Friday, 6 September 2013

Plexi-Drive with Trimmers

Yup. Most of you who know me know that i've had more than enough trouble with JFET biasing on some fixed drain resistor designs... This one also has the input cap and pulldown resistor at the input. Already verified by me. I found the best gain to be with drains at ~5V, but now you can tweak them to the voltage you want. Used 20K pot for tone. 50K should be fine too.


108 comments:

  1. Anyone know a good source for J201's?

    ReplyDelete
    Replies
    1. Tayda has them for $0.38 a piece. So 5 would be $1.90 plus shipping.

      Delete
    2. Tayda has $5 minimum on orders..

      You should check eBay too. Sadly the good deals are gone. You could score 100 piece bag in march for ~$15 but not anymore.
      +m

      Delete
    3. The ones Tayda sells are chinese fakes! Try Smallbear. They should still have some real ones.

      Delete
    4. This peaked my interest.. How can you tell that they are fakes?
      +m

      Delete
    5. I don't think they're fake, but I believe the batch Tayda has are really out of spec, as they've never biased correctly in any build I've ever used them with.

      Delete
    6. Nah. I got a couple hundred from Mouser of the Fairchilds, but there are obsolete now and I'm not selling my stash. Sorry.

      http://www.mouser.com/ProductDetail/Fairchild-Semiconductor/J201/?qs=sGAEpiMZZMvAvBNgSS9LqkIvdxYjVaKX

      Delete
  2. Made 2 plexis before, no joy, 40 j201s, no joy. Now you've answered my prayers! Cheers mate, can't wait to give this another go.

    ReplyDelete
  3. Made 2 plexis before, no joy, 40 j201s, no joy. Now you've answered my prayers! Cheers mate, can't wait to give this another go.

    ReplyDelete
  4. I used J113's and Im not getting a ton of gain. I can hear the Marshall-like quality to the tone but its more of a light light OD right now. Guess I need to get some 201's.

    ReplyDelete
    Replies
    1. For some reason, my builds with J201s are not as distortionish as i remembered this would be either. Those are good though..
      +m

      Delete
  5. Ahh this is great! I tried Plexi-drive before and I could never get it to sound right. The plexi-drive is going to be back in the build list now for sure! Thanks alot!

    ReplyDelete
  6. Thanks Mirosol for this, unfortunately I already built this one without the trimmers but now I might just go ahead and change that.
    Why change the 1N4001 for the 1N5817? any difference in practice?

    ReplyDelete
    Replies
    1. The polarity protection here is in series instead of original parallel configuration. 1N5817 is used because of the way lower voltage drop when compared to 1N400*. Original parallel configuration will burn/explode the diode when wrong power supply is used. This however, won't break anything - it just won't work with wrong polarity. The con to series polarity protection is the voltage drop, but with 1N5817 the drop will stay so low that it won't affect the operation in any way.

      BTW. I think i'm going to build a 18V voltage doubler for mine before boxing..
      +m

      Delete
    2. This comment has been removed by the author.

      Delete
    3. This comment has been removed by the author.

      Delete
    4. This comment has been removed by the author.

      Delete
    5. What do you expect the 18V would give? more headroom or better crunch? I might just put in a DPDT to choose between 9V and 18V using the charge pump which worked so well for the Mu Tron III and the ROG Supreaux Duex.

      Delete
    6. I don't expect anything. I know it gives out more headroom and the overall sound is a bit harder/colder/harsher due to that increase in headroom. Sort of like the highs of Klon.

      Finished two with voltage doubler today.

      Why i use the pumps? Because i can.
      +m

      Delete
  7. Great! I built the original one without trimmers and was a pain in the ass to bias. With this board it has to be a pretty easy build. The only problem is how to fit it in a 1590A :P

    ReplyDelete
    Replies
    1. Just finished this build.... 30 seconds to bias the jfets :) really wonderful sound without wasting hours with the J201s :)

      As I supposed with the previous build, the jfets of the batch I purchased time ago, did bias at about 4,8v with a resistance of ~5k, but this time I had not to try a hundred values of resistor.

