jcm800 split channel mod in progress

[otto]

I'm going to modify my jcm800 pre. Model 2205/4210 '82-'87 version.
Reference schematic is this:
http://www.drtube.com/schematics/marshall/4210.gif
I'll take input for V1b from V1a's anode instead that from general input, in this way I have 4 valves in the crunch channel.
How can I insert an image to show you the new schem of the crunch channel?

[Doug]

I've got the same amp...

So is it a single channel amp now? 4 gain stages or 3 with the cathode follower? How is it sounding?
Mine sounded pretty bad when I got it. I reworked the tone circuit in the normal channel to fender specs, and the entire boost channel to 2204 specs, I used a mosfet as the cathode follower. Channel switching it through a relay now. It's been quite a project, but really does a nice job of fender/marshall in one amp. It drives the reverb a little hard and the recovery is a little weak, so that's next.
Email me your schematic at q6td@netzero.net

[otto]

No, it will be still 2 channels. From anode of V1a I go on one side to the clean tone stack, on the other side I continue with the crunch channel.
I am making the mod, not finished.
Before this I've made other mods, the main are on the 2 tone stacks, not copied but on my tastes.
You've made something similar to what I have in mind. But I think that 2204 has to be both clean and crunch and so cannot be very hard. We have a clean channel, so we can push the crunch one more than original 2204.
Why a relay?? Wasn't possible to you to use the original switch circuit?

[otto]

This is the crunch channel. The clean one goes from anode of V1a to the clean tone stack
The doubt that I have is that the part of the crunch channel between between V1a and V1b will modify the clean tone stack response

[Doug]

old switching sounded bad

I can't remember how exactly but the channels were connected, I started tweaking the amp a couple years ago, but the channels were interactive not completely separate.
So the crunch channel on yours starts with v1a which is also stage 1 for the normal channel? Curious as to which is the cathode follower? Is that the mix amp v3a?
Mine switches at the amp input between v1a or v1b, normal goes v1a to tone stack then through the switch to the mix amp v3a then on through the reverb etc. The boost is through the switch to v1b then 470k/330p into 1meg pot gain control v2a 470k/470k divider, v2b to a mosfet cathode follower, marshall like tone stack, volume, switch, then mix amp, reverb etc.
A 3vdc dpdt switch keeps the channels separate, I eliminated the transistor reverb switching as well and I'll put in something simple like how fender grounds the grid at the recovery. I used the 3 volts for the indicator diode to power the switch and the footswitch just grounds or lifts to open and close.
Sounds really good, I lowered the plate resistor values (some to 56k) to get more voltage (180-230v) on all the preamp stages, the amp immediately sounded better and the voltage on the output tubes only dropped from 480-465v, it's got a great hard rock crunch and lead sound, not a heavy metal sound. I tried the 220k resistors and just thought they were too buzzy, never liked it in this amp.
I'll send a schematic if you'd like.
Doug

[otto]

>So the crunch channel on yours starts with v1a which is also stage 1 for the normal channel?

yes, consider that the 1st stage in the crunch channel is only to amplify the signal, it doesn't clip, why not use the 1st stage of the clean channel to add one stage?

>Curious as to which is the cathode follower? Is that the mix amp v3a?

I don't understand this

>Mine switches at the amp input between v1a or v1b,

I don't have this switch, is the rel่ added by you?
My original schem is the one linked in the 1st post of the thread

>normal goes v1a to tone stack then through the switch to the mix amp v3a then on through the reverb etc.

the same for me, but I don't have the rel่, I have a footswitch circuit that puts to ground one channel or the other, see schem

>The boost is through the switch to v1b then 470k/330p into 1meg pot gain control v2a 470k/470k divider, v2b to a mosfet cathode follower, marshall like tone stack, volume, switch, then mix amp, reverb etc.

this is your mod, I think it's a good idea, but maybe too weak as a crunch channel, doesn't it? Have you seen the schem of my mod?

