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
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.
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
Quote from: duck_arse on January 03, 2015, 09:08:33 AM
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?
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.
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.
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
SWEET! I'm gonna get my Dimebag on!
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.
I built it at 9vdc.
Quote from: caspercody on February 27, 2018, 02:19:52 PM
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.
Quote from: LiLFX on February 27, 2018, 12:09:18 PM
SWEET! I'm gonna get my Dimebag on!
HELL YEAH! (Seems like the only appropriate response)
Wow, this is a layout I've done a few years ago. Actually, got all the parts but never got to made it :icon_redface:. 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.
(https://s9.postimg.org/90t0s177f/layout.png) (https://postimg.org/image/90t0s177f/)(https://s9.postimg.org/kpx0g0dln/schematic.png) (https://postimg.org/image/kpx0g0dln/)(https://s9.postimg.org/r3m3j9arv/traces.png) (https://postimg.org/image/r3m3j9arv/)
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 :icon_twisted:.
(https://s9.postimg.org/xg5u6n0l7/1590bb_layout.png) (https://postimg.org/image/xg5u6n0l7/)(https://s9.postimg.org/4105xnbh7/1590bb_schem.png) (https://postimg.org/image/4105xnbh7/)(https://s9.postimg.org/9cf2icpu3/1590bb_traces.png) (https://postimg.org/image/9cf2icpu3/)
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.
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.
(https://s10.postimg.org/pr2qbf1qd/randallpre.gif) (https://postimg.org/image/pr2qbf1qd/)
(https://s10.postimg.org/ksf7ww5n9/randallprelayout.gif) (https://postimg.org/image/ksf7ww5n9/)
(https://s10.postimg.org/psco49ith/randallprelayout.png) (https://postimg.org/image/psco49ith/)
(https://s10.postimg.org/ha37zwwv9/x2fet_pcb.png) (https://postimg.org/image/ha37zwwv9/)
(https://s10.postimg.org/k46ddsu11/X2_FET_page_1.jpg) (https://postimg.org/image/k46ddsu11/)
(https://s10.postimg.org/ddpw4del1/X2_FET_page_2.jpg) (https://postimg.org/image/ddpw4del1/)
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.
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.
Just for reference, here are the voltages I took from my RG100ES amp the last time I had it open.
(https://s10.postimg.org/ihf1plqwl/Randall_Voltages.png) (https://postimg.org/image/ihf1plqwl/)
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.
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?
If memory serves, I also had low output from that one. I added a JFET stage at the end to boost the output.
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.
Quote from: Marcos - Munky on February 27, 2018, 05:41:39 PM
Wow, this is a layout I've done a few years ago. Actually, got all the parts but never got to made it :icon_redface:. 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!
Quote from: Marcos - Munky on March 05, 2018, 12:22:07 PM
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).
QuoteThat'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.
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:
(https://s15.postimg.cc/lvyhrf23b/randall.png) (https://postimg.cc/image/lvyhrf23b/)
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.
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!
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:
(https://s15.postimg.cc/h85gsjb4n/randallfix.png) (https://postimg.cc/image/h85gsjb4n/)
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!
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.
I surely have interest!
How's the volume output on this one?
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.
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).
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.
(https://s15.postimg.cc/7y1o1s9dj/IMG_20180719_191603.jpg) (https://postimg.cc/image/7y1o1s9dj/)
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. )
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.
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.
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.
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.
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.
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 ;)
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?
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.
Quote from: Marcos - Munky on January 29, 2019, 10:04:42 AM
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
(https://i.postimg.cc/xNKGcvNk/IMG-2956.jpg) (https://postimg.cc/xNKGcvNk)
(https://i.postimg.cc/Sj8fTh1y/IMG-2957.jpg) (https://postimg.cc/Sj8fTh1y)
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).
So, from your Q3 and Q4 voltages, seems like you're using a "18V-ish" power supply. Also, you did the corrections on Q4 and the diodes. So let's figure out what's happening.
