has anyone built this - jcm800 emulator

[wavenator]

http://www.4thlevelmedia.com/JCM800Emulator-Parts.jpg

has anyone built it ?

how is it sounds like
is it really sounds near the jcm 800
it has a tubelike sound?

[ralley]

Haven't built it myself but there is some discussion of it in this thread:
http://www.diystompboxes.com/smfforum/index.php?topic=64038.0

Rob.

[kurtlives]

My buddy could never get it to work but he is still workin on it....

I always liked the design

[drewl]

What's with all the trimpots?
They all need biasing?

[kurtlives]

Ya all the FETs need to be individually biased.

[dschwartz]

you have to put a coupling cap after the vol pot..this thing is giving 4.5V of fet bias right to the output.

[DougH]

And depending on whether the volume pot was connected when the trim pot on the last stage was trimmed, and what is hanging on the output- it may not be anywhere near 4.5v. Lots of potential for a major screw-up there...

[dschwartz]

i´d put a small audio transformer there..just to see what happens..

[rikkards]

I did but I did my own vero layout. It sounds kinda like the thor but more trebly but not as much gain as a BSIAB2. I use it off and on

[wavenator]

it has tubelike sound?

[Bucksears]

That's my original layout - I haven't had the JCM800 in a LONG while. My main distortions these days are the BSIAB II, Dr. Boogey and a modified GGG Big Muff.
If the treble can ever get reduced (the only real problem I had with it), the JCM800 would be a cool unit; it had a few good distortions in there. I remember accidentally putting an 820ohm resistor for the slope instead of the 33k (just before the eq circuit). The EQ did absolutely nothing, but I got an amazing VH1 sound out of it.
There should be a way to work in the BSIAB/Big Muff tone control in there.

Yes, several trimpots, but it had a decent amount of gain.

[petemoore]

you have to put a coupling cap after the vol pot
  Can be put right on the Potlug.

[StephenGiles]

Where is the circuit diagram?

[TELEFUNKON]

Where is the circuit diagram?

http://www.diystompboxes.com/smfforum/index.php?topic=64038.0 fourth posting

[snyder80]

Well i actually build it and it sounds realy good! Its like one of those fet preamps by runoffgroove.
One thing thats important - in my opinion at least- is to add a decoupling cap (22n i.e.) BEFORE the volume pot! Otherwise there will be crackling noise and DC voltage at the output!
I also added the two low pass RC-segments like in ROG preamps.
At least my build sounds very nice in the return of the fx-loop, dont use it in front of an amp. It will not sound as good as it can.
If your amp has no FX-loop/ FX-return u can get the peavey revalver MKIII software demo und use those powerampmodels and speakersimulations for testing your build.




PS: There are two versions of this schematic, one is done by eric hansen and the other one by electrictab. I did use the eric hansen version because its closer to the ROG preamps...

[rikkards]

I did the electrictab one which can be found at Eric's page as well:
http://www.erikhansen.net/?page_id=36
plus it is more accurate to the original schematic that I found of the amp (the amp has the second 470p/470K after the second tube while Eric has it before)

Well i actually build it and it sounds realy good! Its like one of those fet preamps by runoffgroove.
One thing thats important - in my opinion at least- is to add a decoupling cap (22n i.e.) BEFORE the volume pot! Otherwise there will be crackling noise and DC voltage at the output!
I also added the two low pass RC-segments like in ROG preamps.
At least my build sounds very nice in the return of the fx-loop, dont use it in front of an amp. It will not sound as good as it can.
If your amp has no FX-loop/ FX-return u can get the peavey revalver MKIII software demo und use those powerampmodels and speakersimulations for testing your build.




PS: There are two versions of this schematic, one is done by eric hansen and the other one by electrictab. I did use the eric hansen version because its closer to the ROG preamps...

[DougH]

Re. the treble- The JCM800 preamp is fairly bright by definition. I stuck one of these on my Electar10 amp many years ago when I was experimenting with amp preamps. And this pedal circuit will most likely need some low pass filtering on the output to get it equalized for plugging into an amp. Check all the typical 2 pole LPF's you see on these JFET circuits for clues.

[drewl]

The real jcm800 tube amps are bright due to the cap on the volume or gain pot which most people remove (you knew that).

[DougH]

True. But it still pushes a considerable amt of treble with the 2 470p || 470k's.

[dschwartz]

Well i actually build it and it sounds realy good! Its like one of those fet preamps by runoffgroove.
One thing thats important - in my opinion at least- is to add a decoupling cap (22n i.e.) BEFORE the volume pot! Otherwise there will be crackling noise and DC voltage at the output!
I also added the two low pass RC-segments like in ROG preamps.
At least my build sounds very nice in the return of the fx-loop, dont use it in front of an amp. It will not sound as good as it can.
If your amp has no FX-loop/ FX-return u can get the peavey revalver MKIII software demo und use those powerampmodels and speakersimulations for testing your build.




PS: There are two versions of this schematic, one is done by eric hansen and the other one by electrictab. I did use the eric hansen version because its closer to the ROG preamps...

putting that 22n coupling cap before the volume, makes the volume act like a variable high pass filter..if using a 100k pot, at max volume you cut bass at 72 hz.. not bad..but at half volume (resistor to ground =10k), you are cutting 720Hz and below.. and at a quarter volume, you are cutting at 7200hz..man,  you want fizz???!!!..

use a bigger cap..(like a .82uF or 1uF, or even bigger).