>I lowered the plate resistor values (some to 56k) to get more voltage (180-230v) on all the preamp stages

It's good the idea to lower plate resistors. In my mod I've used (in order in the first 3 stages) 100k, 220k, 150k, see schem.
The first is for the clean channel, I think that 100k gives enough headroom, Soldano uses in this stage also 200k.
In the 2nd I mantain 220k because I want to split clipping between this stage and the 3rd one, this value reduces headroom.
In the 3rd I would use 100k or lower but I don't want to shorten life of this tube, in particular the cathode follower half, so 150k seems good to me

>I'll send a schematic if you'd like.

sure, I wait for it and please tell me what you think of my mod ...
don't you need more gain and sustain for solos without a boost?

[Doug]

Cathode follower question.

Which valve is the cathode follower? Is it V3a? Does the normal channel go directly into V4a? In the factory schematic V3a is a "mix amp" stage that feeds the reverb/effects circuit V4a/b.


When you switch to the crunch channel, does it ground v1a? or just the volume pot? Which valve works as the cathode follower? How does the normal channel get into v3a? doesn't 3a end up the cathode follower?

I added the mosfet as the cathode follower since I couldn't figure out another way without adding a tube somewhere. My mosfet scheme was taken from an article by RG Keen. Sounded great.


I did add the relay, wasn't happy with the old switching.

The crunch channel might be a little weak by today's high gain standards, it's no 5150, soldano etc... it's a 2204 style marshall crunch.

I considered ditching the reverb footpedal switch, and having the switch add a bypass cap to stage two or three.

I saw your post on adding a cap at stage 2. I tried this with a 1uf cap and a 1M/220k voltage divider and it worked really well. Couldn't tell you the exact theory behind it, but it's one scheme worth trying for more gain.

[Doug]

Schematic

Here's a look

[Doug]

I misread your schematic

I somehow thought this was a 4 gain stage preamp. I see now. cathode follower is v2b etc...

[otto]

No, it's 3 stages. The cathode follower doesn't add gain

[otto]

>Which valve is the cathode follower?

V2b

>Does the normal channel go directly into V4a?

no, in mix amp V3a, see schem
http://www.drtube.com/schematics/marshall/4210.gif

>When you switch to the crunch channel, does it ground v1a? or just the volume pot?

the volume pot

>I added the mosfet as the cathode follower since I couldn't figure out another way without adding a tube somewhere. My mosfet scheme was taken from an article by RG Keen. Sounded great.

You could also add nothing and go direct in the tone stack, it looses some effect but Engl does in this way, see schematics on their site.

>wasn't happy with the old switching.

I have no problems with it

>The crunch channel might be a little weak by today's high gain standards, it's no 5150, soldano etc... it's a 2204 style marshall crunch.

I'm lookking not for a metal channel, but I'd like to add some harmonics and sustain for solos at high gain settings, this it the reason of my circuit

>I considered ditching the reverb footpedal switch

I don't have reverb switching, my amp is a '82 model

[otto]

1st test

Mod completed. After the 1st test I've removed the 22u bypass cap at V1a because it made noise. I don't know if because it is an old one. Gain also was too much, too buzzy, so without this seems ok.
The main doubt was the interaction between the 2 channels, above all for the clean tone stack, but everything it's ok (you don't have this problem with the rel่ Doug).
Sound is great, maybe bass frequencies with too gain in the first stages but I have to test it better.
I will post the new schematic and further considerations and mods.

[otto]

Modifications on V1b cathode and other things ... see attached schematic.
I've tryied another 22u cap on V1a cathode, it works well but it adds too much gain for the clean channel.
I changed V1a plate resistor to 150k as gain compromise. In this stage headroom is not a problem.
Lowered V1b plate resistor to 150k.
This is the new schematic

[otto]

tone stack

p.s. Doug, it is 'curious' that we are arrived to the same configuration for the tone stack (crunch channel). It's not a scheme seen on other amps or mods ...

[meballar]

Otto,
thanks for posting this mod information. Are there any changes on it since 2007? Can you please make a youtube video of your amp? Thanks.
Mike