The first thing to do is to compare the schematic and layout to see it they matches. I can't do this because my mind can't really understand vero layouts.
But moving on, those voltages on Q1 and Q2 are pretty off. Check those 2 33K resistors, the ones that connect the drain to V+. They're both on the top left corner of the board. Try to remove the solder and solder them again. Or even better, stap them for 50K-100K trimpots and adjust the trimpots until you have about 1/2 of the supply voltage between drain and ground (around 9V).
Also, do you know how to do an audio probe test?
Quote from: Marcos - Munky on January 29, 2019, 12:47:13 PM
So, from your Q3 and Q4 voltages, seems like you're using a "18V-ish" power supply. Also, you did the corrections on Q4 and the diodes. So let's figure out what's happening.
The first thing to do is to compare the schematic and layout to see it they matches. I can't do this because my mind can't really understand vero layouts.
But moving on, those voltages on Q1 and Q2 are pretty off. Check those 2 33K resistors, the ones that connect the drain to V+. They're both on the top left corner of the board. Try to remove the solder and solder them again. Or even better, stap them for 50K-100K trimpots and adjust the trimpots until you have about 1/2 of the supply voltage between drain and ground (around 9V).
Also, do you know how to do an audio probe test?
Yes, I do have one too. I've used it in the circuit and the signal stops right in Q1. Maybe that's all because of a faulty 33k resistor?
Edit: I added a diode for polarity protection in the V+ input, can this diode be part of the issue?
Edit 2: I don't think that's a faulty resistor, voltage across the terminals is the same as the power supply
Since the voltage is getting to Q3 and Q4 drain, the diode isn't the problem.
Using an audio probe, it's expected to the signal stops on Q1, since it isn't getting the voltage to work. I asked because I want you to do an "reverse audio probe test". Instead of conencting the guitar to effect input and connect the probe to the amp, connect your guitar to the probe and connect the effect output to the amp. You won't be taking the signal from part of the circuit, instead you'll be injecting signal on the circuit. Inject signal to the point where Q2 and Q3 are connected together and see how it goes. It may not having sound because of Q2 voltages, if you don't get any sound then inject the signal to that 1uF cap and see how it goes. Just to be sure the part of the circuit after Q2 is working.
And try to replace the 33K resistors with 50K or 100K trimpots and set the voltage to about half of the supply, this is better than using a fixed resistor.
Looking at the picture of your solder side, you might have some shorts. Use a meter to verify you are not shorted in areas.
(https://i.postimg.cc/s1VctkRC/IMG-2956.jpg) (https://postimg.cc/s1VctkRC)
Quote from: Marcos - Munky on January 29, 2019, 03:53:49 PM
Since the voltage is getting to Q3 and Q4 drain, the diode isn't the problem.
Using an audio probe, it's expected to the signal stops on Q1, since it isn't getting the voltage to work. I asked because I want you to do an "reverse audio probe test". Instead of conencting the guitar to effect input and connect the probe to the amp, connect your guitar to the probe and connect the effect output to the amp. You won't be taking the signal from part of the circuit, instead you'll be injecting signal on the circuit. Inject signal to the point where Q2 and Q3 are connected together and see how it goes. It may not having sound because of Q2 voltages, if you don't get any sound then inject the signal to that 1uF cap and see how it goes. Just to be sure the part of the circuit after Q2 is working.
And try to replace the 33K resistors with 50K or 100K trimpots and set the voltage to about half of the supply, this is better than using a fixed resistor.
I've tried the probe with the way that you described and i can get output only from Q4 (when putting the probe in its Gate).
I've also tried to use a pot instead of the two 33k resistors and this way I got the sound from Q1 and Q2 using the classic probe method (from either the output of Q1 and Q2). So is this a bias problem? But it's strange, the layout seems right, to me it's just like the schematic.
Quote from: caspercody on January 29, 2019, 03:56:27 PM
Looking at the picture of your solder side, you might have some shorts. Use a meter to verify you are not shorted in areas.
