Build name: Red Devil
This is actually my third attempt to build this pedal. The first time I tried I messed up PCB while etching. The second time I build it on perfboard with some cheapo pots. It was waiting for an enclosure for a while and I lost interest in it. Over a year later I got etched PCB from John Lyons (a fellow DIYstompboxes.com forum member) just because it looked cool in blue. Another year or so later I finally got some time to finish the pedal. Board I got is made using the improved Dr. Boogey circuit from Gaussmarkov’s PCB layout. Changes from the original design are Miller capacitors added from gate to source, tone stack scaled by 10 and maybe few minor changes.
Before I go into details, a few words on FET emulators in general: the idea behind a FET emulator is to take the preamp of a famous amplifier and replace each tube triode with a FET. Using FETs instead of tubes has a couple of advantages: FETs are waaaay cheaper than tubes and FETs work with only 9V while tubes use at least 250V (in this case Mesa runs preamp tubes at voltages around 400V!!!). Next, FETs are much smaller so you can pack whole preamp into medium sized stompbox and use it as distortion/ovedrive pedal. Finally, unlike tubes, FETs won’t wear out. On the other hand, some argue that FETs don’t sound as good as the real thing. Their clipping characteristics are not the same. In my opinion this amplifier is a perfect candidate for FET emulation because it’s not very smooth sounding amps. Solid state devices tend to clip harsher than tubes.
Below is a typical tube gain stage “translated” into a FET gain stage. To bias a FET plate resistor should be set to make a voltage on the drain a half of power supply voltage, in our case 4.5V. For this purpose 100K trimmer is often used instead of a fixed resistor because it lets you dial in voltage easily and play with it to find the sweet spot. On the other hand, trimmers reportedly induce noise, so I put a socket instead, used a pot to tune the voltage and then replace it with a fixed resistor. The second difference between tube stage and FET stage is a capacitor between gate and source which emulates Miller capacitance found in tubes. All other components remain the same, but now high-voltage components are not required because we work with 9V only.
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Click on a photo to see more details
All FETs are J201 biased as close to 1/2 of B+ as possible with resistor values I had around. There are only two changes I made to the circuit: 47uF filter caps instead of 100uF (I only had those) and 2K pot for Middle instead of 2.5K which was hard to find. I reckon that even 47uF is more filtering than needed and I’ll play this pedal mostly scooped anyways, so having a bit less middle range doesn’t matter at all.
And the result: AWESOME!!! It turned out to be better than I hoped for. It’s very quiet noise-wise and sounds great. I can nail Dream Theater tones rather well, which is what I planned to do with this pedal. It has more much gain than I really need, I don’t expect to crank it above 50% at all.