1977 jmp 50 watt one wire mod help!! PLEASE

[pelvisbuster]

Hey guys,I'm new here...My name is Ben.. I've just started modding a friends amp,and I've kinda hit a wall
I have a 1977 non master volume JMP 1987 model 50 watt plexi. I have installed the LAR/MAR MV mod,and Also cascaded the front end. I cant seem to get the bottem end right..like,there isn't a lot of low mids, but still seems like the preamp is getting loaded up. Also,it's really shrill in the high end. If volume one is dimed,it goes into crazy "wow too much fuzzy treble" kind of area.. I can't seem to get the gain I'm looking for without sounding super ugly. Tubes are KT88's and are fresh. Is there help for this thing? Is there cap/resister values that can help with input gain,and still tame the EQ section the right way? I'm trying to get this voiced for a 412 cab loaded with "MADE IN ENGLAND" V30's... I've searched and searched, I can't seam to find a component layout that shows the actual PCB so while looking at the board I can make sure I'm changing the right thing. . That would be such a help. I'm really good at soldering,and very mechanically inclined, however I do not claim to be an amp builder. SO,while I do know what the components are,I don't always know how the react in a circuit. Please help..

thanks,
Ben

[neikeel]

Hi Ben
Welcome.
In my opinion the one-wire mod is pretty much a one trick pony. If you have the first stage as stock 0.68uF/2k7 on the cathode and use 250uF/820R on V1 you will find far too much gain and mess.
The way Marshall instituted the V1 cascade they used V1b as first stage 0.68uF/2k7 and then ran it through a treble peaker 470pF/470k and then to the gain pot, feeding that signal into a 10k 'cold clipper stage'. That gives you the clarity and crispness (and the sheer balls) of an MV Marshall.
People twiddle and meddle with these combinations to their own taste. I have found that 0.68/2k7 works well with 2200pF/470k treble peaker and just 150pF cap on the gain pot into 8k2 on the second stage. Use shielded RG174 cable and I have also found using 120k plate resistor on V1b helps a little too, at least on a 71 SB which was the test rig my son and I played with - and ended up with,YMMV.
I would suggest doing the 2 in 1 mod that Steve Miller drew up on his site, that gives you a switchable cascade mod:
https://drive.google.com/drive/folders/ ... zd3STVYd3c
If the link takes you to the overall page go to the mods folder and scroll to the Neil Special pdf
Just seen same message on Marshallforum, but as this layout was developed here on Metroamp it should be here!:

[pelvisbuster]

Hey man, thanks for the reply. I definitely want to do the one wire mode thing.. we play higher gained rock gigs anyway..so as long as the amp will clean up by rolling volume off on a Les Paul.. we're good. I just need the right values, and someone to point to where they go.. or give me pictures or somewhere I can read.. anything really.. I just really wanna learn this stuff. It's fascinating to me. I wanna know how and what to change to make the amp sound good to my ears.. and be able.to voice it to my cabs.. thanks so much for your info so far. I'm willing to read, or listen.. or watch, taken direction.. whatever's necessary to get this down.. thank you so much for your response. Greatly appreciated.

Ben

[neikeel]

[pelvisbuster]

Wow.. dude you Rock

[pelvisbuster]

How can I do this with out the push/pull switch? I just wanna have the end result. I don't care to much about having the stock sound..for what we do,it's kinda useless..unless you use an OD pedal or ?? To be completely honest, I've NEVER heard a pedal I like. So for me,why buy a tube amp to use solid state drive? I want the amp to have the drive an tone, and I'll add time based effects later.. This particular amp,just never had any really "drive" to it in stock form. Basically, wide open (no mods) this thing will do super early AC/DC at best,and I do mean AT BEAST. Not good. I want it to be a hot rod.. Any suggestions?

[pelvisbuster]

Now that I look at this picture a bit closer..it seems these values are very stock. Am I missing something here?

[neikeel]

This mod is the one on SDMs website.

The switch is to retain some versatility.

If you build it exactly as it is you will get shed loads of dirt, probably too much.

Personally from your description just rewire it as a 2204, cascaded as stock, maybe drop the cathode resistor to 6k8. Without knowing your original build specs it is hard to recommend any other changes. A couple of pics might help.

[pelvisbuster]

Ok, so here in a while I'll try to figure out how to post a couple pics.. I will say this though.. I do have spare parts in there right now..testing values. Once I find and hot the desired values I will replace tone caps with nice ones. But, definitely feel free to give me pointers of what I could do better. Like I said. I'm very interested in this.. the learning part of it for sure. Thanks for your kindness sir. Folks like you, really help folks like me..and it doesn't go unappreciated..I promise.

[pelvisbuster]

I don't know if the pictures came through..could someone tell me if they see them? If not could you tell me how to do it?

[pelvisbuster]

I also have no idea what SDMs website is.. sorry,I'm a noob man..

[pelvisbuster]

preamp section of 1977 JMP MKII 50 watt 1987 model

[danman]

Here is the link to the sdm website...https://drive.google.com/drive/folders/ ... jdaSFNBOGM just click on the "mods" icon to find the info you want. This is a nice video on a "one wire mod" that sounds pretty good also...http://www.marshallforum.com/threads/th ... 602/page-4. You said that your amp is a pcb model so things will look a little different than the turret board layouts we linked to. Even though they look different, the components and values are basically the same if you trace their order through the amp. The boards for the 4 hole (1987/1959) and master volume amps (2204/2203) are the same also, they just have different components because the circuits are slightly different. The "one wire mod" will also not sound quite the same as the MV amps Marshall made because the MV amps employ a "cold clipper" cathode stage that is not found in the standard Plexi circuit. I know alot of this may sound like Greek but if you keep reading and studying this stuff, you will start to understand it. Focus on learning to read schematics instead of layouts and learn what the tube pinouts are for the premp and power tubes. It will take a little while but there is plenty of good info out there to keep you busy reading for months.

[pelvisbuster]

This mod is the one on SDMs website.

The switch is to retain some versatility.

If you build it exactly as it is you will get shed loads of dirt, probably too much.

Personally from your description just rewire it as a 2204, cascaded as stock, maybe drop the cathode resistor to 6k8. Without knowing your original build specs it is hard to recommend any other changes. A couple of pics might help.

here is a link to the marshall amp forum..for some reason I can post pics there..just not here. The values in my picture are definitely wrong.. I'm waiting on an order for some caps, some 25 or more different values.. the pictures are at the bottom of the second page .


http://www.marshallforum.com/threads/19 ... rry.99150/


http://www.marshallforum.com/threads/19 ... rry.99150/

[neikeel]

My own personal plan would be to leave the PPIMV as it appears to have been done reasonably well

All the non-stock bits I would remove. The extra 2k7 resistor perched under the 0.68uF mustard must go and the mustard soldered in properly.

Do you want owm or a 2204?