Preamp From Hell - Randall RG80/100

[Blackboarcult]

Thought I could share my own take on a schematic that has been rolling around from forum to forum for quite a while now.

It's verified, but won't post it on the Verified Layouts sections until at least one more people successfully builds it, just in case.

Adapted for use with 5mm pitch terminal blocks, as usual

[jarrodthebobo]

Just built this yesterday and I'm having trouble getting it to work properly.

I've rewired and looked this thing over again and again but I just can't get any volume out of it.

For whatever reason, when I'm at 9v, I get mor volume out of this pedal (with drains set to 4.5v) then I do with it at 18v (drains at 9v). 9v is almost doubly as loud as 18v, but yet still not quite as loud as it should be; its not even unity.

It DOES sound like an rg100es however, albeit a very quiet one; so the pedal is definitely amplifying and shaping the sound, but it seems as if there is something strange going on with it.

Could it be because I used 2n5485s instead of 2n5484s? Would the difference really be that much that it'd cause the pedal to not function correctly?

[ibanez48]

I read somewhere long time ago you must use 2n5484. I have never built the layout above but I have breadboarded the original schematic and it’s works and it’s loud hope this helps.

[jarrodthebobo]

The oddest thing is, if I touch the gate of Q4 the volume goes up a LOT; however it's still not unity. It merely goes from whisper volume to 'hearable'. So there has to be something going on I'd assume with the grounding... but again, looking at the schematic AND looking at this layout I cannot find any issues.

I built another audio probe; one that lets me interrupt the circuit and inject it into my amp, and I have nice signal all the way up to the 5nf capacitor before Q4. Of course I replaced this cap but with no avail.

I went ahead and ordered some 2n5484s for arguments sake, but I'm starting to feel like there is something I'm missing here and it's not the trannies themselves.

[ibanez48]

http://www.freestompboxes.org/viewtopic.php?f=13&t=28945#p269562  Check this out

[jarrodthebobo]

After experimenting some more; I think I have some busted trannies.

I had been doing the probe test backwards this whole time; starting at Q1, touching the probe to the bottom-most leg (the input) I got the highest volume of sound; when connecting the probe to the further leg; the volume gets QUIETER, yet a bit more distorted. Now my knowledge involving transistors is rather low; but I'm assuming there is supposed to be a volume increase. I've checked the biasing on this tranny, and I'm getting the correct voltages at the correct spots, but apparently no amplification across the tranny. I've swapped the orientation of the tranny just to make sure that it wasn't backwards, and that just leads to no sound at all.

Are there any tests to see if a tranny is fake or not? I highly doubt that using a 2n5485; which is in the same family as the 2n5484 would lead to a volume DROP; unless the 2.2k resistor is so wildly out of range for this tranny that it can't create amplification.

[jarrodthebobo]

After playing around even more; I've gotten this circuit to work. The problem was; I hadn't realized there were any electrolytic caps on this layout until I looked real close at both this layout and the schematic.

As of right now it's pretty loud without the sustain switch in; but also rather clean. The sustain switch drops the volume a ton, but gives a super cool crunch tone. Still not a ton of gain, but pretty chunky nonetheless.

I can't wait to throw the 2n5484s in when they arrive; I assume the pedal will be a bit more fun with the right parts

[Blackboarcult]

Sorry to chime in that late - glad you could get it to work.
It's not an über-high volume circuit, even at 24V.
FWIW I tested it with other jfets like J201 and 2N5458, and it had a similar amount of output, different gain responses and tones, but not by much.
And yeah... that sustain switch really lowers the volume

However, try the circuit without pulling out the sustain, with a TS or SD in front... that's where the magic happens

[jarrodthebobo]

Hmmm... I've been trying to do just that; boost the front end with a TS and not using the sustain switch but I'm really not getting much distortion at all out of this circuit. With the TS boosting it, the circuit kinda has a Marshall plexi level of overdrive honestly; and without the TS it's almost completely clean with a slight amount of breakup when you hit the strings hard.

Perhaps I just have some bunk transistors?

Also, when you use the sustain switch, is your circuit above unity gain? Because mine definitely isn't haha.

