building the jcm800 emu

[mhk]

hi , i've built the jcm800 emu by electrictabs/vanhansen ,
i still  have to put it in a box though , but the sounds are very good , maybe a little tweaking with the tonestack to get it a little more responsive , but overall : i'm impressed!
so thanks to electrictabs, vanhansen and bucksears , nice work!!!!

[Bucksears]

Ah, yes. The tonestack.

I have yet to alter any of the parts to the stock tonestack (other than the 33k resistor) in trying to keep it as close to 'Marshall' as possible. I currently have the 33k resistor dropped down to 10k, which (to my ears) has opened up the sound so the distortion was more 'amp like' coming through my SRRI.
The down side is, with this configuration, the eq is barely usable. The bass goes from cut (min) to flat (max) and if you turn the mids all the way down, you can play with the treble. If the mids are up, this increases the treble slightly and makes it unadjustable.
With the 33k resistor, the eq (again, to me) is more usable, but at the expense of the distortion sounding more like a pedal than an amp. I've read somewhere that increasing the 33k res up to 56k really beefs up the preamp some, but I can't say for sure because I haven't tried it.
I may try changing the treble pot to 100k and the mids to 50k just for kicks.

- Buck

p.s. thanks for the comments on the PCB!!

[mhk]

Ah, yes. The tonestack.

I have yet to alter any of the parts to the stock tonestack (other than the 33k resistor) in trying to keep it as close to 'Marshall' as possible. I currently have the 33k resistor dropped down to 10k, which (to my ears) has opened up the sound so the distortion was more 'amp like' coming through my SRRI.
The down side is, with this configuration, the eq is barely usable. The bass goes from cut (min) to flat (max) and if you turn the mids all the way down, you can play with the treble. If the mids are up, this increases the treble slightly and makes it unadjustable.
With the 33k resistor, the eq (again, to me) is more usable, but at the expense of the distortion sounding more like a pedal than an amp. I've read somewhere that increasing the 33k res up to 56k really beefs up the preamp some, but I can't say for sure because I haven't tried it.
I may try changing the treble pot to 100k and the mids to 50k just for kicks.

- Buck

p.s. thanks for the comments on the PCB!!

hey , i have the 56k resistor instead of the 33k and for me it makes the tonestack a little more responsive , may i'l try the 10k aswell.......  , to me it can use a little more bass too , but i don't want to mess arround with it too much

-mhk

[vanhansen]

It's been a while since seeing a thread on this one.  I'm glad that folks are still building it and enjoying it.

When I finally get a chance to reorder some parts, I'm gonna make this one minus the tone stack for that "all on 10" sound.

[Doug_H]

I'm wondering- why did you guys include the .1u bypass "presence" cap? That is like having the presence cranked to 10 and will sound much more shrill. I did a sim of this and leaving that cap out gives it a ~10db reduction in mids and highs. Just a thought.

Doug

[vanhansen]

I included it simply because I borrowed that part from the Thunderchief (with permission of course).  My guess is electric did it for the same reason.  Does removing that cap smooth it out?

[Doug_H]

I would think it would smooth it out quite a bit. You would have to try it to see. The way the cap works is it shunts some of the high frequency negative feedback from the fdbk loop to ground. That ends up increasing the overall high freq response of the circuit. I left that out of my meteor and some of my other amp sim stuff when I was experimenting with it. When most people crank their amps up loud they turn the presence down anyway. It's mainly intended as sort of a "low volume helper", kind of like a bright cap I believe. You could always implement a presence control if you wanted, subbing the 4.7k going to gnd with a 5k pot and putting the cap from wiper to ground. But IMO that's an overcomplication for a pedal circuit- but ymmv.

Does anyone have any soundclips of this? I would love to hear it.

Doug

[vanhansen]

I've been wanting to hear it too.  No, I haven't heard it even though I have a page for it...lol.  Wife is still job hunting, still need to get parts. 

EDIT: Scratch that.  Wife now has a job. WOOHOO!!!! Let the parts ordering commence.

I'd like host a clip or two of it on my site if possible.

Thanks for the tips, Doug.

[mhk]

I've been wanting to hear it too.  No, I haven't heard it even though I have a page for it...lol.  Wife is still job hunting, still need to get parts. 

EDIT: Scratch that.  Wife now has a job. WOOHOO!!!! Let the parts ordering commence.

I'd like host a clip or two of it on my site if possible.

Thanks for the tips, Doug.

Hi vanhansen , your site made me wanted too make this , i'm new too this thing of making my own effects but this thing is turning out great! , i changed the 33k resistor to 56k and it turned out good, i get more responce from the tonestack now , it's got the marshall-type distortion to it and i 'm  very pleased with the result , a marshall in a box !!

[Bucksears]

I've tried just removing the .1uF cap and leaving that space on the PCB from the resistor junction (4.7k, 4.7k and 100k) to ground not connected.
It sounds a little more 'blatty', more fuzzy than it did before, so I may just put the .1uF back in.

Or should I just jumper that connection to ground?

- Buck

[Bucksears]

Scratch that last entry. Well, part of it, anyway.
I was looking at the parts layout and comparing it to the actual pedal last night before bed and noticed I had the wrong value cap in that firts 470k/470pF combo; I took the 470pF out for a while, but didn't replace it with the correct value.
I'll replace it tonight and see what I get.
Still wondering about jumpering that .1uF space or if I should just leave it out.

- Buck

[vanhansen]

I'd think that just leaving the pads empty would be fine.

Leaving that first 470p cap out, well you know what it does...lol.  The numbers on some caps can be such PITA to read sometimes.  I've gotten values wrong before.  Wondered what the heck was going on and why the darn circuit didn't work then I'd see it.  DOH!!  lol.

[mhk]

I would think it would smooth it out quite a bit. You would have to try it to see. The way the cap works is it shunts some of the high frequency negative feedback from the fdbk loop to ground. That ends up increasing the overall high freq response of the circuit. I left that out of my meteor and some of my other amp sim stuff when I was experimenting with it. When most people crank their amps up loud they turn the presence down anyway. It's mainly intended as sort of a "low volume helper", kind of like a bright cap I believe. You could always implement a presence control if you wanted, subbing the 4.7k going to gnd with a 5k pot and putting the cap from wiper to ground. But IMO that's an overcomplication for a pedal circuit- but ymmv.

Does anyone have any soundclips of this? I would love to hear it.

Doug

I'll try to make a soundclip over the next few weeks....... , maybe thru my v-amp on a clean amp model.....i'll give it a try

[Travis479]

Does anyone have a link to this schematic?  I wouldn't mind building this, but I didn't see a link on this page.

[vanhansen]

Does anyone have a link to this schematic?  I wouldn't mind building this, but I didn't see a link on this page.

Hey, Travis.  It's on my site (linky in my sig below).  Both mine and electric's schematics are there.  Use electric's schem though.  His is verified.

[StephenGiles]

I've tried just removing the .1uF cap and leaving that space on the PCB from the resistor junction (4.7k, 4.7k and 100k) to ground not connected.
It sounds a little more 'blatty', more fuzzy than it did before, so I may just put the .1uF back in.

Or should I just jumper that connection to ground?

- Buck

"blatty" - which means???
Stephen

[Travis479]

Thanks!

[Bucksears]

'Blatty', meaning, that kind of gated distortion like when transistors are misbiased and it sounds like crap.
I basically meant that the distortion didn't have any 'body' to it and was much more fizzy sounding; i.e. trebley and thin.

Hope this helps,
It's hard describing distortion, much less distortion that sounds bad.
- Buck