jcm 800 4210 channel switcher - which mods to do

[walkman]

hi all,

Picked up a 1983 marshall 4210 , dated 10/11/83 on the chassis inspection sticker. It appears to have the circuit of the early ones , yet also has the pots on the pcb and a di /slave which , are features of the later models. It has the bc182 NPN switching transistors.

http://www.drtube.com/schematics/marshall/4210.gif

after googling i found things suggested to 'improve the tone are'

- removing the diode bounds,
- reroute the eq to after the cathode follower,
- change the phase inverter,
- dropping the anode resistors to 100k,
- changing the slope resistor in the boost channel to 56k,
- changing the 220p in the boost 330p for more dumble like tone,
- adding a 68k before the first triode and perhaps adding a 100mf bypass cap
to one of the cathodes and perhaps a 1uf on another.

Can anyone clarify what are best or most useful changes to make and in what order. It will be a pain to take the pcb out of the chassis do one mod test it, take it out again etc.

i understand it is better to remove a whole part rather than just snip the legs and solder on to them.i guess i'd prefer to mod this pcb than have to replace the pcb with a kit one. However i open to suggestion.

i'd like to make the clean more fender like , and get some smooth classic rock to heavy overdrive from the boost channel.

any comments please?

walkman




edit: i just noticed

http://music-electronics-forum.com/s...ead.php?t=4218

but would still like some guidance

cheers

[walkman]

ok , i've tried to rework the lead channel so that the cathode follower is before the tone stack , i only get a small amount of signal coming through, perhaps via the gain bypass cap ... any thoughts

[Doug H]

I pretty much own the same amp, Canadian white anni that says 83 inside. Probably rare as hen's teech. I've been tinkering recently to try and get a better half gain sound out of it. I haven't been as adventurous as you, I don't really know what all the fuss is about the tone controls being half way through the preamp, I can still adjust the tone as much as I care to, although I know it's "proper" to drive the stack with the CF...

All I've done so far is piggy back a couple resistors on the two lead channel plates and throw a 5751 (I think it's called) in V2. I left the plate resistor on v1b around 150k and dropped v2a to 110. All in all it makes a nice difference, and I might stop there, other than to play with a piggy backing some caps on v2a cathode to let more bass through. I have a V30 in it as well. Mine came with the g12 80, which falls into the "sounds best cranked" category IMO.

Ideally I'm trying to work both channels through v2 to get a early 2 stage Marshall sound out of the clean with and keep the uber lead with maybe some deeper voicing. But I can't quite figure out the switching works, it looks like it's opening a ground path for a specific channel but I do;t get why the other channel doesn't double back through the muted channel and ground out as well. Must be those cap risistor pairs at the end of each channel, but it doesn't add up to me.

Something else I've been meaning to play with is the NFB resistor. It's 100k on the schematic at the 4 ohm tap, which I've read may be overkill if there is already lot's of gain in the amp. I've been thinking of bridging it down to 80k or something like that to see if I can hear a difference.

ONe mod I've read is to turn the DI pot on the back into a presence control instead of flying with the fixed presence, which I've read is set pretty high.

[otto]

look at the thread: JCM800 split channel mod Ottovalvole

[walkman]

Fixed it... was not looking at the mod schematic clearly .... anyway sounds good, now .... thought it need some tweaking till i'm happy with it .

Also i've found now that after about half an hour or so , at loud playing , it starts to fizz/crackle on the clean channel ( maybe both haven checked) when playing notes on the bass E string from about G upwards to E but only on a neck humbucker pickup. I think it could likely be the power filter caps , which i going to replace.

It also needs a new reverb tank perhaps, as it is a bit boingy and the tank vibrates badly when played loud... it needs to be put in a bag like the fender ones and i might mount it on the base , not the back panel of the cab.

What else, rework clean channel to fender style , tweak gain channel , pentode / triode switch perhaps ...... swap the second set of caps around the MV to .1 so as to avoid cutting bass , of course make sure the bias is set right....

i've been working from

http://www.schematicheaven.com/mods/4210_mod.htm
http://www.schematicheaven.com/mods/4210_mod.gif


cheers

[drewl]

I had both the early combo version and a 100w head.
After a few years, several tubes, speakers, caps, tweaking...you name it was easier for me to build a dual channel amp that sounds much better than these than hacking up a perfectly good amp that I could sell for good money!

I didn't want to go through all the trouble of messing with a pc board amp to redesign the circuit.
Remember, at the time these were pretty advanced amps.....

[otto]

it was easier for me to build a dual channel amp that sounds much better than these than hacking up a perfectly good amp that I could sell for good money!
I didn't want to go through all the trouble of messing with a pc board amp to redesign the circuit.
Remember, at the time these were pretty advanced amps.....

I'm sure at the time these were advanced amps, first advanced amps built by Marshall. Maybe they focused too much to the idea to mix tubes and diodes. Not a bad idea, but in my opinion with same components and costs the amp could have been much better. Using the first tube stage in both channels and taking off diodes allows to have a more natural sound (see thread about mod 'Ottovalvole'). I like it much more.
I don't know how could it cost starting from zero, I know that modding one costs some time and few cheap components.

[jimirossdotcom]

I have the 50 watt combo 4210 version of this amp that I bought to use for backup transformers for my 2204. I got this amp for $200.00 and/because it sounded like dog dirt when I got it. I decided to play around with the amp and I took it as a personal challenge to make this amp sound good.

Initially I removed the clipping diodes, reworked the tone stack (including putting after the cathode follower), removed the FX loop (that was right before the mix amp that placement wasn't shown there on any schematic I could find), changed all Ra & Rk values numerous times, changed the NFB tap, replaced all pots, changed the speaker, and retubed it. This was a good start.

I agree that this has been times consuming, but not necessarily costly, mostly because I can reuse what parts I didn't already have extras of in the event that I throw in the towel, which it looks like I won't.

I have this amp sounding way better than it did, better than a 900, and close to where I want it but not quite as good as my 800. I guess I am saying there is hope. Yes, the PCB is a bear to mod on, and most of the traces have broken where I have replaced components multiple times, but I have gone thru-board and jumped without any noticeable sound difference and only compromising out of view aesthetics.

The design of this amp makes no sense to me. They have squandered resources and used 12AX7's for buffering adding no real benefit. My goal is to make the clean sound Fender-y and keep the lead channel sounding like a Marshall. My next move is to bypass the first mix amp for the lead channel and designate it to thickening up while improving volume in the clean channel, which incidentally has a tone stack that I don't care for and will change to that of a Fender.

I'd like to share anything I have learned in detail if anyone wants to discuss this amp to minutia. I am new here and would also like to hear what perspective could be offered from anyone as well.

[otto]

I'd like to share anything I have learned in detail if anyone wants to discuss this amp to minutia. I am new here and would also like to hear what perspective could be offered from anyone as well.

Jimi, please visit this theread:
http://music-electronics-forum.com/s...ht=ottovalvole