop first, I was looking to get some over lapping tremolo with delay and chorus for a Bridge of Sighs type sound. Have it all slightly off time with each other so it hits a phase node at random intervals. We'll see.
     
  4. tubedude

    tubedude Tele-Meister

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    I'd prefer the Merlin design. Also the O'Connor loop from London Power is nice.
     
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  5. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    I think you're going to be pushing the 24dBu limit at full tilt with the first design. Based on the 16k load you do lose a couple of dB. The impedance with the .68u drops about 2k at 100Hz. I think that's good enough. You probably wouldn't want that cap any smaller.
     
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  6. Ten Over

    Ten Over Tele-Holic

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    Kurts's loop is not worth considering.

    Here are a couple of loops for your consideration. One has a FX Return Level control and the other doesn't.
    FX Loop for XTRXTR 1.png
    FX Loop for XTRXTR 2.png
     
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  7. NSB_Chris

    NSB_Chris Tele-Meister

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    A lot of advice on this topic by @Ten Over is captured in this thread: https://www.tdpri.com/threads/tube-driven-effects-loop-and-other-questions.1006952/page-3

    My strategy was to put the loop after the MV so that you could completely bypass it and take it out of the mix. I have built two amps with this and they seem to work well. The intent is to be able to drive long FX cable runs but I don't. On the second build I ended up eliminating the send pot and just use the return FX pot. I set it up so that it is the same volume with the FX loop active or bypassed.
     
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  8. Jowes_84

    Jowes_84 Tele-Meister

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    There is a pretty good FX loop in the Fender Concert II, schematicsare available. I am considering just copying it for one of my upcoming builds. For bypassing, it is using shorting switchcraft jacks. Watching this thread :)
     
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  9. XTRXTR

    XTRXTR Tele-Meister

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    Do you mean to say the .68uf cap in the Kurt loop provides an AC impedance of roughly 2k at 100Hz? If that is so then if I decided to go with this loop I could add a 1uf and relieve the AC impedance to a lesser value as the knee should be at a lower Hz. Thereby providing a slightly better response with clean high headroom bass down to say even 40 Hz or so?
     
  10. dougsta

    dougsta Tele-Meister

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    Here's a tube buffered effects loop by Mr Soldano, I have a JCA22H head and tested on some rack effects units.


    JCA22H.jpg
     
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  11. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    No, I was saying the impedance is roughly 2k lower at 100Hz assuming a 16k resistive load load. The -3dB point for the filter is way down around 15Hz, but the impedance curve is still rising up through 100Hz. The subsequent circuit's input impedance may have a reactive component like an input capacitor, so the two impedances interact and potentially alter the overall frequency response.
     
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  12. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    @XTRXTR the other issue with the Kurt loop, and any based on a cathode follower, is the potential for DC to appear at the output at startup.
     
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  13. Ten Over

    Ten Over Tele-Holic

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    Do you have the component values for that?
     
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  14. Ten Over

    Ten Over Tele-Holic

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    Compounded by the 1M resistor on the Effects Send which radically slows down the drainage of DC voltage on the 0.68uF capacitor. There is no need for a resistor that large.

    The driver is also capable of outputting a 55Vp audio signal that your FX device might not appreciate.

    The driver also chews up 6mA of supply current. There is no need for that kind of consumption.
     
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  15. dougsta

    dougsta Tele-Meister

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    Sure, np

    service manual inc schematics, BOM
     

    Attached Files:

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  16. Ten Over

    Ten Over Tele-Holic

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    Thanks.

    Here are a couple of loops with that sort of topology.
    2204 Loop 1 PNG.png
    2204 Loop 3 PNG.png
     
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  17. XTRXTR

    XTRXTR Tele-Meister

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    Looks unique. It fascinates me how many ways an FX loop can be designed. This one has two things I am inclined to look to into other designs instead. One is the MV is placed before it and the other is there is no gain control on the either the send or return before proceeding to the PI. These combined values could lead to distortion of the effects being returned to the PI without a simple method to place it at a nominal setting to maintain the original preamp tone mixed with the effect. The effect unit used would have to be manipulated to bring the gains at a balance. That is important to me as different effects may have different gains. IOW potentially having to adjust settings between songs or even within the same song/tune is not practical in a stage performance setting.
     
  18. XTRXTR

    XTRXTR Tele-Meister

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    Thanks for clarifying my question and further information that eliminates the Kurt loop design from my FX Loop research. I am inclined to go with Ten Over's loop designs at this point but I have a couple questions for him about that.
     
  19. XTRXTR

    XTRXTR Tele-Meister

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    Thank you for providing 4 different loops for me to ponder. Of the four presented do you still believe the first two are the optimal way to go given the FX loop GSP1101 loop impedance values provided in the post above? Assuming the answer to that is yes then I have another question. The design without a return variable gain control keeps the return signal at maximum at all times - in the case the return signal is clipping woulddn't the one with a variable gain control on the return be a better selection? Disregarding what I just said is there some other elemental lack of understanding I might have that would make the design with no variable return gain control a better fit? Cost or eliminating space for a pot is not really a problem for my DIY custom chassis.
     
  20. XTRXTR

    XTRXTR Tele-Meister

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    I'll need to read this discussion before making a final decision on pre or post MV. Thank you for pointing to the thread.
     
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