Randall RG100ES. Layout inside.

[slashandburn]

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'll report back after I've finished building this but I've just noticed the cap numbering looks a little suspect in the tonestack.   0.002u, 0.2u & 0.22u are all called for.  I'd normally reach for a 2n2 and 2x 220n but ill probably socket them and play with the tonestack calculator if I run into trouble.

Oh and thanks for the layout Marcos!   

[teemuk]

Yes, flipping the switch will result in more distortion and a volume drop.

This.

The overdrive or "Red" channel of that preamp primarily produces distortion by diode clipping. This is produced either with back-to-back Zener diodes, with forward voltage of approximately 4 - 5 volts, or with antiparallel generic silicon diodes with forward voltage of approximately 500 - 600 mV.

The silicon diodes will therefore clip larger portion of signal peaks, and will produce more distortion, but simultanously they limit the peak output amplitude considerably. Hence effect of reduced loudness.

Yes, the FET stage will clip as well. ...at some point. Basically at high enough input signal amplitudes the second gain stage starts to clip the signal peaks asymmetrically. But there's also a very defined threshold for "sensitivity" at which signal magnitudes the distortion is primarly symmetric produced by the diodes (with odd harmonic distortion pattern) and at which point it gradually begins to tilt more asymmetric and also introduce some duty factor modulation (with even and odd harmonic distortion  pattern).

That's how the original RG100 was made. A volume recovery stage is a nice thing to add.

Correct. Randall later made a revised version with a volume recovery stage that fixed the issue. IIRC, Google will find the schematic with "RG80" title.

Word of warning: "Preamp From Hell" circuit diagram contains a note that says: "This circuit may work with lower voltages but the sound may be affected too".
This is absolutely correct. The circuit will "work" even with puny 9VDC supply (variations of JFETS excluded)... but not as it was designed by Randall. With 9VDC supply the headroom of the JFET gain stages will be severily reduced, and they will clip at much lower threshold. In fact, clipping with starved voltages is primarily JFETs instead of Zeners, and even in "Sustain" mode the lower forward voltage will alter the "designed-in" symmetry vs. asymmetry of clipping and the "touch sensitivity" of the design. So, the overall tone and "feel" of the overdrive channel are definitely affected by this! So, if you want to evaluate a real Randall circuit DO NOT operate the circuit at starved voltages. If you want something different then yes, it will "work" and produce sound at lower voltages too. The tone is not THE classic SS Randall tone though.

[Marcos - Munky]

Slashandburn, I took the numbers from the factory schematic. Just go for a 2n2 and 2 x 220n, it will work fine. But if you gonna build it using my layout, I just ask you to use the 2nd one I made. This one:


The reasons for this:
- the 1st one was made a long time ago, with no enclosure size in mind and pots connected by wires. So it'll be probably a mess inside the enclosure;
- the 2nd one was made to fit a 1590BB, with onboard pots, so less things connected by wires;
- the 1st one also may have an error somewhere on the layout. I don't remember where's the error and if it was corrected or not;
- also, for the 2nd one, I'm pretty sure I can fit a charge pump in there. Don't know if the difference from 24V to 18V would be too big, but it'll be better than running it in 9V.

If you want the files, send me your e-mail via private message and I send you the eagle files and pdfs. I have to upload my layouts to my google drive account so I can share them easily, I just need to take some time to do it.

[slashandburn]

Thanks Marcos! I've already started building on the old board or I'd have snapped that up.  My main aim is to put this one up against the X2 that I finished a few months back. The X2 never got a "proper" box and I always wanted to compare them before boxing up my preference along with its own booster and something to bump the voltage.

I'll see how it all pans out. If it gets as far as getting it own pretty box I'll give you a shout for that new layout and I can maybe eventually get around to  putting it on the same board as the booster et al.  Looks excellent btw!   Cheers!

[Marcos - Munky]

Nice. I'm curious to hear your results.

So I checked the layout against the schematic, and did found the error: it's around the volume pot. Easy fix:

[slashandburn]

Oh nice one good catch! I'll need to fix that,  I got the board mostly finised but haven't had a chance to fire it up. With any luck I'll get a chance tonight, might be later in the week!