      Finally I'll be able to offer this pedal at a reasonable price, it gets built in a couple of hours instead 20 :P

      Great layout, Miro!

      Delete
    2. So your trimmers where set at about 5k to get the right bias?
      What voltages did you end up with?

      Delete
    3. Q1 D 5,06v S 0.54v G 0v
      Q2 D 5.02v S 0.48v G 0v
      Q3 D 5.40v S 0.34v G 0v

      Delete
    4. 1590A friendly (as it should be :P) Hope you like it:

      http://imageshack.us/a/img713/3943/f3jv.jpg

      Delete
  8. I must have gotten extremely lucky with my Plexi-drive. I used Tayda's suspect "probably not actually J201s" and it sounded exactly like the demo videos with the first two random ones I dropped in. Never went back and checked the voltage, maybe I'll do that at some point today.

    ReplyDelete
  9. Awesome , been planning to build some wampler amp pedals , building paisley drive and ecstasy next week and if they work out I'll be trying this one and the black 65 , a layout with trimmers for that one too would be awesome.great site:-)

    ReplyDelete
    Replies
    1. + 1 for that "Black 65" with trimmers!

      Delete
    2. Biasing without trimmers is pretty easy and fast actually.
      You just have to socket the drain resistors and try different values. If bias is too high then increase the value and vice versa.
      It took me 5 minutes for the Black 65 to have the 4 drains at +5V +/- 0.2

      The plexi drive was for my to do list for a while, thanks for this great layout Miro

      Delete
    3. Finished it this morning before work, I just had 5 minutes to test it at bedroom level but really like what i heard !

      Delete
  10. Hey everyone, I posted in open chat with no response. Any noon guides on biasing with the trimmers? Haven't biased before.

    ReplyDelete
    Replies
    1. Measure the voltage on the drain of each transistor while you adjust the trimmers. First adjust them so that the drain voltages measure approx. 5V. From there you can use your ears to fine-tune your settings.

      Delete
    2. Simple enough. Now, if I'm working on a layout without trimmers, would I use a pot to find the desired resistance, measure the resistance, and then replace the pot with the proper resistor?

      Delete
    3. Eric....yes, that technique will work

      Delete
    4. Or you can use trimmers in that case too, just connect them to your build with wires. Once you have the value replace with a fixed resistor of the right value. I usually do that too. If you have pots lying around those are ok too. Trimmers are atually variable resistors too like pots, just pots are more durable and are available in different tapers while trimmers are always linear.

      Delete
  11. The pedal sounds soo much better with the 201's I had J113's and they sounded terrible.

    ReplyDelete
  12. This comment has been removed by the author.

    ReplyDelete
  13. Maybe noob question here...

    Would this design also accept 2n5457's instead of j201 since you can tweak the trimpots anyway ? And if so what drain voltage would you recommend ?

    ReplyDelete
    Replies
    1. nvm, has already been answered on the non trimpot version. 2n545x should work fine

      Delete
  14. Guys, asking here because it uses 25k pots too: do you have a good source for them? Tayda doesn't have them, only 20k pots. Thanks!

    ReplyDelete
    Replies
    1. http://www.taydaelectronics.com/potentiometer-variable-resistors/rotary-potentiometer/linear/b25k-ohm-linear-taper-potentiometer-round-knurled-plastic-shaft-pcb-9mm.html

      I've used 50K for this layout and it works well.
      +m

      Delete
    2. Excellent pots, I have tried them they work grate.

      Delete
    3. Ok, I wasn't clear enough... Tayda indeed has the linear 25k pot in plastic housing, but only linear. I'm looking for the usual metal enclosure pots if possible. But if everything else fails I'll probably stick with these from Tayda. Thanks anyway.

