SuperFET Jack

The Granger SuperFET Jack is a high gain, MOSFET driven preamp gain stage that operates at high tube voltages. The entire circuit is mounted on a small PCB (1.625″ x 0.875″) that includes the jack. The level is pre-selectable via internal trim pot that is accessed directly through the jack opening, adjustable from nothing, to “over the top”, and has more gain then a single 12AX7 triode stage. The SuperFET Jack stage is wired in series with the other standard gain preamp stages so it can really push them. Analog, tube-like distortion and soft clipping for a great clean boost-to-overdriven tone. Adding the SuperFET Jack can push just about any low gain amp into fabulous overdrive. High sensitivity allows for lower gain by rolling off the guitar volume, just like a tube stage.

The SuperFET Jack has provisions to be wired with a bypass switch (optional, see below), a separate relay for footswitching (available as a separate product), or by simply wiring it direct (via on-board jumper) and bypassing it by plugging into another normal input jack. Wiring is as simple as removing one of your amp’s jacks, mounting it, and running 3 wires: B+ (tapping off the V1 or V2 plate resistor), Ground, and Signal Out (the wire that is connected to the V1 grid at pin 2 or 7). Three sections of 22Ga 600V PVC Topcoat wire are included (Red, Black, Green)
  • Gain level adjustable through the 1/4″ jack opening
  • Works on nearly all tube amps
  • Metal or Plastic Cliff Jack Nut (choose below)
  • Bypass Switching (choose below)
  • Works from 100 to 350Vdc supply voltage
  • Simple 3 wire installation
  • Easily added to any vintage amp
  • Low noise
  • Gain up to 17dB

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.

Beginning in 2018, instructions are now online for your convenience. 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, and your order history. Please add to your email whitelist to ensure you receive our emails.

DIFFICULTY LEVEL: INTERMEDIATE [Follow safety guidelines for working with electricity. Basic 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. Measure and connect high and low voltage AC and DC. Understand basic working of vacuum tubes. Use all functions of a multimeter. Understand Ohms law and apply to basic circuits. Work with basic hand and power tools.]

Pricing and Options


45 in stock (can be backordered)

Jack Type * 

Bypass Switch *