(https://i.postimg.cc/s1VctkRC/IMG-2956.jpg) (https://postimg.cc/s1VctkRC)
I checked continuity with the multimeter and seems that there are no shorts. I might check it out once again just to be sure.
Try doing a audio probe. Just connect a wire to the output jack and start from the beginning of the circuit and work your way to the end of the circuit and see where you lose sound
Quote from: cla2002 on January 29, 2019, 04:46:54 PM
I've tried the probe with the way that you described and i can get output only from Q4 (when putting the probe in its Gate).
I've also tried to use a pot instead of the two 33k resistors and this way I got the sound from Q1 and Q2 using the classic probe method (from either the output of Q1 and Q2). So is this a bias problem? But it's strange, the layout seems right, to me it's just like the schematic.
Yeah, it's a bias problem. Too little voltage on Q1 and Q2. Yes, the layout is exactly like the schematic, but the thing is one jfet may be really different from another. And nowadays with fake jfets, things are even worse. I built a booster for a friend some time ago, the schematic called for a 10K bias trimpot, but I only got to bias it correctly using a 250K trimpot. And it sounds amazing. So the best option is to use a trimpot and bias them manually.
So, now that you got sound out of Q1 and Q2, is the circuit working? I think you didn't got sound from Q3 before because it's connected to Q2 output, so if Q2 bias was way off this may have affected the funcion of Q3. Now everything should be working.
Quote from: Marcos - Munky on January 29, 2019, 09:40:49 PM
Quote from: cla2002 on January 29, 2019, 04:46:54 PM
I've tried the probe with the way that you described and i can get output only from Q4 (when putting the probe in its Gate).
I've also tried to use a pot instead of the two 33k resistors and this way I got the sound from Q1 and Q2 using the classic probe method (from either the output of Q1 and Q2). So is this a bias problem? But it's strange, the layout seems right, to me it's just like the schematic.
Yeah, it's a bias problem. Too little voltage on Q1 and Q2. Yes, the layout is exactly like the schematic, but the thing is one jfet may be really different from another. And nowadays with fake jfets, things are even worse. I built a booster for a friend some time ago, the schematic called for a 10K bias trimpot, but I only got to bias it correctly using a 250K trimpot. And it sounds amazing. So the best option is to use a trimpot and bias them manually.
So, now that you got sound out of Q1 and Q2, is the circuit working? I think you didn't got sound from Q3 before because it's connected to Q2 output, so if Q2 bias was way off this may have affected the funcion of Q3. Now everything should be working.
I think that the problem is that I'm feeding the circuit with ~17V instead of 24V, which is the voltage needed for the 33K resistors to bias the first two fets, am I right? I'm going to use trimpots, is it better if I use 47K or 100K trim?
I don't think the problem is the voltage - although changing the voltage would mess with bias. It's more to do with the fets themselves.
Since you're getting a low voltage with 33K, we know the correct bias point should be with a resistance lower than 33K. 100K will work, but your adjust won't be so "fine" (nothing to worry about). I'd use 47K, for better adjustment. I believe a 22K trimpot will do the job too.
Just use what you have on hand with values from 22K to 100K.
Quote from: Marcos - Munky on January 30, 2019, 08:58:07 AM
I don't think the problem is the voltage - although changing the voltage would mess with bias. It's more to do with the fets themselves.
Since you're getting a low voltage with 33K, we know the correct bias point should be with a resistance lower than 33K. 100K will work, but your adjust won't be so "fine" (nothing to worry about). I'd use 47K, for better adjustment. I believe a 22K trimpot will do the job too.
Just use what you have on hand with values from 22K to 100K.
Perfect! I bought two 100k trimpots this morning and now I can get sound from the pedal, but there is also some noise (like a high pitched squeal), I guess it's just that I have to set the trims perfectly to get the sound right. Do I have to set the bias to half the input voltage? Are there some standard voltage values?
