JCM800 emulator: tweaking update

[Bucksears]

After trying various values in place of the 33k resistor of the tone stack and a few different combinations of the 470k resistor/470pf cap pairing at the 1st transistor, I seriously narrowed it down to a setting I will probably keep.
The circuit is built completely like the schem (and my layout), but I subbed out the 33k resistor in the tone stack for a 12k resistor and it seems to work really well. I originally wasn't watching what I was doing and put in an 820ohm in there instead of the 33k and the tone controls didn't work at all; however, due to the gain (which you could adjust from low to 'Eruption') and the brightness, IMHO it nails Eddie's sound on VH1 (bright, minimal bass, flat mids).
As much as I love that sound, I'd rather have versatility. With the 12k resistor, the midrange and treble are more interactive, but the bass still doesn't do a whole lot. I'll probably keep on tweaking a little more, but it's zeroing in close on what I want out of it.

Anybody else messing with this circuit and having positive results?

[vanhansen]

Hey Buck.  What values did you settle on in the first 470k/470p network?

[Bucksears]

I actually left that part stock. The only things I've changed so far is dropping the 33k resistor down to 12k and dropping the 250k treble pot down to 100k.
I'm pretty pleased with it.
The funny thing is, when you turn down the midrange, the treble turns down a bit too; so it's a bit interactive. The bass doesn't do much, acts more like a cut instead of a boost. It's right on that 'edge' of the right amount of treble because if I adjust my SRRI's treble from 5 to 4, it's a noticeable difference.
The best thing about this circuit (with the 10-12k resistor) is how OPEN it is; it doesn't sound like a 'distortion pedal'. At 33k, it sounds more compressed and focused, to me anyway.
I'm going to leave it as-is for now and work on other things.
- Buck

[vanhansen]

Excellent.  I'll be sure to include your tweaks in my mods list for this.

[Eddie]

I want a little soundclip!!!


JCM 800 emu better than thunderchief??


Thanks


MXR

[vanhansen]

There is a page on my site about this circuit.  Thanks to Buck for letting me post his layouts and to Electric for the schematic.

There's a small list of mods and tweaks there, some of which Buck has done himself.

It's under 'Projects'.
http://www.erikhansen.net/diy/

[AmpliFuzz]

You might try these too:

https://schematicheavencom.secure.powweb.com/ampmodheaven.htm

scroll down to 'Marshall Mods', some of them seem pretty good.

[vanhansen]

You might try these too:

https://schematicheavencom.secure.powweb.com/ampmodheaven.htm

scroll down to 'Marshall Mods', some of them seem pretty good.

That's actually where I got some of my ideas.  

[Bucksears]

Thanks, Amplifuzz.
Actually, I started on the tonestack mod listed for the 2203/04, but I didn't like where it was going. It started sounding more like a pedal.
IIRC, for the Fender specs it calls to switch to, the 33k resistor is raised to 56k (or higher). I liked the openness of the lower value resistor and didn't want to change it.
For now, I'm going to leave it; I'm going with the Dr. Boogey and Umble next.

[AmpliFuzz]

That's actually where I got some of my ideas.  

They looked familiar indeed

I think that the most interesting mods are the ones that don't mess with the tonestack (cream, touch, more gain, etc.). Then again it would become something other than a 800 emulation

[Bucksears]

"Then again it would become something other than a 800 emulation."

Exactly. I liked the basic sound I was getting right off the bat, plus there are several others out there I want to try.
Hopefully, my site will be back up soon and I'll be able to throw some samples up.

[vanhansen]

Buck, if you want to cut a couple of small MP3 samples, send them to me and I'll gladly add them to my page.  Let me know.

[Ptownkid]

anyone have any photos of this build?