JCM 800 Emulator pedal build

[Brymus]

Hey Pauly I found this yesterday in a link in the "Condor clipping" thread.
http://milas.spb.ru/~kmg/index_en.html
Check out his JCM 800 amp sim,sound clips ,scope pics,and all
You might be able to use what you have and throw his circuit in there if you cant fix that one.

[paulyy]

Thanks Brymus. I looked over the pedal so many times and even swaped out some parts. still the same results. Everything seems to be wired right. Just dont know. I did find another layout with almost the same schmatic. Its based off a marshall type amp as well. Thats all I want anyways. I'll get it going one way or another. Thanks for the link to that site. I remember seeing that some time ago. I'll make sure to check it out when I have more time. Well, Thanks again and I'll post more pics when I get this thing running.

[deadastronaut]

yeah dont give up on it...man.....

i built it ages ago...on a massive board...it wasnt as gainy as i like..but i;m a blues to trash head,,,

but my mate loved it....he still uses it all the time....sounds nice n beefy  through his amp n cab...


rob.

[paulyy]

Yeah, I don't want to give up but I did find another circit that's very close to the JCM 800 layout. I think the board I made was bad. Not sure. I'll revisit it later. I wonder if I had some bad trim pots. Again, not sure but I would like to make another board but this time I'll go to office max and have them print it out. All I have is a inkjet printer. Thanks for your support deadastronaut. 

[deadastronaut]

Yeah, I don't want to give up but I did find another circit that's very close to the JCM 800 layout. I think the board I made was bad. Not sure. I'll revisit it later. I wonder if I had some bad trim pots. Again, not sure but I would like to make another board but this time I'll go to office max and have them print it out. All I have is a inkjet printer. Thanks for your support deadastronaut. 

no worries man.........its all about learning either way..

yeah do a proper pcb..to scale etc...that was a good go at it considering you did the pcb wit h a pen!.
wow...

cheers rob.

[paulyy]

Well Im back and going to give this anther shot. Thanks Brymus for sending me some layouts and for the neo classic overdrive pcb. I figure if your going that far to help me. Then its worth trying to get this to work and Thanks to deadastronaut for your suport as well. Heres some pics of the new layout. You will notice the one I did was much bigger.

I hope have this running in the next week or two. Hopfully I have no problems this time. Thanks again guys.

[paulyy]

Did I say two weeks. I meant two hours. I couldnt wait to put this together and see if it works. It works but Im having the same problem I did before. It just doesnt want to bias at all. No matter what I do It sounds fizzy and I get volume increase depending on how hard I play. Something is not right but I cant seem to figure it out. I'll keep trying.

[Brymus]

Hey Pauly.
Glad you finally got that.
You would have got it sooner if ,I had not used my wifes magazine for the layouts 
that came out good,glad the toner didnt rub off in the mail.
Since I was sending the layouts a second time I just dropped the PCB in there for the Neo 250 clone.

Someone should look at this layout.
Someone who really knows this stuff (you know someone smart).
To me ,I am not understanding the last two stages ,and how the last blocking cap is blocking anything except the MV stages DC.
It looks like the signal from the last two FETs is being mixed outa phase at the V knob.
IDK ,but I think a trace is wrong maybe,I'm just not seeing how the last two FETs are supposed to work.

[paulyy]

Well after a good week of messing with this. Still no luck. It just doesnt want to bias. So I went with my backup plan. I built  the king of the britians and sounds pretty good. Both the jcm and k.o.t.b. have almost the same schmatic and both use the same pot values.
So if you were thinking about building this pedal, I hope you have better luck than i did.

[teemuk]

Someone should look at this layout.
Someone who really knows this stuff (you know someone smart).
To me ,I am not understanding the last two stages ,and how the last blocking cap is blocking anything except the MV stages DC.
It looks like the signal from the last two FETs is being mixed outa phase at the V knob.
IDK ,but I think a trace is wrong maybe,I'm just not seeing how the last two FETs are supposed to work.

Looks like it's a combination of negative feedback (with less NFB at higher frequencies) and an error in the layout.  
The 2.2n cap should be AC coupling the output signal that goes to volume pot, not the NFB path. Alternatively, the feedback path could be also AC coupled with a cap (but this is not overly important as there is a 100K-4K7 resistive voltage divider for the DC making the effect of DC feedback rather miniscule) but the most important thing is to AC couple the output.

So, due to layout error the DC bias potential levels are skewed to begin with,  plus it has those stupid trimmers at drains that are way off the proper bias range likely 90% of their dial. No wonder people have problems to bias that thing.

BTW, that stage is pretty much copied from ROG's "Thunderchief" design (likely rest of the circuit is too with addition of a TMB tone stack), but it was copied with some major errors that make the biasing difficult if not entirely impossible. Ironically, even the original Thunderchied design had a reputation of not getting that stage to bias correctly. Anyway, here's how the circuit really should look like:

As you can see, correcting the error in the layout is pretty much as small and simple task as moving a single trace from one point to another or simply soldering a coupling cap to the volume potentiometer.

