MV mod on 74 JMP50?

[hellwood]

the reason i ask is because i recently bought this amp and was anticipating removing the MV mod but decided to plug it in just for laughs and this thing smokes my 74 SL100? WTF?!! i could not believe it. im still in shock. now im considering doing the same thing to my SL100!! anyway, i know the common MV mod is to interrupt the treble pot to the output driver, but this looks a little crazier/messier and a little harder to undo.

questions: 1) is this the shitty way to do the MV or is it the "right" way?
2) what exactly is this mod and whats the orange cap all about?

also, channel 1's input#2 is bridged to channel 2

[Tom Phillips]

That's only part of the mod circuit.
The photo is nice but can you trace out the schematic too?.

[Enzo]

The right way is any way that sounds good to you and operates as you desire. The shitty way is any way that sounds bad or does not let the amp function as it should or worse yet, causes damage.


I can't work from the photo.

[dai h.]

looks like this one (this is the one I was referring to on the plexi palace bbs) :

http://heysam.com/schem/arredondo1.jpg

[hellwood]

thanks dai for the info! i will have to really study that schematic.
enzo, here is another pic of the whole circuit board if that helps?
Tom Phillips, i will trace it out as the last resort. im hoping its a common mod and one of you recognizes it just by looking at it. i know that might be asking a lot, but im hoping its a possibility. anyway, thanks to all of you. i really appreciate it! the only clue i have is it was done in 76 by Mecktron, a company in Clearwater Florida

[dai h.]

might be another reason that you like this one. If you look at some of the resistors, it appears that Marshall subbed 150ks in places where you would usually find 100ks (if you look at other Marshall chassis, they appeared to just sub whatever they could get when they didn't have the correct value plus the ones shown are the same type as the stock Yugoslavian made Iskra resistors which are one of the stock type parts seen in old Marshalls). 150k plate resistors and 150k for the feedback resistor will both contribute to increased gain, and I'd guess this is part of the reason why you like this one. This amp also has .68uF on the presence cap, which gives more gain compared to the .1uF seen in other amps as well.

[hellwood]

thanks again dai!! could the resistors be related to the fact that this amp has 6550's?

[dai h.]

no not the ones I'm talking about. I think it's just a matter of simple parts substitution when they ran out of 100ks (I've seen the same in another amp--might have been built the same time or period--it was also a PCB amp). Two other resistors are 150ks, and that would have to do with the 6550s but that is the same (usually) in the other 6550 amps. If you like this amp, you might draw it out so you could copy the spec in another amp.

[rockgardenlove]

It looks like that capacitor is to bypass treble around the pot so that you don't lose high end.

Cheers!

[dai h.]

[hellwood]

OK folks! i have to the best of my abilities, drawn out the mod. if anybody has a clue what is happening here in this circuit, please feel free to clue me in!!!!! thanks!

[hellwood]

MV mod on 74 JMP50?

can anyone explain in detail this master vol. mod on a 74 JMP50?
the reason i ask is because i recently bought this amp and was anticipating removing the MV mod but decided to plug it in just for laughs and this thing smokes my 74 SL100? WTF?!! i could not believe it. im still in shock. now im considering doing the same thing to my SL100!! anyway, i know the common MV mod is to interrupt the treble pot to the output driver, but this looks a little crazier/messier and a little harder to undo.

[dai h.]

that is the one in the link I provided above (except I don't see a series resistor in your amp and you need to cut or erase a connection on your schematic). And also, I can't see the values in your pic but judging from the size of the cap, the cap is the same or similar value and I suppose 1M would make sense by not loading down the output(?).

[hasserl]

Unless I'm missing something there is no way that MV would work. It is just a pot wired as a variable resistor w/ cap in series, but it takes off and ends up at the same place in the circuit. That would offer no variation in volume.

[Mars Amp Repair]

I'm certain the idea is to break that connection that the pot is wired across...just not written on the diagram...

I would have thought most would put the MV pot AFTER the treble control, utilizing the coupling cap that is already there...glen