[slashandburn]

Holy crap there's very little between these two!  I'll reserve judgement until I've spent more than 5 minutes with them but its fair to say they're both very similar sounding and very Dimebag if you boost the bejesus out of the input.  First impressions are:   RG100 preamp could be more versatile.  The X2 is a touch harsher, possibly slightly more "modern" sounding. For all i know though, differences could just be down to parts tolerance and/or jfet biasing.  I'm running both at 18v.

IF there's any interest I'll try to get round to uploading sound clips of each. My own ears are probably not to be trusted after all the years of abuse.

[Marcos - Munky]

I surely have interest!

How's the volume output on this one?

[slashandburn]

I've only played with it at bedroom levels so far, it's kinda hard to get a gauge on how far short of unity it falls. Indeed I didn't notice the extent of the X2's shortcomings until I played around at rehearsal levels. I think it.might just.beat the X2 in terms of.output, but it's not something I was paying much attention to during the short time I managed to have a play around. I'll get back to you on that, easy enough to max both out and see which is louder.

I've already a solid preference for the RG100. The X2 seems higher gain but less dynamic as a result. The RG100 was much easier to dial in. Much more control over the bottom end and not so fizzy or ultra-compressed. Hopefully i'll get a chance to play around some more tomorro and submit something  resembling a build report.

Probably important to mention that I used J201's on both builds and my bias resistors on the RG ended up at 22k and 27k.  I don't think I substituted any other parts. The x2 build has some better quality pots and is boxed up in a plastic brick lined with slug tape while the RG is still naked and was thrown together with rusty old pots on a board I forgot to flip before etching.

Early days. I've only given it 20 minutes but round 1 probably goes to the RG.

[Marcos - Munky]

I heard the RG100 preamp sounds different with J201s than with the original fet. Since I didn't got to build it, I can't say how true is this affirmation. I'll give a try to the RG100 preamp some day, I've already etched a board, just need to finish a few things first (and stop to add more things on my to build list).

[slashandburn]

Yeah I know that feeling! I'm also somewhere in the middle of several other builds that will probably now be shelved at least until this is finished.The good thing about being this scatter-brained is that I've just remembered I have a nice enclosure I etched about  2 years ago for a Dr. Boogie that build that never made it. The pot layout and labelling is perfect for this RG100 build. Might be easier to etch a new box than look for it, though.

Regarding my jfet choice, I just ran with the highest gain device I could find. My other options were 2n5457's or MPF102's which might be better suited but with no trim pots on the first board I'm not sure I'll find the motivation to bias and test another two sets of jfets! (At least for now).

School's out and the kids are taking up most of my free time so it could be a few days before I get a chance to do any kind of comparison or record sound samples. This is where I'm at with it, I'd like to get this at least on a switch so I can get them both in the same chain to make A/B-ing a little easier. As it stands it doesn't really seem like a fair fight, but the rg100 is still edging it.





Thanks again for taking the time to do those layouts, man! I'll most likely drop you a line for the newer one if I end up going down the route of etching a box for this. It seems to do what it says on the tin. Its a bit of a hassle, considering it wants a drive in front to get it into Dimebag territory, a boost after to get it to unity, not to mention the 6 pots and 24v. There's certainly much easier ways to achieve a similar tone. It does what it's supposed to though. There's certainly scope for using it to hack together a "Little Box of Dimebag".  Could quite easily turn into a big box. I'm already wondering if a two band gyrator EQ might also be a worthy addition. (The most fun I had with the X2 was with an EQ in front, after the OD. I can't remember for the life of me what I boosted or cut though. )

[Marcos - Munky]

Dime used to push his RG100 with some overdrive (I don't remember if I ever read somewhere which one he used), so it's expected to se a drive to get Dime's tone. As for the output volume, maybe dropping that 2.2K resistor to 1K will help.

