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The Making of the Fat Rat

Super Rat I bought a ProCo Rat and after some testing I decided that I did not like it too well because it had a harsh edge that did not go well with my playing style. It was either sell the box or modify it to produce a better sound and after some deliberation I had formulated a plan. The idea that I finally settled upon was to revise the circuit to make it conform to my Fat Gnat design as near as possible. The original Fat Gnat circuit was published in RECORDING magazine in the October 2000 issue and is available on the AMZ CD-ROM.

Since there are numerous basic similarities between the two circuits I was able to incorporate most of the elements of the Fat Gnat into the modified ProCo box. Listed below is a summary of the changes that were made to the circuit.

  1. Input impedance was lowered... yes, lowered. This reduces noise and helps smooth out the sound. It also changes the low frequency rolloff to make string noises less apparent but still allows the full guitar range to come through.
  2. A socket was installed so the IC can be easily switched.
  3. The op-amp was changed to a high speed, ultra-low noise chip (NE5534AP). This chip also has the advantage of being able to better drive capacitive loads such as the tone control of the Rat. You could alternately use an LM741, which is a single opamp similar to the sections of the 4558, to see if that provides a better tone, but it does not have the drive of the 5534. A TL071, TL081 or similar opamps can also be tried for slightly different sounds.
  4. The gain setting R-C combination was changed to the Fat Gnat values. This re-defines the treble boost of the gain stage and while similar to that of the original, it provides a little smoother response.
  5. Bias filtering was increased 10x for less hum and noise.
  6. Minimum gain was increased - now the sound does not completely disappear at minimum Distortion settings.
  7. The tone filter was altered to the Fat Gnat design values. This reduces the top end response - but still it covers the full range of hearing. This reduces noise and allows for some additional tonal characteristics on the bass end of the spectrum.
  8. An ultra-brite blue LED was installed to replace the stock red one. I may have to cut the current to the LED because it is nearly blinding with the stock Rat current-limiting resistor!
  9. A germanium diode was added to the clipping section to sweeten the sound and introduce asymmetrical clipping for more even-order harmonics. Typically, even-order harmonics are thought to be be more tube-like in character and also more pleasing to the ear.
These changes made a very pronounced change in the sound of the Rat. After modification the harshness was almost completely gone and the sound was very sweet. Noise was greatly reduced and the "Filter" knob has a more useful range. No drilling of the case or other external mods are required.

Furthermore, when the "Distortion" knob is set to its minimum, the Rat now acts as a clean boost! - It's no longer a one-trick-pony. At moderate gain it has a lot of the sound of a Tube Screamer without the mid-range hump (i.e. flat mids). It has slightly more maximum gain than before but with all of the harshness removed.

The modifed Rat can now do clean boost, blues drive and full tilt boogie! What a change in sound these mods have made... it sounds like a totally different effect!

BTW, do not be tempted to run the original Rat at voltages higher than the stock 9v. The power supply filter cap is only rated for 16v and higher values could damage the unit.

NOTE: I am modifying the Muff-Rat eBook to include a schematic that shows all of these changes in detail so that you can make your own Fat Rat.

Update Aug 2017: My Kindle ebook with new information on the Rat and the mods is now available from Amazon:

Guitar FX Mods ebook


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