[Blackboarcult]

You must have some out-of-spec transistors, methinks.
Mine gets above unity gain, even with the sustain switch engaged.
When running a TS into the preamp (switch off) you should get at least Dokken-level distortion, not cleanish plexi.
Try some other transistors, verify orientation of the electrolytics, count the cuts and links again, check the voltage at drain for all trannies, and lastly, use a probe from output to input checking audio signal at every stage

[jarrodthebobo]

All of a sudden now that I'm playing the preamp through a different amp, it feels as if the preamp has much more gain availabe; especially with the TS up front. Still a bit less gain the the Tooth and Nail tones, but much closer; like a jcm800.

The sustain switch however still drops the volume below unity. Will have to see whay happens when the new trannies come in.

[jarrodthebobo]

Got the transistors in; sounds better, but I still have a huge volume drop with the switch engaged. For reference, I'm using the pedal as a preamp into the effects return of my amp, and without the switch engaged volume is just a small bit above unity.

For funsies, I added in a switch to unground the mid pot; taking the tone circuit out of circuit as a whole and the volume goes up IMMENSELY, even with the switch engaged. Now I know that tone circuits suck a ton of gain, but I have a feeling that the tone circuit shouldn't be sucking THIS much gain, should it?

Let me post my voltages for the Q4 and see if it's the same as yours:
D:24v
G: 6v
S: 12v (it WAS 8v, but looking at the schematic of the amp it appears this should be around 14-15v so I put another 1meg resistor in parallel with the 1meg from the voltage source to get a bit more juice)
 

[Blackboarcult]

Hey mate,

Unfortunately I can't check on voltages cause I lent my 24v PSU to a friend these days.
I've rechecked the schem against the layout and all appears to be OK, so unless I'm terribly blind, fault must be found in your build.

I'd suggest the following:

- try a lower value input resistor.
- check electro caps orientation
- count your cuts and jumpers
- double check the value of resistors and caps
- try several combinations of transistors

The volume drop with sustain switch engaged is perfectly normal, and I was told it was corrected on newer series, but I don't know how exactly. As for the tonestack sucking output, yup, that's normal, but it should not be a giant drop in volume either.

Anyway, I haven't played this one in several months, but I remember it had less output than, for example, the JCM800 emulation, or the Sunn Model T. I guess it was made this way in order to avoid overdriving the poweramp transistors, which would have resulted in an unpleasant sound.

Hope this helps!

[ibanez48]

When I breadboarded the original schematic and I was playing it through my fender Princeton amp it sounded great. And when I engaged the sustain switch it had volume drop but not that much. No problems with the tone stack and it’s was quiet to considering it was on a breadboard. So would agree there is something wrong with your build check everything u can. Oh and it had plenty of output.

[jarrodthebobo]

After playing around the the circuit all day it's just plane ol' stopped working and I can't see what went wrong and where. All well

I've junked this vero and ordered some more as I've run out. Maybe I'll have some better luck with this next one.

Thanks for the help guys!

[jarrodthebobo]

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!

[glasspak73]

Did you look very close at Q1 a solid wire is supose to go from row G up to row B.then a shorter solid wire connects row A to row B. I think it look like one solid line from row G to row A but its not.is it possible you may have done the row G to row A for the potentiometer? I had to look very very close befor i noticed it.

[Blackboarcult]

I wired mine to permanently have the sustain engaged, so can't give you a sound sample without it
How do the two transistors bias? Can you get them at half source voltage correctly?
Also check diode and electro caps orientation, just in case.
About building twice or more without getting it to work... I did the Friedman be-od three goddamn times last year unsuccessfully, one from effectslayouts and two from the main page, before giving up on it
Breadboarded it last month without an issue, though. Some errors are bound to be repeated I guess...

[jarrodthebobo]

Nah I have that wire soldered correctly. Also I can get all the trannies to bias correctly no issues. Im not atmy bench currently, but the values were as follows with a 25v supply

Q1:
1:12.3v
2:0.1v
3:24.8v

Q2:
1:13v
2:0.1v
3:24.8v

Q3:
1:24.8v
2: forgot
3: 13v

Q4:
1:24.8v
2:13.5
3:8v

[Blackboarcult]

I'll try to read my voltages ASAP and we'll compare.

Meanwhile, that's how it should sound without the sustain switch engaged: https://youtu.be/yW7qhTQLu4A?t=495

Take a wire from Master 3 directly to output, to check if the tonestack is the culprit here.
Try a 10K resistor at the input instead of the 68k.
Also, recheck your pot values, especially the gain pot ;-)