otto 12/10/2007 1:48 AM
JCM800 split channel mod Ottovalvole
This is the ultimate mod for the jcm800 split channel. I've reached it through pc simulation, circuit comparison and tests. The mod improves the clean channel but above all the crunch one, taking off diodes without losing distorsion. Above all it makes the sound fit both for rithm (as it was) and solo playing, giving the typical feeling of a full tube circuit.
If you try the mod please let me know what you think about.
You find the scheme in images at link: www.myspace.com/ottovalvole
Keep attention: touching tube circuits can be lethal also with power switched off, if you aren't skilled in the subject and you don't know how to discharge the capacitors don't touch the amp and let the work be done by someone of the job.
 
Doug H 12/10/2007 7:45 AM
cool. but I don't have a myspace account.
 
drewl 12/10/2007 7:56 AM
That's in a funny language.
 
Doug H 12/10/2007 8:11 AM
I translated it, Otto just talks about it in general, the pics are what you want, I don't have a myspace account so I can't see them to verify.

btw, it's Italian. I tried German first thinking Otto was a German name.
 
otto 12/10/2007 10:55 AM
Sorry about that.
I post here the schem
 
Doug H 12/10/2007 4:34 PM
I like how you setup v1a into the lead signal, I read your other post a while back googling my amp. If I ever remove the diodes I'll probably give that a try too. Wouldn't there be some kind of voltage drop or something? I guess it doesn't matter, jsut let out the grid resistors a bit, and hey, more gain is more gain
 
otto 12/11/2007 2:09 AM
Quote Originally Posted by Doug H View Post
I like how you setup v1a into the lead signal
Thanks. I'm proud of this simple idea, it's there ...
Re-designing the circuit around this idea solves all the troubles with this circuit and its discussed diodes

Quote Originally Posted by Doug H View Post
Wouldn't there be some kind of voltage drop or something?
no problems with mine

Quote Originally Posted by Doug H View Post
jsut let out the grid resistors a bit, and hey, more gain is more gain
I think the gain obtained is the max one for this number of stages, grid resistors used are designed to balance gain between stages

If you will try the mod don't forgive to write a feedback ...
 
otto 1/25/2008 7:46 AM
I've tranlated in English the myspace page.
www.myspace.com/ottovalvole
 
meballar 7/19/2008 8:48 PM
Please tell me the values of the new components
Hi Otto,
Thank you for posting your recommendations to improve the sound of the early 4210. Can you please tell me the values of the new components, it's hard to read some of the values and I don't want to make a mistake! I thought about buying a 4010 amp, but maybe it's worth the time to first try the modifications. After the modifications, does the amp sound like a 4010 or is it a unique sound. It would be nice if you could make a youtube video of your amp. Thanks bro.
Mike
 
otto 7/20/2008 12:57 AM
Hi Mike. You should ask me which values you cannot read ...
At the moment I have no occasion to make a sample.
The circuit is different respect a single channel like the 4010 so it hasn't exactly the same sound. The crunch channel should be something like a JCM800 single channel 'pumped up'.
What I can say is that respect the standard 4210 you can hear the preamp tubes sound whithout losing crunch and together with this you have a nice clean channel (not a Fender one due to the amp section)
 
Alexmc1988 8/11/2008 5:06 PM
Is it possible for me to get some detailed instructions for this mod? I have never modded an amp before, only pedals. where would be a good place to start?
 
otto 8/12/2008 12:56 AM
Quote Originally Posted by Alexmc1988 View Post
Is it possible for me to get some detailed instructions for this mod? I have never modded an amp before, only pedals. where would be a good place to start?
Hi Alex.
I repeat here that touching tube circuits can be lethal also with power switched off, if you aren't skilled in the subject and you don't know how to discharge the capacitors don't touch the amp or read something about this subject before, to learn how to do.
With this attention in mind I can say to you that the power section is not modded, only the preamp, with the difference respect to pedals that it is a tube circuit, but the mod consists in adding or replacing resistors and capacitors (or blocks of them) so there is not much difference in the work.
There is alto to interrupt the circuit at one point, this is a mechanical job, you have to cut the line on the board.
But maybe you could start with a simpler mod ...
Try to ask some suggestion about your doubt in a more general thread in this forum ...
Good luck
 
Alexmc1988 8/14/2008 10:46 AM
Does the schematic that isn't hand drawn have mods done to it to because it looks a little different from the original schematic I got off a different web site for the 4210
 
