Question about cascading preamps

Necrovore · Post by **Necrovore** » Fri Sep 24, 2004 8:10 pm

What is meant by this and how would you do this to your amp? I have heard tht jumping the I and II channels is the same thing but I dont really think that is how this is done.

Flames1950 · Post by **Flames1950** » Sat Sep 25, 2004 7:07 am

Jumpering the channels is running them in parallel. Cascading them is running them in series, basically like adding an extra gain stage without butchering for another preamp tube. Take one channel's wire from the 470K mixer resistors, and route it to the other channel's input instead.
Never actually done it so I can't give you any comment on the sound.

Guest · Post by **Guest** » Sat Sep 25, 2004 9:01 am

i would like to hear from someone who has actually done this to see what the results were .

Necrovore · Post by **Necrovore** » Sat Sep 25, 2004 4:01 pm

Thanks Flames. Still waiting on the rest of my Sozo's from George to redo all the caps on my board and that other mod you told me about in another thread.
On a side note. Seeing how I am working on a JCM800 SL, I was wondering if the 800 SL's had some tweaks to the tone circuit that older SL's didnt have. I am getting a mid level that I don't seem to hear in other SL's. It's not all that annoying but I dont seem to be able to get rid of it no matter how I eq the amp or use outboard Eq's. You think that I should try a different value pot for the mids? I like the added mids but would like "that" frequency shifted a bit. Then again it might just be those crappy lego caps.

rjgtr · Post by **rjgtr** » Sun Sep 26, 2004 7:49 pm

Some JCM 800s were built to do this. Plugging in the high input cascaded the two channels. In the low input it was like a standard plexi 'normal' input. It can sound good, but it can also sound harsh. It all depends on ow the gain stages are coupled.

I had a Sovtek MIG-60 (JCM800 clone) setup this way and it is a good sound. Check out a JCM-800 2203 or 2204 schematic for how this is done. This was a common mod for 70s marshalls to give them a little more gain and all for the cost of a sorting jack!

Flames1950 · Post by **Flames1950** » Mon Sep 27, 2004 8:23 am

The 2204/2203 series were cascaded, but the low input is nothing like a plexi input unless you do some modding. The input resistor is a 470K with a bright cap, not an unbypassed 68K; followed by the volume control with another bright cap, instead of going straight into a tube grid. A lot of resistance cutting into your lows and low mids while your highs go around through all the bright caps. The cathode resistor on the low input's first gain stage (which is the same as the high input's SECOND gain stage) is way too high to evoke a plexi sound as well, running an unbypassed 10K resistor instead of either the 2.7K/.68uF combo or the 820ohm/330uF combo.
You can mod this gain stage on an MV to be like a standard four input Marshall's so that your low input is more useful; you will probably find a substantial increase in gain from your high input after you're done.
Necrovore, the JCM800 MV's had an extra capacitor on the first preamp tube to suppress oscillation. I don't have a schematic in front of me to know if the Super Leads from that time period had extra caps to solve oscillation problems or not; if they do it may contribute to what you're hearing as well. I don't see a JCM800 era schematic in anything I've got bookmarked on this computer either, I'll look more later.

Necrovore · Post by **Necrovore** » Mon Sep 27, 2004 9:40 am

rjgtr, my Marshall is an 800 series 1959SL.

Flames pm me your email so I can send you the schematic for my amp that I got directly from Marshall. Im still on the lookout for that tone... Understanding that I will never get the compressed MV tone. I do know that I can get at least close to the EV tone/SLide it in era Whitesnake type tone. As far as crunch/grind goes Im almost there. But tonewise like I said the mids spike is more towards a high mid than when listening to either old VH or Slide era Whitesnake. Bright cap might b ethe problem?

rjgtr · Post by **rjgtr** » Mon Sep 27, 2004 10:36 am

I checked the 800 schematics and the 800 series 1987 has a 100p suppression cap on the Bright channel. The plexi version doesn't have this cap.

flames, I agree that the sound isn't like a plexi, I only meant it was just one gain stage like the plexis. There's a lot of differences in a Plexi and 800, some not so great. I know the low input was sort of "wrong", but then again, it wasn't a big issue because almost nobody ever used the low input.

The first mod I did to my Sovtek 800 clone was to "correct' the input resistor values. Since then it was modded with extra gain stages and a second channel, so it has been my experimentation amp. Strangely enough, the Sovtek (which is evidently a Bruno design), didn't have the oscillation suppression cap and I never had a problem.

myker · Post by **myker** » Mon Sep 27, 2004 12:39 pm

necrovore, you might also try changing your slope resistor to 47k rather thatn the 33k that is most likely in there. this fattens up the sound. also for more growling evh sound you can try a .68uf cap across the 820ohm resistor going into the second stage and a 330pf cap across the 68k input resistor that you will have to change from a 470, that will fatten it up.
i have dedicated my life to getting the harshness out of jcm 800's!! cause they rock once you get all the funky no-need capacitors and go more toward the 1987 plexi values in the pre-amp.
you guys in here really know your marshalls, and i love that.
mike..

bmf5150 · Post by **bmf5150** » Wed Sep 29, 2004 3:14 am

use both sides of the v1 tube for the .68uf lead channel..that will give you more gain