49 thoughts on “JCM800

  1. Biasing the Fets – I tried all the voltages posted in comments. What I found was that the best way was to do it by ear. Each pot has a sweets pot – Q5 (VR4) was the first on to do as it created the most hiss and oscillation.
    To do it, I turned the master, volume, and preamp to 10, started with VR4 to get the noise gone and then adjusted each other VR until it sounded right to me.

  2. And you’ll need to bias the drains to thses values or you’ll get farty note decay. These voltages worked on both my builds.

    Q1 – 3.742V
    Q2 – 3.361V
    Q3 – 3.362V
    Q4 – 9.45V ( tied directly to 9V supply )
    Q5 – 4.03V
    Q6 – 3.115V

  3. Connect a 22n – 33n cap from lug 3 of volume pot to ground. This tames the excessive brightness. Sounds like a Marshall now.

  4. Built it. Works fine. I did move mid 1 to ground and bass 1 to mid 3. Mid works as it should. The circuit is very trebly , too much so for my taste. I have to play with the treble all the way down. This has been mentioned in other forums. Working on some mods now to tame the treble….

    Thanks for the layout, Harald

  5. I think my first post got held. It said:

    Hi Harald

    Hope this finds you well. Building since 2010 and have built many of your layouts. Not sure if you’re still answering posts but here goes. I’ve got the board populated and am getting ready to wire up the pots. I noticed that the layout indicates mid 3 to ground. All the other pot connections to ground are lug 1 ( this is what I’m used to for ground connections to pots ). On other schematics, they indicate mid 1 to ground.

    Will connecting lug 3 to ground not make the mid control work in reverse?

    Thanks for all your work here.

  6. …and you would have to change bass 1 to mid 3 if you were going to change mid 1 to ground.

  7. Tom, try searching for “jcm-800 emulator schematic 2004 electric”. It was the second image when I tried just now.

  8. I can’t remember having changed anything dramatically or even at all.
    I suggest building an audio probe (check out the debugging section for more info) as this makes these preamp FET builds much, much easier to build and tune.

  9. I tried this build. Failed multiple times. I get sound past the first set of JFETs but after that it is spotty. Any ideas? Im going to rebuild again and again. And hopefully it will work successfully.

    Another question. Do you change anything in the schematic dramatically or just simple things like the Trimmer and Fet?

  10. A few ideas you might try out:
    – Since you’re seeing +13v I assume you’re running this off a +18v source? Does adjusting the trimmers change the drain voltages at all, but not enough? Maybe switching to 250k or 500k trimmers helps.
    – As someone pointed out, the circuit was designed without an output coupling cap. Perhaps your next effect/amp is introducing a DC bias via the output? Try adding an output coupling cap.
    – Or, as Tim mentions, you could have a short or a misplaced/missing component somewhere.

  11. dale,

    maybe there is a short between your +v and transistors?

    Have you tried an audio probe?

  12. Happy Holidays everyonr.
    I built the jcm800 and am getting ~~13 volts off of all drain legs. When I adjust the trimmers nothing happens on any of them. I get tone when the effect is off and have checked all parts and joints…and suggestions

  13. Hey everyone.

    I built this and it works great. I was just wondering is anyone had any suggestions on tweaking the tone. the high’s and mid’s are pretty rough for my taste, i usually have the mids and high’s all the way down and it almost makes the pedal sound more like a fuzz. any suggestions to swap caps that might increase bass response and tame the high’s and mids.

    Overall it was a fun build though, thanks for they layout and keep up the good work.


  14. Sorry to hear you’ve still got problems, Tim. In light of the latest comments it would probably not hurt to add a film cap between the lug #3 on the volume pot and the circuit board. As for the trimpot, you could try changing it for a higher or lower value; a higher value will give you more of a range but will be harder to dial in, and a lower value will give you less play room but it will be a bit easier to dial. If you’ve got the trimpot maxed and Q3 is still at 8-9V you may have more luck with a 220k trimpot.
    Another option could be to try different JFETS, they can still be the same make and model, but try different individual ones as they vary a lot (why we have those trimpots to begin with). You might be able to find ones that will bias correctly with the existing trimpot. Both Q3 and Q4 might be candidates for experimenting.

  15. This was one of the most problematic builds that I have done.

    I am still having some issues when it comes to biasing the trimpots. I have vr3 set to minimum to give me a saturated tone. If i turn max out the EQ the tone gets ugly…… like a really cheapo solid state amp.

