JCM 800 has some weird issues?

  • Thread starter Thread starter JakeAC5253
  • Start date Start date
JakeAC5253

JakeAC5253

New member
I recently picked up an 83' JCM 800 MV 2203 and I notice some strange performance issues with it. Likely modded, but I am not sure if this is a common mod that someone does or what.

- Presence knob is acting like a second gain knob. When the Presence is all the way down, the amp is off, and raising it makes the amp way louder. Tone is more harsh while lower, and more full at higher settings. Very strange.

- There is an extra knob next to the input jacks, doesn't seem to do a whole lot though... seems fuller at the low end of the knob, but slightly. May be a migration of the original Presence knob?

Just wondering if anyone has some input on this as I haven't opened up the circuit yet and the amp is going in for work soon, so the more educated I can make myself before doing this, the better.
 
Sounds like someone modified the presence control to be a PPIMV. Maybe. Photos of the inside of the amp would help.
 
Yep the Presence was converted to a post PI master, and the new knob looks to be the presence control.
 
That's definitely a PPI MV in the Presence hole. I can't quite tell what that other pot does. Try turning the MV all the way up, then move the "extra" knob. If it is Presence, you won't hear it much if the MV is low.
 
Ick. Going to get rid of that PPIMV asap and mod it back to stock presence knob, then I'll use the extra knobs' hole to play with some mods of my own :)

Thanks!
 
Yeah, that's a PPIMV. Looks like the "extra" knob is the presence. PPIMV should be totally out of the circuit when turned up 100%. So no need to remove it really. But when turned down low, the presence control won't do anything. Can't see the rest of the amp chassis, though. But the PPIMV should have leads going from the pot to the power tubes. This is one I installed on a Super Lead:

ec806f84b578e86f8b61bc70858f7a72-d33fw0a.jpg
 
charveldan":2o7hroaq said:
JMP/800, best amps ever ... :thumbsup: :thumbsup: :thumbsup:
Click to expand...

Haha, yeah man. Can't wait to get this circuit restored and biased for EL34s (came with KT88s) and put some new preamp glass in it as well (came with EH 12ax7s in all positions). It's already sounding pretty good at some settings, but it's yet to really sound appropriate I think.

Speaking of which, I'm very familiar with the Marshall circuit and I have a schematic for the amp, but does anyone have a nice chart or PDF that relates the schematic components to the actual board components? Or am I going to have to not be lazy about this and actually trace it out?
 
Anyone else notice the shielded lead going to V1? Looks like somebody soldered the shield to the plate (pin 6). :confused:
 
Actually B+ is ground as far as AC (audio signal) is concerned, it is pretty common to see the shield of wiring attached to B+, the ADA MP-1's did this as well. I wouldn't do it, and you don't see it much anymore due to safety concerns. So yes a :scared: is appropriate :)
 
I would first swap the kt88 for el34 before removing the ppimv as they can sound great on 800's, especially if you play at home with it. I never like kt88 in Marshall's.
 
JakeAC5253":1w7xheg9 said:
charveldan":1w7xheg9 said:
JMP/800, best amps ever ... :thumbsup: :thumbsup: :thumbsup:
Click to expand...

Haha, yeah man. Can't wait to get this circuit restored and biased for EL34s (came with KT88s) and put some new preamp glass in it as well (came with EH 12ax7s in all positions). It's already sounding pretty good at some settings, but it's yet to really sound appropriate I think.

Speaking of which, I'm very familiar with the Marshall circuit and I have a schematic for the amp, but does anyone have a nice chart or PDF that relates the schematic components to the actual board components? Or am I going to have to not be lazy about this and actually trace it out?
Click to expand...


SDM has one I used and the Metro Wiki's have one as well....

http://home.comcast.net/~mamp17/2204%20Layouts.html

http://metroamp.com/wiki/index.php/100_ ... lume_board
 
baron55":37am9qiy said:
Actually B+ is ground as far as AC (audio signal) is concerned, it is pretty common to see the shield of wiring attached to B+, the ADA MP-1's did this as well. I wouldn't do it, and you don't see it much anymore due to safety concerns. So yes a :scared: is appropriate :)
Click to expand...

Yes, this is the way they did it, and it is perfectly acceptable. No safety issues.
 
You must log in or register to reply here.
Back
Top