      Delete
    4. They have metal shaft ones too. Those are Alpha, so on the surface they should feel and work exactly the same as 16mm base pots, even though those are with 9mm base. Inside europe you could check out banzai and musikding, but you won't find anything at the same price as tayda.
      +m

      Delete
    5. True, banzai has them for about 1.5x the price :( I'll try how the linear ones feel instead of the log ones, though in this case I'm a bit surprised to have a log one for tone, those are usually linear...

      Delete
  15. Hi all!

    I built this pedal three times already, sounds great, but never seems the original pedal. I set all of drain voltages to 4,8V, that’s OK. It sounds like the original pedal but with much less gain.. If i set gain pot to 12′o clock, the sound is still clean, not crunch as we hear at the original..
    Does your pedal works so? or have you any idea how can i increase the gain?
    Thanks!

    ReplyDelete
    Replies
    1. Try tuning the trimmers by ear. The voltage may not tell you if it's biased for maximum gain.
      +m

      Delete
    2. As miro said, try to set the trimmers by ear. Also after checking the original's inside (only on pictures from the web) it seems that one electrolytic cap is missing. Most probably from the source of one of the stages Q1 or Q3 to ground. I tried both, adding a 22u-47u cap from the source of Q3 to ground gave me the best result. However it is only possible on this layout if you have some components crossed above each other. You can relatively easily add a similar cap (22u-47u) from the source of Q1 to ground, and that will also increase the gain, but not that much. If you want you can try adding both caps, but to my ears that was a bit too much. Someone from a Hungarian forum also reported that adding a 47u cap from source of Q1 to ground plus lowering all three source resistors from 1k to 680R or even to 330R gives a good result too.

      Delete
    3. Tweaking the source resistors may be a good idea.

      Only bypass cap shown on the schematic is the 47µ from Q2 source to ground... There may be two electrolytic filter caps on some original units.
      +m

      Delete
    4. Check the original demo by PGS I think, there are some shots from the guts. Here is one picture (taken from that demo): http://members.upc.hu/dudas.csaba5/original_pcb.jpg
      I assume the fets are Q1 to Q3 from the upper right corner to the left at the bottom. Now if you check the upper right fet, there is an electrolytic above it. The biggest one on the left is the 470u filter cap (which is substituted with a 100u in your layout), the one in the middle is the 47u source cap on Q2 and I think the upper right one is a source cap on Q1 or Q3. From the size I'd say it is 10u or 22u. I could imagine that Brian changed the schematic a little, say a v2 maybe. Or maybe I'm completely wrong, but anyhow adding a source cap to Q3 and/or Q1 definitely helps.

      Delete
  16. Hi all. Trying to get into this whole vero thing and built this up tonight. It works but sounds really fizzy. It biased fine, and the resistors are correct. Any suggestions where to look?

    ReplyDelete
  17. I see FSB has the Velvet Fuzz schematic dissected so am thinking about using this as the base of that pedal and see how it comes out... Great Layout mirosol and hopefully a Velvet Fuzz will come up soon that fits inside at least a 125b enclosure :)

    ReplyDelete
  18. Mine doesn´t sound right. Mostly clean, only a little bit of distortion. Biased at 4,8V...

    ReplyDelete
  19. Wrong resistor, damn. Now it sounds as it should! But it´s too clean!

    ReplyDelete
    Replies
    1. Try to bias them by ear and with gain maxed.
      +m

      Delete
    2. will do, hope it´s hoter then...

      Delete
  20. Did this and it sounds better now. But the trimmer from Q3 (in the lower right corner?) doesn´t change anything when trimming. There must be something wrong, have to check this. But perhaps I need a booster. Which one to choose, must be a linear one, want that sound but more distortion. Or I try the cap from Q3 to ground...

