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Easiest Way to Mod an Amp with an FX loop?


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  • Easiest Way to Mod an Amp with an FX loop?

    Some of my Tube Amps have no FX loop so I am thinking of inserting (2) 1/4 inch jacks before the Master Volume control and using (2) low noise Op Amps for impedance match to the effects that will plug into the FX loop. I do not have room for Vacuum Tube Send/Return circuits so Op Amps will have to do with high S/N Ratio. Anybody done this kind of circuit on have electronic knowledge if this will work? Not sure of the rails +15 to -15V or can I use +12 to +15V to pass the signal without distortion? Any Op Amp recommendations?

  • #2
    You may not need to insert those devices in the chain unless you are running a parallel effects loop. If you're running a series loop you may not need them. If you haven't installed an effect loop before, you may want to buy a kit that has the parts and instructions. It really depends on the specific amp however.

    This site has mods for loops last time I checked. There are others but knowing what amp you have is an important key.


    • #3
      I am only inserting a NanoVerb in series so may not need buffering at all with 2' cables. Just need to insert before the PI with a series device and HI Z in and Lo Z out. One of the Amps is a Jet City 20 Watt model.


      • #4
        I found an Op Amp circuit buffer design in a schematic I have of the Laney VC30 that could be used for General purpose FX loop Apps if I need it, Thanks for your help WR!


        • #5
          Here's a site that sells the effect loop mod but their price is really high. http://www.fjamods.com/JetCity

          If you have some technical skill, there's a thread here with pics installing a loop on that amp.


          • #6
            I am an Electronic Engineer like you WR. Thanks again and for one Effect with In, Out level Pots I am not going to bother buffering the FX loop unless I want to drive 20' of cable. LOL Thanks again WR! I got my answer.


            • #7
              Thanks, but I cant take credit for being an engineer. I only took three years of college and have a technical degree. I'd like to think my 45 years working in electronics makes up for that lost year and the 200+ training courses I've either taken, written or taught for major manufacturers kind of makes it honorary, but I do live by a standard. I never got that degree so no matter how much experience I may have, or how much knowledge I've accumulated, I have enough respect for others in the industry not to steal a title I don't have a degree for.

              I have had the title of Customer Service Engineer working for major corporations but companies are big at giving out fancy titles to empower their people. Its not the same thing as a formal education degree. Its one of my pet peeves in audio because anyone and everyone into recording give "themselves" an engineering title. That wasn't so in the past, because the guys who worked in major studios actually had college degrees, could and did build the consoles and gear from scratch. Today you cant even call them out on not having any degree in electronics without them getting bent out of shape, but thats the way it is.

              Anyway, I humbly accept your thanks and hope things go well for you.
              The only thing I might suggest is check the DC volts between the hot terminals and ground. If they don't read zero, you may want to put some high value caps of 1~ 2MF in series before the jack. This should pass all audio frequencies yet prevent from passing into the loop hardware. Its kind of a poor mans buffer. I also haven't studied the schematic and I'm not sure it does use frame ground for everything. If it does than you're probably fine. If not it may be wise to use plastic jacks or rubber grommets to prevent the loop from grounding to the frame if there's hum issues.


              • #8
                Well I think you know a lot about Electronics, Musician, Tube Amps; and experience does count toward Knowledge and Wisdom. I understand about the FX loop precautions and that is pretty close to what I was going to do but good advice about the safety DC caps. Thanks again WR and will rap with you later.
                Last edited by Maveric Manic; 06-05-2014, 09:33 PM.


                • #9
                  Here are some pics of a Bass rig using a built in Effects loop that is working very well for me.
                  Acoustic 200 Watt Solid State for clean Bass Guitar amplification. Model B200H has very versatile EQ using knobs instead of Graphic EQ slider controls. You can play Electric Guitar also for clean sound through this rig. The (2) Effects are the Vintage Alesis NanoCompressor (top) and the dbx 463x Noise Gate (bottom).