    I think i finally biased it to the point where i am getting a decent tone, but the attack still sounds a soft and the sustain sounds like there is a noise gate that is cutting it out a bit too early.

    judge for yourself: http://youtu.be/PRfrZStYnwM

  16. anyone know what voltages i should be expecting at the S&G pins of the FETs? turning up VR5 causes static and makes the board microphonic (presumably either a FET or the alpha pot i stuck in in lieu of a trimpot). i found a last MPF102 and put it as Q6, but no improvement. there should be a 100nF cap right before the volume pot btw – it’s on an updated version of the schematic and without it i was reading 4-5VDC on the output

  17. thanks for the tips, Harald – i’ll bear em in mind once i get past some boring debugging. so far i have a combination of problems others have had: very weak signal and loads of interference (though hopefully shielding the in/output will help with that). i ran out of MPF102s and used a 2n5952 as Q6… wonder if that could be a problem. i should’ve used that for the source follower instead, for more uniformity. *sigh* lol
    at least i can bias everything, even if VR3 is insanely sensitive

  18. MPF102, though I have no experience with them, should do just fine. As for the 470pF to 1nF swap, give it a try. It’ll probably sound a bit different, but that doesn’t necessarily mean bad. And if you’re not happy with that, two 1nF caps in series makes for 500pF which should be close enough 🙂

  19. If you look at the J201 datasheet they should have no problem handing that voltage, and as long as you use properly rated caps etc. you’ll be fine. Yes, you need a different bias voltage, but it’s still quite simple; you’ll want to adjust the bias on each JFET so that the drain is idling at approx. 1/2V+, so if you use a +15V supply adjust for ~7.5-8V. Give it a try, it might end up being not that great of an improvement, but it’s certainly not detrimental.

  20. couple more questions – sorry to be annoying lol: assuming i don’t mind the sound deviating a little bit from the JCM800’s frequency response, would subbing the 470pF caps for 1nF and using MPF102s instead of J201s cause any significant problems?

  21. hi,
    if i were to use this as a preamp in a DIY amp build and had a between 10 and 15V available (i’d be keeping the preamp single-rail even though i have a bipolar supply, just for simplicity’s sake) – would this be detrimental to the circuit in any way? i’m guessing i’d need to tweak the biasing differently from everyone else at 9V. assuming all caps have the right voltage rating ofc. i know the whole higher-voltage hype is slightly BS with pedals, but just wondering whether going for a 15 or 12V regulator instead of a 9 would offer a bit more headroom/pwnage in this circumstance, as an amp is pretty much the last stand when it comes to tone.

    any advice appreciated! cheers

  22. Hi s3b. Not sure I can be of much help, but do check out the debugging section. I’ve had good experiences using an audio probe to fix preamp emulator circuits, figuring out what stage is problematic by listening in on the drain pins.
    As for the problematic trim pot, you could try a smaller value one to get a bit more control, maybe 50k or even 25k.

  23. Hi, I made it but I have no sound or very very low.
    VR1 = 4.95V
    VR2 = 4.93V
    VR3 = 4,71V
    VR4 = 4,98V
    VR5 = 7.32V (impossible to set the voltage accurately, it is either 0V or 6-7V, turning the axis of less than a millimeter).
    Can you help me?

  24. *FINALLY* got this thing working! I think the problem was a cold/loose solder joint on one of the socket pins. Such a relief to get this working after all of the problems. It sounds great, too 🙂 demo recorded and uploading to youtube…

  25. UPDATE: I set all my drain voltages to 4.7-4.8 v and WHAM, it sounds great. Gives that old school Marshall sound. I went as far as putting my Ibanez tube screamer clone in front of it and I couldn’t believe the sound. It was way cool. Definitely sounds like a Marshall. GREAT BUILD! Now I need a good overdrive to go in front of it that has good sound (burnt out on the tube screamer). Any suggestion?

  26. Got done with this project and it works. One possible mod is to use in c6 a silver mica and c10& c11 orange drop. I did this in mine to give the tone stack a more amp like quality. Still trying to figure out the fine tuning on this though. To be honest, I have no idea what I’m shooting for.

  27. Hi Harald,
    I had a short in my board which I identified and fixed. Now it works and actually sounds pretty wild. Thanks for this great project.