    ReplyDelete
  21. Just finished this one. Sounds awesome. Biased all the trannies @ 4.5V. One the best OD's I've ever played with I'd say. I guess I'll just leave the "bass" switch engaged all time tho. Really liking the fatness. Gain is ok, probably just like the demos (I haven't checked them in a while)! It can do low gain quite well, up to a medium, compressed, plexi-esque (duh) OD. Not something you'd play metal with but for Classic Rock it's a killer. Thanks, Miro!

    ReplyDelete
    Replies
    1. why not make the "bass" as standard and test some other caps to switch to?
      maybe something that gives nice sound with high volume on amp with rolled back volume on guitar and neck pu?

      i deffo gonna build this one!! cheers for the trimmers!! they sure give us more hope to score with our builds :)

      gotta love this place :P

      Delete
    2. Yeah, the "bassy setting" might as well be my "stock" setting. I wouldn't add anything more to it though, the pedal sounds great like it is, maybe use a footswitch to change from bass mode (lead) to the actual "stock mode" (for rhythm). ;)

      Delete
  22. will we win anything by matching the fets for this circuit?

    ReplyDelete
    Replies
    1. Shouldn't really make a difference, the trimmers will allow you to bias unmatched fets as easily as matched. It's really things like phasers where matching is much more important.

      Delete
    2. thanks!

      i am almost done with this PlexiDrive, but lacks the 1N5817 :(
      any substitutions for that one?
      i think i'll jump over to the Fuzz Master until stuff arrives :)

      i can't see you guys making any money from this site!??
      you all should!! this place rocks! :P

      get some affiliate with Tayda or something and i'll support it :)

      Delete
    3. Try a 1N4001 or really any silicon diode will do, even a 1N4148. The site will never make anyone a millionaire but if you click on ad links or buy a kit from bitsbox or the soon to be announced US kit seller then we will get a contribution for that :o)

      Delete
  23. This comment has been removed by the author.

    ReplyDelete
  24. I decided to play with the trimmers a bit. Biased Q2 @ 3V or so. Sounds killer with the extra gain. This circuit really impressed me. Might make the bass switch a footswichable feature one day. Cheers!

    ReplyDelete
  25. i got only two J201 that bias 4,5V at 15K, third best biased 3,56V at 15K so i used the trimmer to get it up and gave it the Q1 position. many of my J201's needed 4,7K to get to 4,5V and that gives not a clone of the plexi-drive. in next Tayda order i order some more J201's. the pedal sounds VERY much like a JTM-45, it beefed up my Blues DeVille 410 on clean, so... it's gonna get boxed soon, killer sound :P

    next pedal to be built is MXR Distortion + :)

    thanks all at tagboardeffects, cool project!!

    ReplyDelete
    Replies
    1. It's still a clone no matter the bias settings. Don't let Mr. Wampler fool you ;)

      Delete
  26. Is it important to use matched 201's for this. What's everyone else doing?

    ReplyDelete
    Replies
    1. No. The trimmers are there to disable the need for a matched set. Each JFET can be biased separately with the trimmers.
      +m

      Delete
  27. Wasn’t sure where the best place for pics are, Ive built a multi with 5 vero circuits from layouts from here with a relay switching mechanism from TH Customs with a 1 of X function. 1 of the circuits is the Plexi so I thought I’d post it in this thread. The others are MXR Dist+, TS9, catalinbread Rah and midifi fuzz gun
    .
    Anyway, heres some pics:

    https://www.dropbox.com/s/7d0lh6ou9m1nk1i/IMG_6029.JPG
    https://www.dropbox.com/s/kj892yoypwgkdxq/IMG_6036.JPG
    https://www.dropbox.com/s/ezppt8fhokqh7wz/IMG_6037.JPG
    https://www.dropbox.com/s/z35f9lxx821xoqj/IMG_6039.JPG
    https://www.dropbox.com/s/lnnchkh5dikfym7/IMG_6040.JPG
    https://www.dropbox.com/s/gxxsm8mlbp25k6y/IMG_6044.JPG
    https://www.dropbox.com/s/cayge6pxft1zei0/IMG_6046.JPG
    https://www.dropbox.com/s/tknok2p5cmjd2jz/IMG_6047.JPG
    https://www.dropbox.com/s/f0xxu2iko0vzqr8/IMG_6050.JPG

    ReplyDelete
    Replies
    1. INSANE !!!!! man.....in a good way...!!!

