FET Input Board

New, by request! We now offer our FET Input board for installation into your own ODS style builds, or for an extra gain boost in the front end of any tube preamp. We offer several options:

– Bare PCB only, for those who want to add their own components
– Unassembled board with all components
– Assembled board with all components
– Assembled board with components, except the JFET (for those who want to provide their own)
– Full installation kit that includes the assembled board, wire, plastic mounting standoffs, and a TRS Cliff jack to provide a dedicated FET input.

The board will function per schematic in any tube preamp that can supply 300-350VDC. To use a lower voltage B+ node, the dropping supply resistor will need to be lowered in value. In place of the now-hard-to-get NTE452, we’ve opted to use the PN4392 JFET (datasheet here). For those who want to experiment with other FETs, we’ve also added a unique terminal arrangement, so that FETs with pin-outs of Gate-Drain-Source, or Gate-Source-Drain can easily be used. Finally, the power supply filter capacitor mounting has terminals to facilitate either a radial or axial electrolytic.

Please note, due to the wide variety of possible applications and styles of amplifier, we do not offer installation instructions. There are many available resources online as reference. If you need assistance in installing, please contact a qualified amplifier tech.

Q: “How does this compare to your SuperFET Jack?” A: Gain results are similar. The main differences: 1. SuperFET is easier to install, and works with a wider range of voltages. 2. The FET Input Board is derived directly from Dumble’s board, so it’s better implemented in Dumble-style amps. 3. SuperFET utilizes the high-voltage LND150 MOSFET as the active gain device, whereas the FET Input Board uses a low-voltage JFET (we use PN4392). Both have trim pots to adjust sensitivity/gain.

DISCLAIMER: Please review your amp and/or schematic to determine if this mod kit can be installed before ordering. User assumes all risks. Performing these modifications WILL void the warranty on your amplifier. Working on amps can be deadly if you do not take proper precautions. Follow the instructions very carefully. Basic soldering skills 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. 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


4 in stock (can be backordered)


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