Edit: also the tone stack and the clipping/boost switch don't work, any change in both of them won't affect the tone
Quote from: cla2002 on January 30, 2019, 09:41:20 AM
Perfect! I bought two 100k trimpots this morning and now I can get sound from the pedal, but there is also some noise (like a high pitched squeal), I guess it's just that I have to set the trims perfectly to get the sound right. Do I have to set the bias to half the input voltage? Are there some standard voltage values?
I remember the voltage indicated on the schematic I had was slightly asymetric when I built mine (I used my own vero, but the schematic should be the same). Something like 15v out of 24v for the first transistor, and slightly less for the second.
You'll usually read "set for 1/2 of the supply, then fine tune by ear" when searching for how to regulate fet bias.
The squeal may be a lot of things. Long wires, circuit outside a metal box, bad filtered power supply, just to list some of them. Use the wires as short as possible, shielded wires are better.
For the tone stack and swicth, check your wiring, they should do something (at least the tone stack).
Quote from: sixthfloor on January 30, 2019, 12:46:43 PM
Quote from: cla2002 on January 30, 2019, 09:41:20 AM
Perfect! I bought two 100k trimpots this morning and now I can get sound from the pedal, but there is also some noise (like a high pitched squeal), I guess it's just that I have to set the trims perfectly to get the sound right. Do I have to set the bias to half the input voltage? Are there some standard voltage values?
I remember the voltage indicated on the schematic I had was slightly asymetric when I built mine (I used my own vero, but the schematic should be the same). Something like 15v out of 24v for the first transistor, and slightly less for the second.
Quote from: Marcos - Munky on January 30, 2019, 01:11:45 PM
You'll usually read "set for 1/2 of the supply, then fine tune by ear" when searching for how to regulate fet bias.
The squeal may be a lot of things. Long wires, circuit outside a metal box, bad filtered power supply, just to list some of them. Use the wires as short as possible, shielded wires are better.
For the tone stack and swicth, check your wiring, they should do something (at least the tone stack).
I put mine at 1/2 of the supply and now it is working good! The squeal disappear when connecting a boss pedal before it (maybe because of the buffer of the boss pedal), so I think it's a circuit problem, maybe an input buffer in the pedal will fix it. The switch is now working, the old one was broken, that was the reason. The tone stack is still not working, I checked the connection and they seems ok, the only pots that are working are volume, gain and presence (this last one works in reverse, it opens the sound when going to zero, but I will fix it just by reversing lug 1 and lug 3 of the pot), turning bass,treble and mid does not affect the sound.
I believe the squeal have something to do with your build. Maybe it's beacause of the veroboard. The original amp doesn't have any buffers, and slashandburn didn't related any squeal problems on his build. Also, the tone stack not working may be an error on the layout or a short on your build somewhere near the tone stack. Like I said, I can't compare it to the schematic since my head can't get used to vero.
Quote from: Marcos - Munky on January 30, 2019, 06:31:36 PM
I believe the squeal have something to do with your build. Maybe it's beacause of the veroboard. The original amp doesn't have any buffers, and slashandburn didn't related any squeal problems on his build. Also, the tone stack not working may be an error on the layout or a short on your build somewhere near the tone stack. Like I said, I can't compare it to the schematic since my head can't get used to vero.
I think that maybe the squeal is due to the fact that the pedal has not a case for now, maybe if I put it into one then the squeal may disappear. But it's very strange, because the squeal appears only when the switch boost is turned off, as soon as I turn it on (and then I use the two diodes instead of the zeners) the squeal disappear.
As for the tone stack, can the fact that I replaced the 0.005uF (4.7nF) cap with the 100nF cap (the very last cap before the last jfet) be the cause for the tonestack not working? Seems that only the bass pot and the mid pot (this last one changes the sound very little) are working; also the bass pot works in reverse like the presence pot, I will reverse lug 1 and lug 3 of this one too.
Quote from: cla2002 on February 01, 2019, 08:52:53 AM
As for the tone stack, can the fact that I replaced the 0.005uF (4.7nF) cap with the 100nF cap (the very last cap before the last jfet) be the cause for the tonestack not working?
