The Four Knob Rat
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Big Throbbing FKR Update, 10
December 2007
For months and
months now, the FKR production
line has been silent, a sorry
casualty of two factors: a) they
are a bitch to build because of
my crazy design, and b) I don't
have the time free in evenings
and weekends to build the
things. However....
The Much Simplified FKR Mark
III
I have just
finished the redesign of the FKR
to make it much easier to build.
The new version is identical to
the previous one except that I
have replaced the two PCBs with
one, and changed that big ass
collection of toggle switches to
rotary switches. "Well what does
that mean to me Beavis?" I hear
you saying....
It means two
things. First, I'm one step
closer to having the FKR in a
state where I could send it off
to a production company. Which
means that there is actually a
chance in heck that these may
actually see the light of day.
Second, the new
Mark III offers a bit more
versatility. The previous
version only offered LED or
MOSFET clipping in the overdrive
stage. Since I've built to
identical clipping stages into
the new model, that means that
stock symmetrical and
asymmetrical clipping are now
available in the overdrive stage
in addition to the hard-clipping
stage. In a nutshell,
there are now more options in
the same box. Plus the rotary
switches with knobs should be a
bit more road-hardy than the
toggles found on previous
versions.
So, I'm
continuing to hack away at this
beast. I hope to have some good
news to post in January
regarding availability. Until
then, if you've been waiting for
a FKR, thanks for your patience!
If it is any consolation, I own
neither a FKR Mark I or a FKR
Mark II at this point. They were
cajoled off my board months ago.
So believe me when I say: I
can't wait to have these monster
boxes see the light of day.
Back to your
regularly scheduled FKR page... |
The Proco Rat is probably one of my all-time favorite
pedals. Nothing comes close, as far as I’m concerned, to
the raunchy irreverent growl that comes out of that box.
A few months ago I built a Rat clone and started
studying the circuit. What I discovered is that the Rat
topology is an ideal laboratory for mods. It uses a
cheap nasty LM308 opamp with a pathetic slew rate, and
bog-standard 1n914 diodes that clip the output. So why
does it sound so good? I think a combination of things
like the interesting feedback loop to ground and the
fact that the LM308 adds a lot of its own clipping.
I’ve long wanted to try and create a sort of
distortion pedal “synthesizer.” No single pedal meets
all of my needs. Wouldn’t it be swell if I had a
distortion stompbox that I could flip switches and turn
knobs so as to create a whole array of sounds. And if
the Rat is a great lab for mods, maybe that’s a good
place to start? Indeed! That how the Four Knob Rat (FKR)
started.
The FKR started out as a clone of the Rat, but I
deleted the input buffer stage and added true-bypass
switching. From there, I’ve moved the stock 2-diode
clipping components off to a separate circuit stage that
allows switching between:
- Stock: 2 x 1N914 diodes
- Stock/Asymmetric Clipping: 1 x 1N914 + 2 1N34
Germanium diodes
- LED: 2x red LEDs for much more intense clipping
(incidentally, this is the difference between the
Rat and the Turbo Rat)
- MOSFET: 2xIRF520 MOSFETS + 1N34A Germanium
diodes for boutique clipping as found in various
pedals including the Fulltone OCD and Hermida
Mosferatu.
Additionally, I implemented the
Ruetz Mod whereby you swap one if the fixed
resistors in the feedback loop with a pot. This allows
you to mellow out the Rat—you can go from slightly dirty
boost, to mild overdrive to full out face-shredding tone
pownage.
Finally, I added an additional clipping section to
the feedback loop of the LM308 opamp. This clipping
section allows the choice between no loop clipping, red
LED, or MOSFET clipping. So in essence you have a new
section that is more like a Tube Screamer overdrive. And
you can have it turned on at the same time as you have
any of the other clipping mods turned out. Outrageous!
The FKR Mark II
Currently, I'm producing the FKR Mark II.
This adds a few interesting things to the
original Mark I. First it adds a switch to
put the Ruetz mod (called Lube on the FKR)
in or out of circuit. This makes it easy to
switch between stock and modded Rat.
Additionally, there is an effects loop which
may or may not make it into full production.
FKR Family Foto Fun
The Mark One was the first production
version. Here are some photos of various
pedals and batches.
The First Production Batch
Ready to Ship
That's a Lot of Wires
Here's the Standard Knob Rat--A straight
clone with true bypass in a small box.
This one now belongs to JJ Pistols of Austin
Texas.
SKR Guts
FKR Tech Stuff
If you want to see more pictures and
words about earlier versions of the FKR and
the development process, including
schematics,
go
here.
Special Thanks To
There is no way I would have come up with the FKR
without the huge pool of wisdom and ideas out there in
the interwebs. Jack Orman's awesome article on the Rat
was good starting place. The folks at DIYStompboxes also
had a big impact on the design with their ideas and
contributions. And thanks to Philip Ruetz of
http://www.diyguitaramp.com/index.html for his
awesome pot mod.
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