Randall RG100ES. Layout inside.

[fishbone_vet]

Hello guys. I found layout and circuit of rg100es preamp, its also calls like "preamp from hell". Does anybody knows does it work??

http://obrazki.elektroda.net/57_1260128683.gif
http://obrazki.elektroda.net/34_1260128762.gif

[duck_arse]

welcome to the forum, fishbone!

looking at the layout diagram, I can tell you it won't work as shown, bacause all the fets are backwards. all the pins marked "D" are actually "G". but you probably woulda found that when you was checking the datasheet, no? other than that, you might be the first.

[fishbone_vet]

It's not  a problem, i'll check fets pin before mounting. maybe i will make another layout but with using this diagram. Just want to know  if its working

[Jan Irace]

welcome to the forum, fishbone!

looking at the layout diagram, I can tell you it won't work as shown, because all the fets are backwards. all the pins marked "D" are actually "G". but you probably woulda found that when you was checking the datasheet, no? other than that, you might be the first.

So technically I would just need to rotate the mosfet to have the correct orientation?

[duck_arse]

technically, I see no mosfets on that schem. I suggest you check the datasheet for the manufacturer of the jfets you have in your hand, but when I checked, the 2N5484 had the gate pin on the opposite end to that shown on the layout. as to the goodness of the layout, I can't say 100%, but the fets in the right way will get you a good long way.

[samhay]

I would breadboard this first.

There are no trimmers to bias the FETs, so you will either have to audition (possibly many) JFETs to find two that bias sensibly and/or replace the 33k resistors with appropriate alternatives. If it isn't clear, it is the 2 left-most FETs that will be troublesome.
As good JFETs are not easy to find any more, I personally wouldn't waste then using them in the other 2 positions. You could use  (2N7000 or BS170) MOSFETs or high-ish gain NPN tranistors instead and I don't think they will sound very different.

[caspercody]

I made this using that PCB and it works. I used sockets for the JFETs. Do not remember if I had to try different ones

[LiLFX]

SWEET! I'm gonna get my Dimebag on!

[slashandburn]

Ah crap. Another highgain pedal I don't need but now really want.  Not sure where to start.

I take it the 24v is mostly required for headroom?   And idea if I'll get by with just 2x  9v batteries in series?   

I guess the short answer is "build it and see".   I'll probably do just that. Sometime. Eventually. 

[caspercody]

I built it at 9vdc.

[slashandburn]

I built it at 9vdc.

Did it work? Any idea what the extra 15v would provide? I'm absolutely guessing when I venture than it might just be headroom.

SWEET! I'm gonna get my Dimebag on!

HELL YEAH!    (Seems like the only appropriate response)

[Marcos - Munky]

Wow, this is a layout I've done a few years ago. Actually, got all the parts but never got to made it . I thought there was some error around the tone stack. Do yours works ok, caspercody?

I made a layout with all pots onboard, to use it in a amp as a preamp. It's unverified but based on the factory schematic.


Also, I can try to make a 1590BB layout with onboard pots, just give me a few hours.

Edited: here's the 1590BB layout. All pots onboard, soldered at the solder side of the board. The space on the corner near the presence pot can fit a charge pump circuit .


Edited again: I just saw how ugly those transfer images went. I can e-mail the eagle files and/or a transfer image in pdf if anybody wants them.

[caspercody]

I built these years ago and do not have them anymore, but they did work. I built them at 9 vdc.

First one is the X2FET, which is the better to get Dimebag sound.

Second is the pre-amp from hell.

[JebemMajke]

I guess when you flip that switch for more ( adding diodes ) there must be some sort of reduction in volume.

So, how about adding one more FET stage after the diodes, maybe a buffer stage, to recover volume?

Using a dpdt switch to remove additional FET stage + those diodes.

[Marcos - Munky]

Yes, flipping the switch will result in more distortion and a volume drop. That's how the original RG100 was made. A volume recovery stage is a nice thing to add.

[Derringer]

Just for reference, here are the voltages I took from my RG100ES amp the last time I had it open.

[slashandburn]

Thanks again for those pcb images Casper! I got the x2fet made yesterday, it does what it says on the tin.

I'd planned to build both but forgot to flip the image of the Preamp From Hell. Might come back to that at a later date.

[slashandburn]

Appologies for double posting. This has been on my to do list for too long. I've been having a bit of fun with the x2fet recently but it's a bit shrill and doesn't quite hit unity even with the volume maxed out. With an OD in front its definitely in the right ballpark but it's still left me wanting even after considerable time playing around and stacking with drives and EQ pedals. I'll have a stab at the preamp from hell and put them head to head. The victor can get boxed up with a boost and maybe a charge pump.

Is there any consensus on the error from the original Preamp from Hell?  Is it just the missing 68k on the input?

And if I run with trim pots, is 100k a good value or would that be overkill?

[caspercody]

If memory serves, I also had low output from that one. I added a JFET stage at the end to boost the output.

[slashandburn]

Ah cool!  If I make it far enough to a/b them I'll dig out my mosfet booster. 

The shit news about Vinnie has me binging on 90's metal and nudged this near to the top of my to-do list.  With 6 pots, a switch and possibly also a charge-pump I'm already feeling a bit apprehensive about boxing it up.