JCM-800 Fet Emulation

[ethrbunny]

... all those trim pots.. for biasing the drain on each fet? to what value(s)?

[vanhansen]

Start with channel one.  See how it sounds.  Then try channel two.  With something like this, experiment to find which channel sounds better to you.

As for biasing each fet, it can be different depending on the circuit.  The Thunderchief biases at 4.5 volts as do a few other of the ROG emulators.  You can start there and tweak to find the "sweet: spot.

Good luck and Happy New Year!

[panasonic_youth]

thanks van hensen. i just drew up #1, if anyone cares about it, i have it.  looks like i'll have to break in and buy a multimeter after all.  the question i have is this: do you have to change any of the component values to suit the differant voltages? and the rectifier doesnt use 12AX7s in the preamp, that wont mean anything as far as the JFETs go will it?

[Bucksears]

On the JCM800 emulator, I see that there's three volume controls: pre-volume (gain), master volume and volume.
Since I really don't need two volume controls, can I ditch the one of the two and still have a usable circuit?
Thanks for giving me yet another circuit for my 'to do' list that will never finish!   :wink:
- Buck
www.bucksears.com

[Johan]

On the JCM800 emulator, I see that there's three volume controls: pre-volume (gain), master volume and volume.
Since I really don't need two volume controls, can I ditch the one of the two and still have a usable circuit?
Thanks for giving me yet another circuit for my 'to do' list that will never finish!   :wink:
- Buck
www.bucksears.com

if you ever got the chanse to play a jcm800 on full, you probably know that the trick to get them to sound GREAT is to balance the preampgain and the poweramp ( mastervolume ) against eachother. ofcourse that would be VERY loud and you probably need a powersoak of some kind..think of the gain and master as two different gains, giving different carracter, and the last volumepot as tha powersoak...
...I probably wount build this one since I allready have the amp , but the concept has been proven to work with all the R.O.G emulators, so..great work..

Johan

[electrictabs]

On the JCM800 emulator, I see that there's three volume controls: pre-volume (gain), master volume and volume.
Since I really don't need two volume controls, can I ditch the one of the two and still have a usable circuit?
Thanks for giving me yet another circuit for my 'to do' list that will never finish!   :wink:
- Buck
www.bucksears.com

if you ever got the chanse to play a jcm800 on full, you probably know that the trick to get them to sound GREAT is to balance the preampgain and the poweramp ( mastervolume ) against eachother. ofcourse that would be VERY loud and you probably need a powersoak of some kind..think of the gain and master as two different gains, giving different carracter, and the last volumepot as tha powersoak...

you couldn't answer better
thanks

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?

this is what you ask for
http://diystompboxes.com/sboxforum/viewtopic.php?t=25327&highlight=mesa+boogie


edit:i also have to note that i omitted the dual lpf on the output of the jcm800 emu cause i prefer it that way.feel free to try for yourself which one is better.

[ethrbunny]

...sooooo.. how does the sound compare to the real-meal-deal?

[Branimir]

and how do these fet emulators work driving a power amp into a guitar cab?

or are they to be used guitar->fet emu->soundcard for recording?

i never managed to get that cleared up heh  :lol:

i mean sure they can be used as preamps but what is the original idea, and for what purpose do you guys use them?

[ethrbunny]

from TubeFreak.com
By no means use it to try to built a replica, it won't work. These schematics are only here for information & repair. Trying to replicate it is useless

Does this mean that something critical has been left out on purpose to prevent replication?


Branimir -->

from the ROG FAQ
Are the FET-based amp emulators used differently than other pedals?

No, they're regular stompbox circuits. Use them in the same manner as any pedal.

[Branimir]

Branimir -->

from the ROG FAQ
Are the FET-based amp emulators used differently than other pedals?

No, they're regular stompbox circuits. Use them in the same manner as any pedal.

oh. i always get confused with these devices since i know the same thing won't sound the same on a fender, marshall or a ss randall  :wink:

dunno, i don't like tonestack when a tonestack is already present in a chain aftewards.. although these designs have simple tone controls.. hm...

well..

