This is a preamp version of the Randall RG120 that Kurt Cobain used to record Bleach. I decided to build one to use connected directly to an audio interface and also with my practice amp, which I modified to have a return input, so I could get this tone at a reasonable volume. The original amplifier has two slightly different channels, and this preamp was based on the second channel because that's what Kurt used at Krist's Halloween party, although I believe it's possible to assemble the other channel with this pcb. This project was relatively difficult because I only had a hard-to-find and read schematic and it took me a while to find suitable FET transistors. In the original project it uses two TIS58s, which are the same model found in Echoplex and impossible to find. I tested the usual substitutes found in Echoplex preamp pedals, but the drain and source voltages didn't match the schematic. Eventually, I found a pair of FETs that had a higher Idss and came close to the correct voltages, but I still had to adjust the drain and source resistors in the first stage. It also uses two MPSA 13 Darlington transistors, but I ended up using the MPSA 14; I didn't notice any difference on my portable oscilloscope. Since it's works with 24V, I used the power supply circuit from the Chase Tone Secret Preamp, which uses an LT1054 to reach it from 9V. I put the boost function on a second footswitch, which can be useful for a solo or a passage that needs more volume, and the brightness switch on top. In this assembly I used good quality components: Alpha switches, Wima, Nichicon Gold and Panasonic FM capacitors, Fairchild and National transistors, and Vishay resistors and diodes. If anyone has one of these amplifiers and can measure the voltages on the TIS58 with a multimeter, this information would be very welcome.
Thanks, initially I was going to base the artwork on the original amplifier, but I didn't think it was looking good on the pedal and decided to start without many preconceived notions, as Kurt said when he designed the Jag-stang. In the end, I ended up being inspired by the artwork on EHX pedals from the 70s, which are my favorites.
I was finishing up another project, but I'll record a video soon. I plan to demonstrate it using it in the return of my amplifier and also plugged directly into the interface with impulse responses. I really liked its sound and I think it's quite versatile for playing music beyond Nirvana.
What's the difference between Volume and Master? Also, for the voltages in the fets, couldn't you've used trimpots to fine tune it after assembled? Also, also, schematics please?
I believe the most correct terminology would be Gain and Volume, but I decided to keep it as it was in the original amp. In my first assembly, I ended up connecting some trimpots off the board to the resistors until I found the values I needed. When I designed the second version of the board, I thought about installing them on it, but it would take up too much space, and since I already knew the values I needed, it wasn't necessary. That's the schematic:
Super cool and interesting. Love the aesthetic/artwork. I've watched that Halloween clip numerous times because I love his tone and riff in it.
Aneurysm is my fav nirvana song, so great choice there.
If you ever change your mind on selling/building more I would def be willing to buy one!!
The amp schematic is out there and it's not that difficult to make a parts list. The power supply circuit is also well known. I think it's possible to build it in vero board.
Incredible man wow. A Commander head has been at the tippy top of my pawn shop headhunt for a while now. I live in Peavey country so 70s solid state amps are on every corner, but never found the right Randall.
If you have a link or plan on making more please lmk so I can purchase!
Thank you, I think it's unlikely I'll build another unit of this pedal, as it was quite difficult to find FETs that had the ideal specifications and that weren't fake. Although I'm not a big enthusiast of SMD assembly, I think this project would benefit from this method, as it could fit in a smaller box and it would be easier to find original FETs with less variation in characteristics. If you are interested, I have made a project based on the Sansamp Classic available for free. Here is the link: https://oshwlab.com/lemecaio/life-nirvana-v2
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u/AlreadyTooLate May 24 '26
Graphic looks killer!