Randall RG100ES. Layout inside.

[Marcos - Munky]

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?

[cla2002]

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

[Marcos - Munky]

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.

[caspercody]

Looking at the picture of your solder side, you might have some shorts. Use a meter to verify you are not shorted in areas.

[cla2002]

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.

Looking at the picture of your solder side, you might have some shorts. Use a meter to verify you are not shorted in areas.

I checked continuity with the multimeter and seems that there are no shorts. I might check it out once again just to be sure.

[caspercody]

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

[Marcos - Munky]

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.

[cla2002]

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?

[Marcos - Munky]

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.

[cla2002]

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

[sixthfloor]

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.

[Marcos - Munky]

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).

[cla2002]

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).

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.

[Marcos - Munky]

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.

[cla2002]

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.

[Marcos - Munky]

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 

[jarrodthebobo]

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.

[slashandburn]

Ive played around with this circuit, needed a buffer added afterwards. See Marcos comment from earlier in the thread.

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.

[jarrodthebobo]

Ive played around with this circuit, needed a buffer added afterwards. See Marcos comment from earlier in the thread.

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?

[slashandburn]

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.