Mad Bean Pedals Green Bean, Быстрый старт

Страница: 7 / 9

7
The Ibanez® Tube Screamer™ is probably the most well known and widely used overdrive in the history of
guitar effects pedals. Vintage TS units can fetch several hundred dollars on the used market while new units
remain cheap and plentiful. The Green Bean is based on the vintage unit, known as the 808, however it
incorporates several mods to increase its flexibility and tonal range.
The 2015 version in a new layout. The two toggle switches have moved to a more convenient location and the
Tone knob has moved to one side.
________________________________________________________________________________________
There are two versions listed from you to choose from . The Stock values give the original TS in all its glory.
The "madbean" version is the one I always build. Here is an explanation of why I choose the different values.
R1 - This pulldown resistor is not needed on the Tube Screamer since it is a buffered bypass (not present in
the Green Bean). In the absence of the buffer bypass, a pulldown decreases the chance for any popping when
the foot-switch is engaged.
R3, R13 - These resistors do two things: set the input impedance on the input and output buffer transistors and
provide current to the transistor bases. The value here is not terribly critical so I use 470k because it's more
common. You could even go up to 1M if you wanted but I probably would not go higher than that (if the
resistance is too high the resistors may not provide adequate current to the transistor base for proper
operation). Conversely, you would not want to use too low of a value for these....there is no reason to go below
470k.
R7 - The value of this resistor sets the minimum gain when the Gain knob is all the way down. I use 47k
because it is a more common value. It will lower the minimum gain slightly but almost imperceptibly.
C1 - Who has a 20n film cap? Not me. 47n also lets a little more bass into in the circuit. 27n is another popular
alternative.
C3 - I use 100pF instead of 51pF because it smoothes out the high frequency range a bit more when the Gain
pot is all the way up. This helps balance out the Tone pot, IMO (the Tone pot at full up can get a little "ice-
pickey" with some guitars).
D1 - D6 - The vintage TS used two back to back silicon diodes for symmetrical clipping. This is fine. It is also
boring. The madbean version instead uses the silicon diodes asymmetrically and gives the option for a
different pair of diodes for symmetrical clipping. I generally use LEDs for the symmetrical clipping, but you can
use anything you like here; silicon, LED, germanium, etc.
Q1, Q2 - Since these two transistors are used as buffers the actual transistor we use is not critical (IOW one
particular type does not impart any magic tone more than another). The only thing that matters here is that they
are reasonably low noise and reliable. So, I pick the more common C-B-E pinout type rather than the 2SC1815
which is E-C-B (note if you use 2SC1815 on the GreenBean you are going to have to twist the transistor leads
around). I generally use either 2N5088/89 or BC550 (these have the opposite pinout from the 2N5088). The
BC550 does seem to be slightly lower noise but that is purely anecdotal.
VOL - Just a personal preference. I like audio taper for the volume control more than the linear type. YMMV.
________________________________________________________________________________________
Other Mods
The Fat switch changes between the stock clipping frequency cap (47n at C4) and a larger value cap (120n at
C5). This increases the bass response of the overdrive by moving the mids emphasis of the Tube Screamer™
clipping from around 720Hz down to approximately 282Hz. This is what could be considered the lower