[paulyy]

Thanks for pointing that out for me. After doing this project twice, I thought something wasnt right with it. As of right now. Im happy with the king of the britians layout, That and I took some parts away from the jcm layout to finish building the K.O.T.B. layout. Not sure I would revisit this layout but at least if anybody else wants to try there hand at doing this layout. They will have some good info on how to fix the problem thanks to you.

[RRIOS77]

Hi, first soory for my english is not good enough. So, I have the same problem, the pedal distortion when I touch a riff hard but the sound during 1 second. I start to thing is because the pots values....I don't know what value is for each pot. I f you see the layout the layout some pots are conected between them and maybe if we used the wrong value they make a cort.
I sent an email to Buck and I'm waiting for his help.

Please Buck what are the right values for the pots?

Thanks so much

[stratman7000]

Hi you all.  I have also worked on this particular pedal build. The  JCM800 pedal from electriclabs.  This is a very tricky build. The pedal it self is thin sounding unless you crank the master and level controls all the way up. Here are some popular mods that have been applied to the JCM Amplifiers that work very well with this pedal...
1) Remove the 470pf Cap from input stage. It will make the tone controls more useful, and the pedal less bright
2) Remove the 470pf cap from the preamp stage. It will further make the controls more useful, and the pedal less brittle
3) To increase the Gain Remove the 10k resistor on the PREAMP and change it to 4.7k NOTE anything lower is overkill.
4) My personal favorite: use a 2N5457 Transistor for Q4 it will add squishyness and dynamics and a hotter output.

Last but not least don't adjust the trimpots with a DMM(volmeter) use your ears. The trimpots are too sensitive. Your ears will not fail you =)
Watch out for trimpot (3) it is the trickiest of them all. If you did good you should have a pedal that has a Volume boost of up to 8-10 dbs above the bypassed signal. By the way this pedal works best with Humbuckers

[stratman7000]

Thanks for pointing that out for me. After doing this project twice, I thought something wasnt right with it. As of right now. Im happy with the king of the britians layout, That and I took some parts away from the jcm layout to finish building the K.O.T.B. layout. Not sure I would revisit this layout but at least if anybody else wants to try there hand at doing this layout. They will have some good info on how to fix the problem thanks to you.

Thanks for that Fix.. I tried it and it worked, The volume pot is not staticky anymore. I also Wanted to let the users to DOUBLE CHECK The PINOUT  of Q4 FET its the only one that is REVERSED. =)  if everything is well you will have an awesome pedal with good Distorion and a great Boost output.

[Joseeduardosn]

Hi Stratman7000,
I'm doing the pedal and let me know if you think must be removed codensadores of 470p. This emulator sounds like Dr. Boogie?
Regarding the knobs, you used 1Meg Log? what does 25K L?
linear?

[J0K3RX]

Hi Stratman7000,
I'm doing the pedal and let me know if you think must be removed codensadores of 470p. This emulator sounds like Dr. Boogie?
Regarding the knobs, you used 1Meg Log? what does 25K L?
linear?

FYI - This is a really old post "September 05, 2010" was the last reply and Stratman7000 has not had any activity on here since Nov. of 2010... Soooooo, you might not get a reply from him I don't know that this is a very good layout? I have not heard of many good results... And, no this would not sound like a Dr. Boogie.

[Magnus]

Hello all,
I will build this pedal too...

...only the pcb is finished yet, I have rebuild the layout for axial parts
- for those mojo and vintage authentic-freaks  

I hope I won't have biasing-problems...




Greetings
Magnus

[J0K3RX]

Hello all,
I will build this pedal too...

...only the pcb is finished yet, I have rebuild the layout for axial parts
- for those mojo and vintage authentic-freaks  

I hope I won't have biasing-problems...



That's a real nice looking build!! Hope it sounds as good as it looks!
Greetings
Magnus

[Joseeduardosn]

Hello all,
I will build this pedal too...

...only the pcb is finished yet, I have rebuild the layout for axial parts
- for those mojo and vintage authentic-freaks  

I hope I won't have biasing-problems...




Greetings
Magnus

wow
And why these capacitors so big?
You are almost there, only the potentiometers.
Some soundcheck?! Please! ... And also you could comment if you applied the changes proposed by Stratman7000.

[Magnus]

Hello,
thank you  

@Joseeduardosn :
Those caps are bigger ones because their capacity is 630Volts DC and they are used (for example) in Marshall-Amplifiers
but they can be used in pedals too and its a little bit more authentic compared with the original amp we are emulating here...
They are manufactured by Mallory, called "Mallory 150" and are metalized polyester.

Sorry, i can't make soundfiles because I don't own any software/hardware ro record...

I will try all the mods from "Stratman7000" for sure!
I think I will use toggle-switches for the "less bright"- and "more gain"-mods.


Greetings
Magnus