JCM-800 Fet Emulation

[electrictabs]

Well there are two reasons that made me do this.
First of all i really miss that happy feeling everytime the R.O.G. guys release a new emu-circuit.
The second reason is that i read in an article that Slash used a JCM-800 master volume preamp (not the jubilee) for the recording of sweet child o' mine (one of my favourite licks).Grab your Les Paul switch to neck pup and you can nail that tone.
What i really did was modding my thunderchief clone so nothing really difficult here...
Samples as soon as i get my Cubase to work again...

Haaaappy New Year!!!

[vanhansen]

Hey, that's the exact same thing I'm doing.  I'm working on a 2203 JFET Emulation also.  I have the schematic laid out, almost identical to yours.  My tonestack is in a different place though.

Once I get it on breadboard and tested out I'll post results.

[electrictabs]

My tonestack is in a different place though

well i followed the amp schem for placing the tone stack.looks and sounds good to me.

[chokeyou]

I'm working on one as well.
*cough*orange 1974-era MKII graphic 120watt*cough*
(you can imagine my excitement)
I'm waiting on my caps and FETs and enclosure bits and peices from smallbear to finish it up, but my layout is done and i have the majority of it hooked up already.
I can't freakin wait.

[vanhansen]

My tonestack is in a different place though

well i followed the amp schem for placing the tone stack.looks and sounds good to me.

I just looked at mine again.  It actually is in the same spot.  I some errors in mine though after comparing the amp schematic with it.  It's my first shot at an emulation and it's been a few weeks since I've looked at it.  Gotta study it some more (learning experience for sure)

Can't wait to hear some sound clips.

[vanhansen]

I see the difference between your schematic and mine.  I was using only half of each preamp tube, like the Thunderchief, but yours has a J201 for each half.  Off the first J201 I put a 10K to ground rather than the resistor/cap combo.  Same with the second J201, only a 1K resistor to ground.  That and I had the second 470p/470k junction between the preamp volume pot and the second J201, then on to the tonestack and on to the end like the Thunderchief.

Once I acquire all the parts to do this, which I hope I can do soon, I'll have to see how my schematic sounds versus yours.

Is the voltage bias for the J201's 4.5v in your circuit?

[ragtime8922]

I love what you guys are doing. I would love to make an emu of the Mesa-Boogie Mark IIC+ but the schematic I have isn't clear and I can't read the values. Anyway, in order to do this do you guys use the exact same values for every component and just simply replace each half of the preamp tubes with an FET? If so are the FETs chosen by ear or another method?

PLEASE, post the results of that 1974 Orange :!:

[vanhansen]

Anyway, in order to do this do you guys use the exact same values for every component and just simply replace each half of the preamp tubes with an FET? If so are the FETs chosen by ear or another method?

Pretty much, yes, the same values are used.  Some tweaking with values may be necessary here and there but in my comparisons with the emulations out there from ROG and the original amp schematics, the values are almost all the same.  The fun part is in the phase inverter stage and the power amp stage.  If you search the forum, you'll find a post I put up earlier this month or late last month about this as well.  Bryan from ROG answered a lot of questions for me and explained the different stages.

The J201, MPF102 and 2N5457 are the ones I've seen most commonly used.  Their pinouts match that of the Anode (Drain), Grid (Source) and Cathode (Gate) of a preamp tube.

Runoffgroove has some good info on this and diagrams in the FAQ. http://www.runoffgroove.com/faq.html

[ragtime8922]

Thanks Vanhansen. What do I search for? ROG Emulations? Also, I didn't realize that the emus went further then the preamp. Are you saying that the power section is included in the emus? I was going to ask what was done after the master volume section of the jcm800.

Anyway, thanks!

                      -Chris

[vanhansen]

Thanks Vanhansen. What do I search for? ROG Emulations? Also, I didn't realize that the emus went further then the preamp. Are you saying that the power section is included in the emus? I was going to ask what was done after the master volume section of the jcm800.

Anyway, thanks!

                      -Chris

Chris, here's the post.

http://diystompboxes.com/sboxforum/viewtopic.php?t=27611&highlight=

If you look at the Thunderchief schematic, the second to last JFET is the phase inverter, the last JFET emulates the power amp section, plus there's a dual low pass filter in there.  So yes, the power section is included.

In the JCM800 that electriclabs did, and mine is similar, after the Master Volume comes the phase inverter, or the 3rd preamp tube.  Then, after that, from the 22n to the last J201 is the power amp stage.  The 2n2 off the volume at the end that eventually leads to ground through the 100k resistor looks like the low pass filter.

Now, you can rip out the tonestack for that "always on 10" sound.  Again, referencing electriclabs' schematic, that would mean removing the components starting at the 4th JFET right before the tonestack through to the Master Volume.  The 3rd JFET would then go right to the Master Volume or remove that as well and go right to the 22n cap at the start the phase inverter.

I hope all this makes sense to you.  This is a new road for me.  Actually, all of it is but the emulations have caught my attention big time since listening to the Thunderchief, Eighteen, Professor Tweed and the others from Runoffgroove.

Anybody, please correct me where I'm wrong, if I'm wrong (probably am somewhere a little bit).

[RDV]

Somebody build this thing already. I would now but I don't have time.

RDV

[vanhansen]

Somebody build this thing already. I would now but I don't have time.

RDV

And I don't have the parts yet.  :cry:

[RDV]

Parts I got, time I don't. It really looks promising.

RDV

[vanhansen]

If anybody has parts, I knew you would, Ricky.    Being relatively new to this I have yet to build up my parts bins.  My mother-in-law may help me there with old VCR's and stuff she has in closets.  Being that I have a breadboard now, I'm looking forward to building it.

Hey electriclabs, got your Cubase working yet?

Ricky, was my explanation a few posts up on the stages correct?

[panasonic_youth]

how does one go about making a JFET simulation of an amp? i would like to make a mesa boogie rectifier sim....

[Torchy]

crp

[panasonic_youth]

alright, but what JFET to use?

[Torchy]

crp

[vanhansen]

Thanks for helping out, Torchy.  Happy New Year from across the great big pond.

panasonic_youth, not sure if you read the whole post but most of what Torchy mentioned I posted for another member earlier on.

Once you read the stuff posted here, study the ROG stuff, study the original amp schematics, it will all start to make sense.

[panasonic_youth]

alright, i still need a good referance for biasing, but aside from that, all i need is the money now. i have this lined up after a punch-in-the-face build, so who knows when i'll have the cash. haha. oh, also, which preamp version should i use: http://www.tubefreak.com/mesa.htm#recto . version 1 or two? the main differances seem to be for noise reduction, if im reading them right.