JTM Plexi Overdrive

[jojokeo]

Another pedal boxed up but this time using "white" decal laser paper.  Big difference with the white in that the colors show up much better and w/ whites too. The box color underneath doesn't affect the graphic colors like the clear stuff does. This graphic may not be the best example as it has a majenta /  darker blood red base color which sort of came out looking brownish against the black enclosure. The black enclosure was a bit of a pain in the ass in that the labels had to be "black outlined" so that a white name would show then cutting it out very closely because of the white paper background. Live and learn, maybe a Brother label maker or = would be better for printing white labels?

The pedal itself is solid and sounds really good complimenting the tubes, embellishing harmonics with sustain and gently entering and exiting very soft clipping just right - very much tube-like. J201 jfets get the job done really well here. I love these little things. All they need is proper biasing like triodes do. It takes being slammed w/ other pedals nicely kicking it up to great singing Santana-ish lead tones and w/out compressing the signal. The boost simply uses a bypass cap across the source as you would a cathode giving the same results for added gain, just right for that solo-punch. It'd be temping to leave this thing on through a set or even a whole night's jam just working the guitar's controls and the boost switch when needed.

[tubenit]

very much tube-like. J201 jfets get the job done really well here. I love these little things. All they need is proper biasing like triodes do. It takes being slammed w/ other pedals nicely kicking it up to great singing Santana-ish lead tones and w/out compressing the signal

That sounds quite impressive! The interior sure looks very neatly done.

With respect, Tubenit

[Geezer]

Very nice!

Do you have a link to the circuit or website the design came from? Or is this your own design?

G

[jojokeo]

I used the plexidrive circuit as the basis for this. I had to change the values on almost everything as J201s are sensitive and not very "tolerant" (bad tolerances on these as you may know?) little critters. But, if you build the fetzervalve jfet tester (or use Tillman's method) it will help you in designing for them properly.  I also made a spreadsheet similar to what's on that site, just in case the site goes down or away in the future. If you build that circuit the way it is, it won't work. Or at least will be a venture in frustration and poor performance unless you're extremely lucky somehow. So the road map is good (circuit in general) but the shrubs, trees, and signs are not (values). It sure helps immensely to have our tube amp building skills, circuit knowledge, and experiences to draw from. Ofcourse I'm biased here but I can humbley & honestly say that if you played & compared the JTM to the plexidrive side by side you'd choose the JTM every time. I think we have an inherent and distinct advantage when it comes to making these things as builders and players both.

*geezer & tubenit: Sorry if it may seem I'm being vague it's just that the jfets have such a wide tolerance that you really need to tailor each one specifically for the circuit they're used in. Once you know the measured pinch-off voltage and saturation current then you can arrive at the values to use for that particular device. Do this for each one you use and you're fine.  You can make a long tail circuit where it's not supposed to be sensitive but you need to generate a neg voltage and extra parts for each device. This isn't necessary when doing it correctly for each one in the first place. There's a lot of info out there on how to do this:

The one I like best is: http://www.runoffgroove.com/fetzervalve.html
others are here:
http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp1.htm
http://www.till.com/articles/GuitarPreamp/
http://www.geofex.com/
http://www.qrp.pops.net/fetbias.asp

[jojokeo]

I took apart my jfet tester to show how easy it is to build and to assist with testing the little buggers. Yeah, you could bread board it but what's the fun in that, and what about your next project? I cut a dip socket down to three lugs w/ a small piece of perf board, use a momentary switch to apply power after inserting the jfet and between settings of the switch. You only apply power quickly to device & test, no need nor do you want a switch really. Testing goes very quickly and no need to hold power button and wait for value to "equalize" - it won't. Take the first reading you see and write it down and proceed onward. I installed the battery connector externally because you need to know exactly what the voltage is as that's the third component needed to go along with the other two readings for Idss and Vp. Also no need to disassemble for replacing it if it's not used for a while or the battery is needed for something else. Hope this helps?
Joe

[zendragon63]

Nice and neat Jojokeo! The FET tester looks way handy--I need one of those. I had way too much time dinking around with a couple of 2N5457s on a similar project. Thanks for sharing. Regards

dennis