Alexmc1988 8/14/2008 6:04 PM
OK so I am fully confident that I can do this mod. I have gone threw the schematics and highlighted all of the changes that I can find. I must admit this is a bigger mod that I thought it was going to be but I am still confident. I have all of next week off and then I am moving back to school so I will have time to work on it. I know how to discharge the amp by placing alligator clips on the first pin of a pre amp tube then on to the chassis or some other ground. I am just having trouble following the schematics. For one Are the letters, such as "A" were the lines just stop will I make a connection to a solder pad or trace there and the then trace goes to where the other "A" is? I am also not sure where you have cut the board. It looks like the reverb is cut out of the signal switching. Will I still be able to turn the reverb on and off with the foot switch? Also I to am having trouble reading a lot of the values are you sure you can't just make a parts list? Also is there additional information on the hand drawn schematic? It looks incomplete to me and I don?t know what the "TS" with a circle around it means. Also what wattage of resistors will I need and do you suggest metal film caps and resistors like with effects pedals?
Thanks Alex.
 
otto 8/22/2008 4:29 PM
Quote Originally Posted by Alexmc1988 View Post
Does the schematic that isn't hand drawn have mods done to it to because it looks a little different from the original schematic I got off a different web site for the 4210
in internet you find one schematic for pre-1987 version and one for post-1987
(see www.drtube.com)
my mod is for the first one
 
otto 8/22/2008 5:02 PM
Quote Originally Posted by Alexmc1988 View Post
For one Are the letters, such as "A" were the lines just stop will I make a connection to a solder pad or trace there and the then trace goes to where the other "A" is?
"A" connects to the other "A", B to B and so on. I've used letters only because there was no enough space on the original schem to draw the adds.
It's a representation only, it's not important how you make it, you can use a wire soldered point to point. For a portion of the circuit I've used an additional little board with components connected point to point and then point to point to the master board.

Quote Originally Posted by Alexmc1988 View Post
I am also not sure where you have cut the board. It looks like the reverb is cut out of the signal switching. Will I still be able to turn the reverb on and off with the foot switch?
The reverb circuit varies in different versions of the jcm800. I've drawn it only to represent mine. Don't touch it if you havn't problems with it. It's not important in the resulting sound. You are still in time to mod it later, in case.
I've cut the circuit of the board in the zone of the double pot of crunch channels's gain. This means that if you want to go back with the mod you must reconnect the 2 parts with a wire.

Quote Originally Posted by Alexmc1988 View Post
Also I to am having trouble reading a lot of the values are you sure you can't just make a parts list? Also is there additional information on the hand drawn schematic? It looks incomplete to me and I don?t know what the "TS" with a circle around it means.
You can check values with the hand drawn schem. If you have doubts with a particular value ask me.
The hand drawn schem is a semplification of the important part of the mod, the one that gives the crunch channel. TS is for "tone stack", it is the tone section, you can find it on the other schem. As first job you can leave it as it is and change it in a second moment if you don't like it.

Quote Originally Posted by Alexmc1988 View Post
Also what wattage of resistors will I need and do you suggest metal film caps and resistors like with effects pedals?
Thanks Alex.
Resistors are half-watt (1/2 watt), metal films are ok. Capacitors 400V. For "sound" of different types is the same as for pedals, I think the mod is far more impostant than their material.

Go with the hand drawn mod first and try it.
Let me know ...
 
otto 8/22/2008 5:22 PM
I suggest other 3 little mods to prevent possible noise.

1) 47 pF capacitor between cathode and anode of V1a, soldered directly on the tube socket.

2) 470 pF capacitor in parallel with 150k anode resistor at V1b

3) 22 nF capacitor in series with the 220k resistor in the clean channel tone stack. If you don't change your tone stack values leave the resistor you have but add a 22 nF cap in series with it if it is not present, if it is present leave the one you have as first attempt
 
Sandfobs 10/12/2017 1:57 PM
Hey Otto hoping your reading this quickly I have my tech guy doing the mod right now, Im hoping if there is any problems your here to help me walk through. Thx in advance.
 
The Dude 10/12/2017 2:36 PM
FWIW: otto hasn't logged in since 2011.
 
Sandfobs 10/12/2017 2:57 PM
Well letís see if we can awaken the dead, by the way this amp was sounding live it could have. Lol