      Delete
    2. Fantastic!
      How do you mount those LEDs without a bezel? Glue?

      Delete
  28. I have big crackling or "horn" noise here. I tried to bias by ear, 4 - 4,5 - 4,8 - 5. I cant make this one sound like it should. I always have big noise. Please help me... i'm working by 2 days. I'm goin' mad. I double and triple checked everything!

    ReplyDelete
    Replies
    1. It could be anything based on your description, we always need all pin voltages so we can see if something looks amiss.

      Delete
    2. Ok... I have this readings from left to right upper couple first, then the lower one:

      D 4.50 S 0.32 G 0.00
      D 4.56 S 0.37 G 0.01
      D 4.52 S 0.27 G 0.00

      https://fbcdn-sphotos-g-a.akamaihd.net/hphotos-ak-xpa1/t31.0-8/10620065_778718858837808_8846163737579090255_o.jpg

      Delete
    3. THE ABSURD AND INFAMOUS thing is that after i took these readings for you I closed the pedal, plugged it and it played perfect. After a minute of playing it began to crackle as crazy. Now if I open it i'm quite sure i'll find all the readings totally different from before.

      Delete
    4. Your voltages look ok and judging by the fact it started working and then stopped again it strongly suggests a bad solder joint or connection somewhere. You're really going to have to go over it with a fine tooth comb, reflow solder if necessary and make sure all the connections are true

      Delete
    5. I spotted something wrong on the switch. I used a SPDT as SPST. Now... when I engage it the voltage raises to about 8 volts. So... i cant get what's wrong. All the solder are good and i have no false contacts.

      Delete
    6. I've given my ideas. If it works intermittently then it has a bad connection somewhere or possibly an intermittent bridge that makes an occasional unwanted connection, but it's something that we can't diagnose for you unless we had the build in front of us.

      Like I said this is something that you have to go over with a fine tooth comb. Sometimes cold joints aren't obvious and so reflowing the solder may be necessary, especially around any connection that doesn't look perfect.

      Delete
  29. I built this circuit and had a fair amount of success - but not complete so I hope people will bear with me on some questions. I thoroughly tested the circuit for layout, cold solder joints (did find two & fixed them) - slivers, etc.

    I am able to bias the J201s to 4.24 v. - But I am getting no gain.I get "tone" and can hear the pots and switch working, but its a clean guitar tone, no gain at all. So, two questions.

    1) I believe some of my J201s came from Tayda which I have heard are ?able. If they are not good, would I still be able to bias them. Since I can bias then, can I assume they are good? Or not?

    2) The 1uF Cap with no polarity. I do not have such a cap. (Ironically I just bought one to make the test box from last week). But I only had one. That box will be done by Monday, I hope. How critical is this cap spec? I tried using an electrolytic, tho I don't know which way to orient the +/-.

    I tried both ways and it makes no difference, still no gain.

    I know when my audio probe is working I make have better luck with this but if anyone can answer my questions now I would appreciate it.

    I also saw some people recommending tying some sources to ground with caps, but in those cases it was when they could not bias a J201. I am not having that problem. There was one guy who said he had the same problem as me - but if he fixed he did not come back to report how it was fixed (something that people should try to do).

    Thanks for any help at all.

    ReplyDelete
    Replies
    1. 4.24V sound a bit low. Try tweaking the trimmers by ear instead of any particular voltage.

      1µ cap - plus side down.
      +m

      Delete
  30. Just got this working a cold solder joint in first 1M R (sometimes you see them at different angles when you turn the board). Found it with an audio probe.