:shock:

back to the drawing board.

happy new year.

[william]

Quote:
from TubeFreak.com
By no means use it to try to built a replica, it won't work. These schematics are only here for information & repair. Trying to replicate it is useless


Does this mean that something critical has been left out on purpose to prevent replication?

On some of the schematics yea.  Mostly, its useless to replicate because mesa amps use double sided PCB boards, and lots of parts.  Also, it can potentially be very noisey if not layed out well.  When decideding to do a jfet emulation of a mesa amp, you need to decided what fuctions you wish to settle on.  For instance, do you want the red or orange channel?  How will you handle the Negative Feedback loop, which is where the mesa's control the Vintage, Modern switch.  Also worth mentioning is that the fets will sound diffrent than the tubes.  You may need to alter some parts before everything sounds good.

[ethrbunny]

How close (ly?) can FETs match tubes? Isn't this the basis of the whole SS v tube debate?

[vanhansen]

That's a whole 'nother subject.   We're not trying to recreate the wheel, just putting a good sounding amp circuit in a stompbox form.  Emulation circuits may sound somewhat tonally similar, but not exact 100%.  They'll get in the ballpark though.

I was inspired to make an attempt at the JCM800 2203 after hearing the Thunderchief and studying the schematic with the original Marshall Super Lead schematic (1959 model).

I have a big Marshall amp already, but something like this would be great to use for practicing and noodling around without the neighbors hearing it.

Just waiting for Smallbear to reopen so I can get some parts.

[chokeyou]

smallbear is great for some stuff...

but i tried mouser out the other day and all i have to say is wow.

as soon as you get over the intimidating catalog, the prices and shipping are very nice. the packaging even gave me a warm fuzzy feeling in my heart...the appropriate static-guard baggies, an invoice indexing all the parts and so on. TWO THUMBS UP FOR MOUSER. if smallbear was a small army and he could get his stuff shipped faster I'd love him just as much. I'm waiting on a shipment i ordered like 3 weeks ago  :roll:  

PS - to you guys thinking there's no application for FET emu's with larger amps... I've got a 1974(or so, i havent dated it yet) silverface bassman that is cleaner than clean...so a nice FET in front of it gets it the dirt and dynamics it needs in those gainy situations. still EAGERLY awaiting my parts from smallbear to finish the '74 orange graphic 120watt FET. WOO

[electrictabs]

hello
if you're gonna breadboard this circuit try these out
they are not exactly mods but maybe sound better to your ears
since everyone is using different gear

1.the all knobs to 10 mod bypassing the eq
(just like in thunderchief)
resulting in just a little more gain

2.the dual lowpass filters on the output emulating some speaker response (refer to thunderchief schem if you don't know to do)

[vanhansen]

Thanks for sharing.  My first schem eliminated the tonestack then I decided to add it in.  Once I did that I also decided to put on the dual LP filters.  Gonna be breadboarding it soon.

[space_ryerson]

Has there been any progress with this? I'm starting to feel the burn to build this sucker.

[vanhansen]

Not on my end unfortunately.  We're saving for a new house some a majority of my DIY has gone to the backburner.  Pretty much have to use what I have and salvage parts.  I'm sure someone has built this.  I'd love to hear some some clips (dry, no reverb or delay) to get a general idea of how it sounds.

[Branimir]

anyone have any clue how will this thing sound into a SS 100 poweramp and into the 2x12" celestion vintage 30s box?

[NaBo]

I built it and when audioprobing it sounded pretty great up to the "phase-splitter stage", which I couldn't get to bias properly with any of the MPF-102s that I have.  It inherited the same hard-to-bias problem as the thunderchief which it was somewhat based on.  Tried the ROG fix with little success.  Hopefully the J201's from small bear that are waiting for me at home will work, but I won't know til May  

Oh, and if you haven't seen it, there's a nice pcb layout that Bucksears made here:  http://www.diystompboxes.com/sboxforum/viewtopic.php?t=28551

(Scroll down a couple posts for the PCB)