New to stompboxes, and a question about JFET amp sims...

[JonMan94]

Hello, my name is Jon and well besides being a musician, I'm studying to be an industrial engineer and I also do quite abit of gaming... but back on topic here, I've been getting very interested with building my own pedals, and at my university, I have access to a open laboratory that has 3D-printing and circuit board printing capabilities. My electronics experience is decent, for I have a good understanding of how guitar harness wiring works and I have done wiring jobs on almost all of my guitars. Nowadays, I'm looking for a bigger challenge, mainly in the form of taking the schematics for a DIY Mesa MkII C+ Preamp (Bancika's Preamp) and turn that into a JFET-based pedal similar to the Dr. Boogie, but in addition to the preamp it would also have the five band EQ that could possibly be switch on and off with the ability to place the EQ before or after the entire preamp section (heck, maybe make it so it can be turned on independently from the preamp section so people can have that Mesa EQ tone.)

Before that however, there's a question that I haven't been able to find a clear and concise answer for: How does one interchange the tubes in a circuit for a JFET? I've read up on what JFETs are and have browsed the forum, searching for anything that might help, but I'm still rather confused by the concept... I understand how a JFET can potentially replace a tube in a circuit, but when it comes to applying concept to design, what exactly gets replaced?

Realistically though, for my first project, I want to do a Klone and design a signal booster pedal that has a switch that replicates the PRS 'Sweet Switch' so any guitar plugged into it can get that sound instead of having to put it inside the guitar.

those two seem simple enough with the resources I have access to. I hope to learn more about DIY building and to be able to not only make my own, but possibly share or even sell my own designs if I stir up enough interest with them.

[mth5044]

There was quite the craze for replacing tubes with FETs in pedals for a long time. Seems like it's on its tail end at this point. While it does give you a sound, it doesn't always give you the sound of the amp you wanted without substantial circuit changes. Run Off Groove did a lot of these types of pedals to excellent results, but they have begun to make circuits that use bits and pieces to get the sound of the whole amp, rather than just the preamp, which IMO sound a lot better than previous attempts when considering the sound of the amp. I don't mean the previous circuits sounded bad, in fact I have used quite a few, but when considering the ultimate goal was the make a pedal that sounded like an amp, their new approach blows the old way out of the water. See their Azabache (fender), Brittania (Vox) and Thunderbird (Marshall) for examples. You can look at all their other amp in a box pedals for the tube to JFET type circuits.

They have a wonderful write up about the Fetzer Valve which might help what you originally posted about before I wrote that story

http://www.runoffgroove.com/fetzervalve.html

[Transmogrifox]

+1 For runoffgroove fetzer valve.  Good basic run-down for the considerations for replacing tubes with FETs.
http://www.runoffgroove.com/fetzervalve.html

One thing not considered in that article is the dynamic nonlinear effect of the cathode bypass capacitor.  When you are operating this circuit in an approximately linear type of amplifier (booster, amp with a little "compression") the effect of this capacitor likely has a small effect on the sound.

Once you go into clipping (really nonlinear) then this cap turns into the storage tank on something that begins to look like a 1/2 wave rectifier.  This causes a dynamic clipping duty cycle modulation (basically changes the device on threshold).  I like to think this effect is really a more important attribute in the expressive type of break-up that occurs when you start pushing an amp over the edge.  I would consider it more important than attempting to match the x^3/2 power law. 

Probably a good amp emulation would use the Fetzer as presented at the input stage, and follow it by stages with bypass cap.  Runoffgroove Brittannia is a good example of this, although I don't know if that is the effect they had in mind.  The low bias current on that stage seems to point in that direction.

[J0K3RX]

I don't think fet emulation of tube amps is on it's tail end...? If it is, then it shouldn't be! The AMT Legend series 2 is a great example of what can be done if the knowledge and skills are there... Those pedals are spot on and they do sound and feel every bit like their sometimes $3000 to $5000 couterparts! The other forum has documented almost everyone of them and I have to say they are brilliantly designed!

There already is a jfet version of the MkII C+ Preamp otherwise known as the "Boogeyman" but I am sure it could be done a little better if somebody put their mind to it... I have to admit I have never built this preamp before but I have seen a few that have an it was not too bad.. Again I think it could be done better and perhaps totally redesigned but for starters it ain't too bad... The 5 band EQ would be interesting...

Can be found here
http://www.4thlevelmedia.com/stompboxes.html

Video of a build

The same guy did a demo of the Black Forest he built over at Tagboardeffects and I have to say it is one of the best demos I have heard yet! Guy's name is Tim, don't know his last name but he is a wiz at this stuff!

[Kipper4]

My recent B52 project was a hoot to do. It sounds awesome too.IMO.
My guy Dave doing a demo

[Cozybuilder]

Nice demo- came away having a good idea of what this pedal is capable of. 

[Kipper4]

Thanks Russ.
Credit to Jok3r for all the help he gave during development of the B52 also. Cheers man your a diamond.

[J0K3RX]

Thanks Russ.
Credit to Jok3r for all the help he gave during development of the B52 also. Cheers man your a diamond.

C'mon man... It was all you bro! I may have threw you a bone here or there but, you totally banged this one out on your own... You should be proud of it man cause it's totally original IMHO and I had no idea where you were going with it.. I really like the changes you made! Top Notch!!!  And another testament to the mighty power of the Field-Effect Transistor!