New/First Design: JubileeDrive

[rockhorst]

So after getting to grips with JFETS (thanks PRR), I've been breadboarding JFET stages the past week. I've long wanted to do a Marshall Silver Jubilee (2550) to JFET conversion. Also, a friend of mine keeps subtly mentioning the Wampler Plexi-Drive everytime I see him (he bought a Plextortion) so I decided to see if I could fuse the two. I took the 3 stage Plexi-Drive as a starting point, changed some values and added a clipping section, inspired by the Silver Jubilee. I'm quite happy with it so far, but might tweak a little more. Here is my very first pedal design, the JubileeDrive 1.0:



Schematic was made with CircuitLab, which I used to run some simulations for part values. Power filtering not shown. I started out with three 2N5457s but I like the J201s better (a bit warmer). For now, I've kept the 2N5457 in the first stage. R3, R7 and R12 may have to be adjusted for different FETs, I haven't optimized it yet, but values are pretty close to those in the PlexiDrive. I was able to swap 2N5457 and J201 around without additional biasing.

Sw1 enhances the bass response. If you want to have a crisper sound lower C4 by about half. Clipping LEDs were 8mm green LEDs (the were in the neighbourhood). Sw2 smoothes the clipping. For a crisper sound with the switch open remove/lower C8. Sw1 and Sw2 go together nicely on a 2PDT as a switchable voicing.

Short Soundclip Here
Little rhythm part on the neck pickup of a Fender CS Strat. Amp was a DIY 18W which is quite dark sounding. Recorded with a Sony PCM-M10 (great little recorder!) at about 1 meter from a 2x12 Mesa cabinet. The riff was recorded with a Boomerang Rang III looper, looped 3 times:

• Clean
• Sw1 open Sw2 closed
• Sw1 closed Sw2 closed

(Sorry, no clip with S2 open)

Tone and Gain at about 2 o'clock, level matched to clean sound (barely open, 25% or so, so there's plenty of volume left).

I've kept the original PlexiDrive tone control, but I think I might swap it for something a bit more exciting. It works well enough though.

Please treat her with respect. I'd love to hear some comments and/or suggestions.

[oh7hhi]

Looks and sounds interesting!

How much gain does it have?
Is C12 really 1uF? Seems quite large, corner frequency would be 10,6Hz. Also, I guess volume pot is 100kOhms - not 100Ohms...

[rockhorst]

Well spotted, that's a typo! Should be 10nF, so 100x smaller
Haven't actually measured gain, but it's plenty, barely opened up the level pot. First gain stage could be a bit hotter, currently you could just put the gain pot at max and still be happy (or a bit short). I recall that simulated, a 100mV input went up to 3.5V output. Compared with audio testing, it seems FETs aren't hitting the power rails very hard, but slightly distort and the diodes do the rest (but subtle). I tried it with just the LEDs and it sounded gainier (ie more distorted) but less pleasing.

I think I'll lower the source resistors to 1k and try all j201, for a bit more agressive sound. Also, I imagine it could really soar with a tube screamer hitting it in the face. Sadly, I have to wait a couple of days before I have the time for more experimention, that's why I posted it now, see what a few days of simmering on the forum might bring up.