King of the Brits (KOTB)
Most JFET amp simulator pedals are cool. Not necessarily because they sound like real tube amps, but rather because they can sound pretty good in their own right. A friend of mine needed a Marshall-sounding circuit, so I looked around the web and picked K.O.T.B. (stands for “King of the Brits”). It aims at hot-rodded Marshall Plexi kind of tone – crunchy, raw, saturated.
I built the pedal for a friend who doesn’t intend to use it as a stompbox, so I hosted it in a nice Hammond BB-sized plastic enclosure and used a DPDT toggle bypass switch which is easier to flick by hand. Also, one of the requests was to include guitar-style volume and tone controls separately from the effect because the guitar doesn’t have any controls on it. To prevent EM noise, I shielded the whole pedal with copper foil normally used to shield guitar cavities. I gave up using home-made PCBs and similar vero/strip boards altogether, so I got a professionally made PCB from Guitar PCB Shop. Professional PCBs are not that expensive and are much more reliable than home-brew boards.
To speed up the wiring, I printed out control layout, glued it on a piece of thick cardboard and punched out all holes with a pen. That gives you much more freedom to work than than working in the tight space of the enclosure. I always liked the neat look of ribbon cables to connect the pots, so I got a 10×0.25mm ribbon cable and split it into 3 lead ribbons.
How it Turned Out?
I was lucky (or experienced enough? :)) to get it up and running form the first try. Just needed to go through a couple of J201s and find the ones that suit the circuit best, as there is no way to adjust the bias with trimmers. I was aiming for roughly 4.5V on the drain (half of B+) for Q1, Q2, Q3, Q5, about 4.5V on the source for Q4 and about 7V on the drain of Q6 (because of a much smaller drain resistor). KOTB has surprisingly high amounts of gain and compression, even with gain pot set at the lowest. So much that I realized it was a great idea to have input volume and tone controls to tame the gain if needed.
It seemed a bit redundant to have separate Master and Level controls, but after playing with the pedal, having both actually makes a lot of sense. Cranking Master brings out even more distortion from the last two gain stages, simulating what happens in a power amp of a real tube amp maxed out. You can also do the reverse – set Master low to reduce distortion and crank the Level up to boost the output without distorting the sound further. Very cool.
Voltage Reference (taken from GuitarPCB.com forum)
Qx: D/S/G – measured in V
Click on a thumbnail to play the video on YouTube.