#1
Hey guys-
I have a 1981 JCM 800 4210 combo and the previous owner moded it so that instead of a master volume it has a presence control. Anyone know how to put it back so its Master or give me tips on how to overdrive the amp without the master volume? Or should i just take it to a tech to change it back?
thanks
#2
turn the pre-amp knob all the way up and use a boss super overdrive if your low on money
#3
^HAHAHA!
n00b!

I say leave the presence, and add a new master volume.

http://www.drtube.com/marshall.htm#JCM800

Find your specific amp schematic, and just add the pot back in there. It should be fairly obvious on the PCB what parts are lacking/have been replaced.
I'm not very active here on UG currently.
I'm a retired Supermod off to the greener pastures of the real world.
#5
Why you are a n00b:

1.) He wants a master volume, and you told him to get an overdrive pedal.

Is that a good reason?
I'm not very active here on UG currently.
I'm a retired Supermod off to the greener pastures of the real world.
#6
^^
he also asked how to overdrive it without the master volume


are you sure the amp has got no master volume??? http://www.drtube.com/schematics/marshall/jcm800-sc-manual.pdf
It says its present there, and its in the schematic as well

in any case, taking out the prescence would probably be pretty confusing as its basically some kind of feedback loop from the end to the beginning of the power amp, thing would sound very different without it. Though you could just move the pot to the inside of the amp (if you dont feel like drilling another hole or have enough space for the master volume)
As far as adding a master volume: just find the end of the preamp and put in a 500kohm/1Mohm log taper pot there.
#7
Yeah, I dunno how he might have added a presence, but if he used the old volume pot for the presense knob, he must have jumpered the connections that went to lugs 2+3 on the old MV. Just find an old schematic, and stick the pot back in


And damnit, I kind of ignored the not so technical aspect. I gotta start reading the whole post!
I'm not very active here on UG currently.
I'm a retired Supermod off to the greener pastures of the real world.
#8
Quote by seljer
^^
he also asked how to overdrive it without the master volume


are you sure the amp has got no master volume??? http://www.drtube.com/schematics/marshall/jcm800-sc-manual.pdf
It says its present there, and its in the schematic as well

in any case, taking out the prescence would probably be pretty confusing as its basically some kind of feedback loop from the end to the beginning of the power amp, thing would sound very different without it. Though you could just move the pot to the inside of the amp (if you dont feel like drilling another hole or have enough space for the master volume)
As far as adding a master volume: just find the end of the preamp and put in a 500kohm/1Mohm log taper pot there.


Yeah well he replaced the Master Volume knob with a presence control. How difficult is it to put back to stock yourself- especially since i dont have much electronic experience?
#9
Since we don't know how he did the presence, just add the master back in. It's not that hard, just read the original schematic.
I'm not very active here on UG currently.
I'm a retired Supermod off to the greener pastures of the real world.