Probably. Also, I often get reversed tone pots too :icon_lol:
Sorry for bumping this old thread, but I just put this pedal together yesterday based on this vero layout here: http://guitar-fx-layouts.42897.x6.nabble.com/Preamp-From-Hell-Randall-RG80-100-td45585.html#a47244
I can't seem to get the pedal to function properly.
At 9v, and with the drains biased to 4.5v, I get a louder sound than with the pedal at 18v, drains at 9v, almost doubly so. The pedal definitely SOUNDS like an rg100, but with less than unity volume. All the voltages seem to check out on the jfets, so I'm not too sure what could be going on.
I did use 2n5485s instead of 2n5484s, would that cause such a strange issue?
I made a little probe to inject audio into the circuit at different points, and the loudest audio is present at Q4. Injecting audio at the other trannies produces much quieter audio; although the closer you get the Q1, the more distorted the audio tone.
I can't find any solder bridges, and I physically cannot check my layout any more than I already have; what could be going on here?
UPDATE: After some more fiddling, I've found that if I touch the gate of Q4, the volume jumps up ALOT, however it still doesn't appear to be hitting unity volume. I've made a plug to plug directly into my amp and interrupt the signal to hear the gains at each tranny individually; and all seems fine up until just after the 5nf cap before Q4... I've checked all around this part of the circuit and havn't been able to see anything wrong.
Ive played around with this circuit, needed a buffer added afterwards. See Marcos comment from earlier in the thread.
Quote from: Marcos - Munky on March 05, 2018, 12:22:07 PM
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.
Quote from: slashandburn on September 04, 2019, 07:19:37 PM
Ive played around with this circuit, needed a buffer added afterwards. See Marcos comment from earlier in the thread.
Quote from: Marcos - Munky on March 05, 2018, 12:22:07 PM
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.
Alright, barring the big volume drop, I'm noticing that compared to other clips of people playing real RG100's, they have quite a bit more gain then the pedal I built does even with a TS up front... is this circuit really not as gain-y as the real thing with its included power amp and the like?
I wouldn't have thought it would be lacking in gain, that's for sure. Doesn't take much to drive this into Dimebag territory with something like an SD1 or a TS in front, even without the buffer stage to make up for the volume drop. Just run a booster in front (or anything like an SD1 or TS with the gain low and the level cranked) and you should be in the ballpark.
If it's not I'd say somethings not quite right with your build. If all else fails, socket those trannies and swap in whatever other jfets you have at hand.
Completely rebuild the board and still have a lack of gain; the volume appears to be a bit better, but the gain is still very weak.
Have tried all different combinations of jfets to no avail; I'm so confused as to what could be causing this.
I mean, I can't have made the same error twice, could I?
Does anyone have a sound clip without a boost pedal in front of it so I can hear what it sounds like?
Thanks!
Hard to tell man, depends on what your perception of high-gain is and what you run it into.
It's not gonna turn a crystal clean amp into a dual rectifier. With these preamp circuits my philosophy is basically boost the @#$% out the front end (and in this case if the volume drop is ruining your fun then also run a buffer after).
Bias those trannies. Turn it up. Also check Marcos correction to the pcb earlier in this thread. Seems you've used a different layout but worth checking the pot wiring is correct as per Marcos correction.
Ive just build this circuit for the second time and i cannot get it to work. I can turn it on and hear a quiet pop but nothing else happens. Im running it qt 24.6 volts and im reading about 22-23 volts at the output and im getting voltage through all transistors.
welcome to the forum, Bennett99.
you will need to show us the stuff. photos of what you have built, component side so we can see what's there, copper side and off board wirings. and you need to post your voltage measures on all transistors. and post your circuit diagram and the layout you used [please don't make us chase the stuff you have in front of you].
Are any of you aware of someone selling a board for this circuit, say, made on JLCPCB or similar manufacturer?
Quote from: Share on July 28, 2021, 03:29:49 PM
Are any of you aware of someone selling a board for this circuit, say, made on JLCPCB or similar manufacturer?
This?
https://pcbguitarmania.com/product/vandal-from-hell/