Ultimate FX Loop


Now shipping version 3.1 (red boards) of our best selling kit to date! Our loop features zener diode protection for both MOSFETs, metal film resistors, and a smaller board size. There are now three options for bypass switch configuration, via dropdown below.

The Ultimate FX Loop is our best selling modification kit. Our high voltage, zero-loss, true-bypass effects loop, available in kit form. Our Ultimate Effects Loop works perfectly for most vintage Marshall, and Marshall-style amps, as well as many other tube amps, that have extra mounting space in the chassis, and a B+ supply of 300VDC or greater. The board dimensions are 3.25″L x 1.56″W x .875″H – very small chassis footprint.

Our effects loop is optimized for insertion between the preamp and driver stages. Our low noise, non-clipping, analog effects loop utilizes a discrete, high voltage buffer and a return gain stage. To be the most compatible and easy-to-install effects loop available, we made the mounting requirements simple (4 wires – Power, Ground, Signal In, and Signal Out), and also features internal trim (level) pots that can be accessed directly through the jacks (no chassis removal required to adjust levels) so it doesn’t require a lot of drilling for the controls.

The footprint is small (with 2 or 3 holes for jacks and optional true bypass switch), yet both the send and return levels can be adjusted for nearly any loop requirement. In factory configuration the send stage has 10dB of cut and the return stage has 16dB of gain, for a net gain of 6dB. Low cost stomp boxes work just as well as thousand dollar rackmount effect units. The Send is configured as non-interrupt, so it can be used as a Preamp/Slave Line Out. Jack spacing is 1.5 inches, center to center. The board can be easily mounted component side up (Send jack on right side), or solder/trace side up (Send on left), with power, ground, and signal terminals clearly marked on both sides.

We offer the option to have the PCB-mounted bypass switch soldered to the circuit board (default), PCB-mounted bypass switch, not soldered (requires soldering before installation), a DPDT panel mount switch, if an alternate mounting location is desired, or no switch included, for wiring the loop “always on” (Select from the dropdown below). For builders and techs, we now offer a “board only” option. Please note: Wire is not supplied for the separate panel mount switch, if chosen. 22 or 24 gauge wire is sufficient for wiring a separate mini toggle switch. If installing a switch more than 1-2 inches from the board, shielded wire is recommended.

This is the most versatile effects loop on the market. If you are interested in a custom configuration, just contact us.

NEW! Check out the new installation video to see just how easy it is to install our Ultimate FX Loop.

Board dimensions: 3.25″L x 1.56″W x .875″H
Send, Return jacks spaced: 1.5″
True Bypass switch centered between jacks (default)
Operates from existing B+ supply (275-400 VDC is optimal, usually taken from phase inverter B+ node)
Version 3.02 (red board) currently shipping

Kit includes:
(2) Power wire leads are 18 inch sections of 22Ga TopCoat (pre-tinned) 600V in the following colors: Red, Black
(2) Signal wire leads are 18 inch sections of RG-174 shielded cable (shipped as one 3 ft section, please cut to length as needed).
(1) 6 inch piece 24ga stranded ground wire (for shielding ground connections)
(1) piece heat shrink tubing

To aid in multiple configurations, switch options now include: PCB-mounted bypass switch (default), PCB-mounted bypass switch, not soldered (requires soldering before installation), separate, chassis-mount switch (requires soldering wire leads from board to switch), and no switch, for “always on” configuration, or to use your own switch.

Beginning in 2018, instructions for modification kits are now online for your convenience. Download links are available after purchase. This allows us keep our prices competitive, and helps all of us reduce paper waste. Links to PDF files will be included in the order status email when the order ships, and always available in your order history. Please add grangeramp.com to your email whitelist to ensure you receive our emails.

DISCLAIMER: User assumes all risks. Performing these modifications will most likely void the warranty on your amplifier. Working on amps can be deadly if you do not take proper precautions. Read and follow the diagram or instructions very carefully. Basic soldering skills and basic user-supplied tools are required. If you are at all unsure of your ability to perform these modifications STOP and take your amp to a professional to be modified. You can also send your amp to us for modification. Contact us for pricing.

DIFFICULTY LEVEL: ADVANCED [Follow safety guidelines for working with electricity. Advanced soldering skills and knowledge of working with AC/DC circuits. Identify all terminals of electronic components using data sheets. Read resistor color codes and capacitor values, as well as identify PNP, NPN, JFET, MOSFET semiconductor devices. Measure and connect high and low voltage AC and DC. Understand advanced working of vacuum tubes. Use all functions of a multimeter. Understand Ohms law and apply to basic and advanced circuits. Work with advanced hand and power tools.]

Pricing and Options


5 in stock (can be backordered)

Bypass Toggle Switch *


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