Marshall JCM800 Preamp
« on: April 27, 2019, 05:44:31 AM »
I just wanted to report, i  am done with this project and it works like a charm. I used 6n2p.

Here are the documents:

I made it in a rack, point to point without PCB, added balanced input with transformer so i can send line signals to it, and measured flattest EQ setting for sending other stuff to it, like vocals for some heavy vocal warming, or just about anything if i want to go crazy.

It works great as a guitar pre, i am thinking about ripping apart a Moer Radar, and puting it inside the same rack box. This way i get cabinet sims as well out of one box.

« Last Edit: January 02, 2020, 12:36:41 PM by kingkorg »


Re: Marshall JCM800 Preamp
« Reply #1 on: May 09, 2019, 03:07:49 PM »
Great work Kingkorg!!!


Re: Marshall JCM800 Preamp
« Reply #2 on: December 23, 2019, 06:34:59 PM »
Just an update. I use it now with a 25w Greenback cabinet and this 2.5$ output board with a lap top power supply as output stage. I am just stunned how good it sounds. I used it for couple of gigs, and studio work. It is simply amazing and tons of fun!

Only regret i have is i didn't have this back in the day when i was 15. I would have blown away all of my friends with their solid state amps and DS1 pedals.


Re: Marshall JCM800 Preamp
« Reply #3 on: January 24, 2020, 12:51:38 AM »
Can confirm, this pre works and sounds very nice. Even for metal its good preamp (in FX return of 6505, guitar with EMG pickups). Our bass player tried this preamp in FX return of his EBS amplifier and the bass sound  was...ehm...very Lemmy style :-)

Ive modified "volume" pot. I dont like that output was too hot. So I swaped 1M potentiometer for 100K log and add 1M series resistor before this pot. Volume pot is now much more usable.

Use shielded cable for wiring "Lead" switch and dont solder 1uF capacitor to this switch. I have here some minor noise. I solved this when i soldered one leg of1uF cap directly to PCB and to other connected cable.

Ive soldered 220K 2W resistor between legs of first 47uF capacitor (the one after diode rectifier). So 47uF capacitors discharge in about 1 min.

Input connectors wiring was for me a little bit harder. I incluede wiring scheme. I didnt use switch for Hi and Low input switching. I used two separated connectors. You need connectors with switching.

Ive used 250mA fuse (i think lower- 200mA could also work).

I modified a little bit pcb design (i edited a little bit ground pads and add another aprox. 1 cm to pcb for mounting holes. I attached this pcb layout (its solder bottom side). PCB final dimmensions are 135x83mm. Enjoy!!

Thanks Kingkorg for typo!!

« Last Edit: January 24, 2020, 05:13:41 AM by Vac11 »


Re: Marshall JCM800 Preamp
« Reply #4 on: November 07, 2020, 09:11:07 AM »
I shall be attempting this shortly. This project is all over the internet but mostly in Spanish and, as far as I can tell, with a lot of people asking for help with transformers but no replies.

One question I have: how do I work out what kind of transformers are needed? I understand that the first transformer takes 230v from the wall and puts out 12v which is split to the board and to transformer 2. Transformer 2's 230v output is also fed to the board.

What kind of amperage and wattage should the transformers have? How do I calculate this?


Re: Marshall JCM800 Preamp
« Reply #5 on: November 13, 2020, 04:03:32 PM »
Look at Gyraf G9 to learn how he used 2 transformers wired back to back to get HT and heaters. Switch should be switching both mains wires because of safety. Be careful with high voltage in this project and maybe find help to make first, it can be dangerous.


Re: Marshall JCM800 Preamp
« Reply #6 on: November 15, 2020, 05:48:48 AM »
Thanks for looking out  - think I have worked out how to make sure everything is safe. Good tip about having both live and neutral wires on the switch. I shall also adapt the circuit to be able to discharge the capacitors, as per Vac11's suggestion earlier.

It looks like I shall be going for the cheap halogen transformers (240, 12, 4A) for V1. I reckon I have a decent understanding of how it will all work and how to keep myself safe.


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