An EQ will be a fun addiction. Dime used to have 2 EQs, one to cut the mids and other to boost the mids. Both used at the same time, so he could get that mid cutted heavy tone and the mid boosting one to make sure the mid cutted guitars won't disappear in the mix.

Well, after reading your build report, now I need to build mine! I just put it a few positions early on my build list.

[slashandburn]

I've read similar accounts of his EQ setups. Two Furman PQ something or other, in front of the amp.  I haven't really looked into them but I'd assume they're big rack mount parametric units and not something we could fit into a 1590BB even if we knew his settings.  If memory serves I've also read he favoured the Zakk Wilde "Beserker" distortion in front of the eqs. I'm skeptical but since it's essentially just a modded Boss SD-1 I'm inclined to assume there could be some truth to this. I don't have a ZW or an SD1 but have had decent results with a few different Tubescreamer variants. Low gain, high volume seems to be the pre-reqiuisite rather than any specific flavour of overdrive.

Despite his father being a record producer I really don't think there was any rocket science to his tone. By most accounts, hit an RG100 with an overdrive (as a booster) and Dime (excuse the pun) the gain on the amp you're most of the way there. So far these build have mostly confirmed that theory.

[Marcos - Munky]

The ZW overdrive is indeed a modded SD-1. I used to have a clone, that a friend borrow to test it and the pedal never came back...

Yeah, no rocket sciencie. And actually I got pretty close to Dime's tone using other Randall (which I don't remember the model, but it surely wasn't the RG100) and a Sweet Honey OD.

[space_ryerson]

Long time no post here — had two kids and got a crazy busy job, and suddenly years have past me by

I've been consistently using different flavors of RG's since around 1994 or 1995, and am very familiar with the circuit. While I'm not at all a player like Dimebag, I have a few words of advice: consider using Jack Orman's warp control on those 1N914 diodes, and perhaps try adding a red diode in series with on of the 1N914's to create asymmetric clipping. That's a common Metal Zone mod, which is easily applied here. The 'contour' control found on the Century heads are also worth investigating.

After listening to the red channel hitting those diodes hard for almost 20 years, you get a bit tired of that sound, so adding some variability here will go a long way. Personally, I've stopped using the pull out knob (disabling the 1N914 clipping diodes). It doesn't sound substantially different, just a lot more 'open' and less compressed. When I need that extra level of grit, I use a booster. The fun thing about boosters is that you can have a few different flavors on your board, which bring out different tonal characteristics.

I plan to put the warp control into one of my RG's; but citing the first sentence, I haven't gotten around to it On one of my RG heads, I've swapped out the 1N914s for red diodes, which get hit HARD. If you're breadboarding, definitely give it a try.

I've found that J201's do sound a bit different than the stock 2N5484 JFETs. Not necessarily good or bad; just different. I should also mention that being a JFET circuit resembling a grandfather of the ROG designs, each Randall I've used sounds different, and has different amounts of gain. This is largely down to differences in the JFET's, so definitely socket the JFET's, and try mixing an matching.

When it comes to Dimebag, I'm not sure he used the pull out knob to enable the extra clipping, or just used a lot of boost in front of the head. The RG's do sound better with a pretty hot signal; perhaps look at the input impedance. If you haven't tried yet, a considerably part of his sound came from the Bill Lawrence pickup. I have one in one of my guitars, and it definitely has some of that Dimebag 'thing'; more so than my other guitars with EMGs, Dimarzios, or Duncan JB's.

[slashandburn]

Thanks Tim, thats some good food for thought! I've just been looking at the schematics again and noticed your name attached to the Pre-Amp From Hell, so thanks for that too.

Sorry for the delay with this one Marcos, life got in the way but I'm back at it now.  I've just started populating the board for your most recent PCB, with a view to boxing it up properly.  I will A/B them and upload the results, eventually!               

Anyway, the real reason I'm posting here, I've noticed a discrepency between schematics, the eagle files Marcos sent over for his newer PCB and the Tim's "PreAmp from Hell" from the first post on the thread both show the volume pot wired differently.  The a Capacitor across lugs2+3 of the master volume that appears to be missing from Marcos schematic and PCB.  I should be able to get away with wiring the Cap straight to the lugs, but just throwing that out there! There's also the 68k resistor on the input thats not on Tim's drawing of the schematic. I think thats already been discussed but I'm not sure why it's there or if its necessary.

