PLEXI/JCM800 hybrid pre

Marshall Amp Discussion

Moderators: pompeiisneaks, Colossal

Post Reply
andresound
Posts: 701
Joined: Mon Dec 01, 2014 8:48 am
Location: Durban South Africa

PLEXI/JCM800 hybrid pre

Post by andresound »

Hi guys. I'm new here. Would this work? Please feeel free to advise/comment etc.
IMG_0951.JPG
You do not have the required permissions to view the files attached to this post.
If it sounds good, it is good! Trust your ears
strelok
Posts: 276
Joined: Wed Mar 26, 2014 5:46 pm

Re: PLEXI/JCM800 hybrid pre

Post by strelok »

Yup lots of people have done that mod. Though I would recommend using a DPDT switch and placing the second pole where you have the two arrows coming together. This will keep the noise of the second gain stage out of the circuit when its bypassed. You'll probably also have to put some 1M resistors across the switch to keep it from popping when you use it. You may also consider adding a 1m resistor to the grid of the second gain stage so that it still has a ground reference when not used. I also noticed in your diagram the output goes to the CF, traditionally marshalls had one more gain stage before the CF, is that how yours is too?
andresound
Posts: 701
Joined: Mon Dec 01, 2014 8:48 am
Location: Durban South Africa

Re: PLEXI/JCM800 hybrid pre

Post by andresound »

Thank you for the reply. I will redraw and post. Maybe just pm you so as not to waste valuable space here.
Most appreciated!
If it sounds good, it is good! Trust your ears
strelok
Posts: 276
Joined: Wed Mar 26, 2014 5:46 pm

Re: PLEXI/JCM800 hybrid pre

Post by strelok »

No problem, I'm still not 100% certain if what I suggested is the best option. Will be building something very similar in the next week or so. Will give it some thought and try a couple things as I go.
andresound
Posts: 701
Joined: Mon Dec 01, 2014 8:48 am
Location: Durban South Africa

Re: PLEXI/JCM800 hybrid pre

Post by andresound »

Are you going to post your build?
If it sounds good, it is good! Trust your ears
strelok
Posts: 276
Joined: Wed Mar 26, 2014 5:46 pm

Re: PLEXI/JCM800 hybrid pre

Post by strelok »

Sure will, the best way to do it I think occured to me last night. I just had to draw it out and think about it for a second. Here's a quick drawing I made:

Image

If you can find one I highly recommend using a make before break dpdt switch. This should alleviate the need for resistors across it which will unavoidably alter the circuit characteristics. Whether or not that will be noticeable I'm not sure but if you put a 1M resistor across the first pole and you have it on the right position now there is an extra path to ground in parallel with the volume pot, with the pot all the way up its now the equivalent of having a 500k instead of 1m pot. It will also mess with the taper since its a log pot. May or may not be noticeable but this is as close as you could get I think to having both circuits all in one without actually building the two channels separately.
andresound
Posts: 701
Joined: Mon Dec 01, 2014 8:48 am
Location: Durban South Africa

Re: PLEXI/JCM800 hybrid pre

Post by andresound »

I finally have a few days available to put some work into this project for a friend. Here is the donor and the board so far. This was an unfinished atttempt by the friend (who handy at this type of work). New filter caps fitted.
IMG_1654.JPG
You do not have the required permissions to view the files attached to this post.
If it sounds good, it is good! Trust your ears
andresound
Posts: 701
Joined: Mon Dec 01, 2014 8:48 am
Location: Durban South Africa

Re: PLEXI/JCM800 hybrid pre

Post by andresound »

Re did the drawing of schem
IMG_1725.JPG
You do not have the required permissions to view the files attached to this post.
If it sounds good, it is good! Trust your ears
andresound
Posts: 701
Joined: Mon Dec 01, 2014 8:48 am
Location: Durban South Africa

Re: PLEXI/JCM800 hybrid pre

Post by andresound »

The 470k to ground needs to go after the "network". Won't bother posting a redraw. Mod works great and amp sounds great!! Now need to do a PPIMV and clean up. Thanks for all the help guys.👍
Last edited by andresound on Tue Sep 19, 2017 11:59 am, edited 1 time in total.
If it sounds good, it is good! Trust your ears
andresound
Posts: 701
Joined: Mon Dec 01, 2014 8:48 am
Location: Durban South Africa

Re: PLEXI/JCM800 hybrid pre

Post by andresound »

IMG_2624.JPG
You do not have the required permissions to view the files attached to this post.
If it sounds good, it is good! Trust your ears
framos
Posts: 20
Joined: Sat Jul 22, 2017 2:19 pm
Location: Rio de Janeiro, Brazil

Re: PLEXI/JCM800 hybrid pre

Post by framos »

Hi all, this is my first post in this forum. I am just starting out, and I have been reading and learning a lot lately in this forum. Thanks! :)

I have built a 2204 Ceriatone kit a few months ago (my second build), and I plan to do this "mod" to it. Thanks a lot for posting the schematic.

However, I was considering one thing. As the V2A's 820ohm is not bypassed by a .68uf (as in 68-69 plexis) , perhaps the 22nf would be a bit too dark for the plexi sound, and 2.2nf would be preferable, once in plexi mode (as in the 70's plexis)?
User avatar
pompeiisneaks
Site Admin
Posts: 4222
Joined: Sat Jan 14, 2017 4:36 pm
Location: Washington State, USA
Contact:

Re: PLEXI/JCM800 hybrid pre

Post by pompeiisneaks »

I'm not a plexi guru, but I do know that getting an extra capacitor of that range is only a couple dollars, and if you can afford that, it's worth it to try both and see what you think. Part of the fun here is finding what YOU think sounds best :)

~Phil
tUber Nerd!
framos
Posts: 20
Joined: Sat Jul 22, 2017 2:19 pm
Location: Rio de Janeiro, Brazil

Re: PLEXI/JCM800 hybrid pre

Post by framos »

Indeed! :D
Post Reply