    Great circuit. The tone does not do a lot for me, the switch does far more. Together they give some variation. Varying the bias does more for the tone than anything. Lower V seems to mean more gain & noise, so it is a trade-off in there. But this is an ear-bleeding circuit just like a real plexi - one the best yet I have personally built from here.

    ReplyDelete
  31. By the way, I socketed and just changed the 10n cap (103) to a 47n (473) - and it now works in the manner I would call a tone control. You still get all the highs but also a workable range of tones in between to a very mellow sound.

    ReplyDelete
  32. This comment has been removed by the author.

    ReplyDelete
  33. Does the third lug on the tone control get connected to anything? Thanks

    ReplyDelete
    Replies
    1. It's not connected. If you want, you can chain the tone 2 with tone 3. Result will be exactly the same.
      +m

      Delete
  34. Great Great layout and a great sounding pedal whether you bias the JFETs at 4.5V or 5V.

    Thanks guys,

    http://fuzzquest.blogspot.com/2015/04/rocking-in-plexi-world.html

    ReplyDelete
  35. I had high hopes for this build but sadly mine sounds very clean compared to the demos. Wonder if it's something with the BF245As I used, they worked well in the Marvel Drive but not in this one no matter how much I twinkered those trimmers. Maybe a bad FET..

    ReplyDelete
  36. First I want to thank you Miro for this "handy" layout with trimmers. I made this and sound very nice! I didn't get much time to bias JFETs by ear. But i have one question: The original gain is just fine, but i'd like to get a bit more. Is that possible just changing some components? Cheers from Brazil!

    ReplyDelete
    Replies
    1. There's an aforesaid mod that you can add an electro cap to Q3's source to ground to get more gain, or playing with source resistors.

      Delete
    2. Cool man, it worked! I've tested with several cap values, like 1uF, 10uF, 47uF, etc..., but the result was always the same. I ended up using 47uF and a switch on/off to get the cap connected to ground when i want.

      Delete
    3. This comment has been removed by the author.

      Delete
    4. Great! Does it affect the tonality/frequency response? if so, how?

      Delete
    5. No at all, it just increase gain and volume a little bit, like a clean booster pushing. No squeal, humming or any other annoying problems. Nice and easy mod. ;)

      I tried add another cap from Q1 to ground, but this time the tonality change becoming too bright, and i didn't like it.

      Delete
  37. Tried those tiny MMBJ201s and yeah, they woke this thing up quiet much. The bass boost was too much for me and the lows overwhelmed everything so swapping that 2n2 cap to 1n really helped to get a nice tone there and to hear a bit more mids. Thanks for the tip!
    Any more components to change for getting more mids? Thanks!

    ReplyDelete
  38. Hi there, first of all thanks for this great sight. I'm pretty new at this and this site is invaluable. Built the triple wreck and its awesome.

    This build is not going so smoothly. I have breadboarded the above and biasing j201 is proving to be a challenge. The drains are biasing between 4.8-5.0v however the source voltages are very low all three around .09v

    Why would this be? My understanding is that they should be around .5v.

    Source resistors have been double checked and all are the correct values.

    There is no distortion. I have audio probed the drains of each j201 individually and biased until i heard clipping without the farting but it cleans up by the time the signal reaches the output.

    Any help would be appreciated.

    ReplyDelete
  39. Great layout !!! The best diy pedal I've ever made ... sound much better with trimmers... although i adjust them only by ear (these one never lie) sounds very organic.
    It's the firSt time that I put my clean channel on with a smile.
    It's like it was a marshall that I've never had in my room.

    Thanks

    ReplyDelete
  40. Great layout !!! The best diy pedal I've ever made ... sound much better with trimmers... although i adjust them only by ear (these one never lie) sounds very organic.
    It's the firSt time that I put my clean channel on with a smile.
    It's like it was a marshall that I've never had in my room.

    Thanks

    ReplyDelete