  28. That sucks, Tim. Maybe if you put it aside for a while and look at it with fresh eyes. Or maybe start all over.

  29. Harald, yes, all of the pots are connected, and triple checked. I don’t know what’s going on with this build. I double checked everything again for the 3rd time in 3 weeks and i still can’t find a mistake. i might have to abandon this build 🙁

  30. Hi Tim,

    I built mine this weekend, and the pedal doesn’t staturate a lot. With all preamp vol, vol and master volume to max, it barely sounds like a tubescreamer (nice sound but not the one expected, right?).
    I then checked the drain JFET and I have values between 9.70 and 11V for Q2! Any suggestion on how to fix this. I checked the VR2, but it works fine.
    Many thanks in advance and for your great website.

  31. Hi Tim.
    Q4 is actually set up as a source follower so you will hopefully find your signal (unity gain) there. That’s the signal that goes to the tone stack (treble/mid/bass pots), via the volume pot and finally to Q5 gate (and then Q6). Could there be a wiring problem related to the tone stack pots or the volume pot (I assume you’ve wired all the additional connections, written in the lower right-hand part of the layout)?

  32. Hi Harald.

    I finally built an audio probe and found that as soon as i probe the drain on Q5 the signal goes completely dead I get a signal when i probe Q1, Q2, and Q3 on the drain. Gate on Q4 has signal.

    I’ve tried swapping in a new transistor for q5 but nothing. When i adjust the trimmer i can see the voltage change. I’ve double and triple checked the schematic (a week apart) and cannot find a misplaced component. I’ve checked for solder bridges but can’t find anything.

    Any suggestions would be greatly appreciated

    (the tone i get at q3 sounds *really* nice 🙂 )

  33. Tim,
    I assume you mean Q4 (which is the source follower). Your best bet would probably be to build an audio probe and listen to the signal as it goes from the input all the way to the output. Somewhere along the way there’s probably a mistake and this will help narrow it down (see the debugging section for more info).

  34. Hi Harald

    finished the final assembly on this last night and… nothing.

    Biased all the jfets (except q3) to about 4.8 V.

    sometimes there is a very faint sound coming thru, but it is orders of magnitude lower volume than the bypassed signal.

    If i touch the copper side of the vero board or tap some of the contacts with a metal screwdriver i can hear buzzing at the same volume that i would expect the guitar signal to be at.



  35. Hi There,

    My emulator is not working. i dont know why, but i cant bias the Q3. i can do it just if i haven’t pluged in the q4. q4 raises the voltage of the q3 drain to 8.13volts and i adjust it with the trim pot.

    my values are: q3 D 8,13 S 1,47 G 7,47 q4 D 8.9 S 8.8 G 8.13
    I think i have a problem with q4. i have tried to switch the fets, but nothing changed.

    can anybody help me?


  36. This was an exelent layout!
    I made it and works perfectly. I never before had biased fets but after setting them to 4.5 to 5 Volts I had a sound. Then play a little and micro adjust by ear.(it can sound differend depending on biasing)

    This sounds perfect and can do high gain things very easy.
    Thank you Harald!

  37. I don’t think it’s a bias problem, those tend to either give no- or choppy sound. Have you double checked all components and made sure they’re in the right place and of the right value? Check for solder bridges, even tiny almost invisible ones. Try looking up “motorboating” on one of the forums and see if you can figure out anything from that.

  38. Hello, Harald! Almost done with the pedal, but have encountered some noise. When I plug in the guitar, it sounds almost like a car engine running. Is it because the biases are wrong, or is it a problem in the circuit? I get normal volt values on the trannies.

    And one noob question: Why is Q4 the only one which connect the drain directly to the 9v source? Why is it the only one which gets 9v at drain? What is the function of Q4?

  39. Hi, DaGa. Not a stupid question.
    First off, if you’re totally new to this I recommend you start with something simpler. I would consider this at least an intermediate build because of all the tuning required. That being said there’s no reason you can’t pull this off with some patience and persistence.

    Because of the great variance of individual JFETs you need to either a) go through a whole bunch of them finding just the ones that work with your fixed drain resistance, or b) use any JFET of the appropriate type and adjust the drain resistance to match each individual one. Those trim pots are used for option b). As a rough tuning, given that you’re running the circuit off +9v, each JFET drain should be at approx. +5v. Then you can fine-tune each stage by ear (an audio probe helps) or ideally with a scope.

  40. Hi, I have a question about the layout… I’m sorry if it is a stupd question, but I’m not an expert 😉

    Why did you use trimpots, and how can I set them up?

    Thaks !

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