[sixthfloor]

I've built a RG100 circuit on vero some time ago. I remember removing the master volume cap because it changed the harshness of the sound depending on the volume, and I wanted a more consistent behaviour.

As for voltage, I ran it at 25v, 18v and 9v, using 2n5484 transistors. There was no difference I could hear between 25v and 18v; 9v didn't work very well.

My 2 cents 

[cla2002]

Hi, I built the preamp from hell following this vero layout http://guitar-fx-layouts.42897.x6.nabble.com/file/n13955/Randall.png. I think that this layout has some problem because I can't get no sound from the pedal, I double-checked everything and seems to me that I did everything right, but still no sound at all. I also tried to see if there are any differences between the vero layout and the schematic and I found four: https://imgur.com/a/bdcLLWG

1) this 220uf is not in the schematic, is it there to smooth out the DC?
2) shouldn't this 10uf cap be a 1uf cap instead?
3) why is a 0.005uf cap replaced by a 100nf cap?
4) that seems a 22k resistor instead of a 2k2

I can't see any other difference, can you please help me to figure out why the pedal doesn't work?

[Marcos - Munky]

1) that 220uF cap is between V+ and gnd. Just a cap to filter some possible noise from the power supply.
2) yeah, you're right. It's a 1uF cap. But using a 10uF cap won't make it "non working", it will just make it have "more lows".
3) maybe who did the layout though a 5n cap is too small. And indeed it's a small value, compared to what you'll usually find on pedals, but it's the value on the factory schematic. A higher value cap will let more lows go thru. No problem on using a 100n cap, you can make up for the differences on tone using the tone stack.
4) yeah, the correct is a 2k2. The one in the layout is wrong.

We can't help you without more info, because we don't have your build to check and make measures. So:
1) is this layout verified?
2) what did you used for the fets? Did you checked the pinouts?
3) how's the voltages on the pins of the fets?
4) photos of your build will help.

[cla2002]

1) that 220uF cap is between V+ and gnd. Just a cap to filter some possible noise from the power supply.
2) yeah, you're right. It's a 1uF cap. But using a 10uF cap won't make it "non working", it will just make it have "more lows".
3) maybe who did the layout though a 5n cap is too small. And indeed it's a small value, compared to what you'll usually find on pedals, but it's the value on the factory schematic. A higher value cap will let more lows go thru. No problem on using a 100n cap, you can make up for the differences on tone using the tone stack.
4) yeah, the correct is a 2k2. The one in the layout is wrong.

We can't help you without more info, because we don't have your build to check and make measures. So:
1) is this layout verified?
2) what did you used for the fets? Did you checked the pinouts?
3) how's the voltages on the pins of the fets?
4) photos of your build will help.

Thank you for answering! I think that this layout is not verified, I found it here: http://guitar-fx-layouts.42897.x6.nabble.com/Pramp-from-Hell-Randall-preamp-td12271.html#a13955. I used four 2n5484, I checked the whole schematic (included pinouts) and it seems to be correct (but I'm not sure). Give me 10 minutes and I'll post photos of my build and the voltages on the fets' pin.

Edit: these are the photos




and these are the voltages:

Q1:
D--> 1.08
G--> 0
S--> 1.05
DG--> 0.53
DS--> 0.04
SG--> 0.52

Q2:
D--> 1.03
G--> 0
S--> 1.01
DG--> 1.02
DS--> 0.02
SG--> 0.99

Q3:
D--> 16.40
G--> 1.05
S--> 3.80
DG--> 15.37
DS--> 12.62
SG--> 2.77

Q4:
D--> 16.36
G--> 5.47
S--> 11.04
DG--> 5.41
DS--> 5.40
SG--> 2.72

The first three of every fet are the voltages between the pin and ground.

My power supply is an universal power supply at 12V (but testing it with a multimeter it says ~16-17V).